Lesson:
Classes and Objects
With the knowledge you now have of the basics of the Java programming
language, you can learn to write your own classes. In this lesson, you will
find information about defining your own classes, including declaring member variables,
methods, and constructors.
You will learn to use your classes to create objects, and how to use the
objects you create.
This lesson also covers nesting classes within other classes,
enumerations, and annotations.
This section shows you the anatomy of a class, and how to declare
fields, methods, and constructors.
This section covers creating and using objects. You will learn how to
instantiate an object, and, once instantiated, how to use the dot operator to access the object's instance variables and methods.
This section covers more aspects of classes that depend on using object
references and the dot operator that you learned about
in the preceding section: returning values from methods, the this keyword, class vs. instance members, and access control.
Static nested classes, inner classes, anonymous inner classes, and local
classes are covered.
This section covers enumerations, specialized classes that allow you to
define and use sets of constants.
Annotations allow you to add information to your program that is not
actually part of the program. This section describes three built-in annotations
that you should know about.
Classes
The introduction to object-oriented concepts in the lesson titled Object-oriented
Programming Concepts used a bicycle class as an example,
with racing bikes, mountain bikes, and tandem bikes as subclasses. Here is
sample code for a possible implementation of a Bicycle class, to give
you an overview of a class declaration. Subsequent sections of this lesson will
back up and explain class declarations step by step. For the moment, don't
concern yourself with the details.
public class Bicycle
{
// the Bicycle class has
// three fields
public int cadence;
public int gear;
public int speed;
// the Bicycle class has
// one constructor
public Bicycle(int startCadence, int
startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}
// the Bicycle class has
// four methods
public void setCadence(int newValue) {
cadence = newValue;
}
public void setGear(int newValue) {
gear = newValue;
}
public void applyBrake(int decrement) {
speed -= decrement;
}
public void speedUp(int increment) {
speed += increment;
}
}
A class declaration for a MountainBike class that is a
subclass of Bicycle might look like this:
public class
MountainBike extends Bicycle {
// the MountainBike subclass has
// one field
public int seatHeight;
// the MountainBike subclass has
// one constructor
public MountainBike(int startHeight, int
startCadence,
int startSpeed, int
startGear) {
super(startCadence, startSpeed,
startGear);
seatHeight = startHeight;
}
// the MountainBike subclass has
// one method
public void setHeight(int newValue) {
seatHeight = newValue;
}
}
MountainBike inherits all the fields and methods of Bicycle and adds the field seatHeight and a method to
set it (mountain bikes have seats that can be moved up and down as the terrain
demands).
Declaring Classes
You've seen classes
defined in the following way:class MyClass {
// field, constructor, and
// method declarations
}
This is a class declaration. The class body (the area between the braces) contains
all the code that provides for the life cycle of the objects created from the
class: constructors for initializing new objects, declarations for the fields
that provide the state of the class and its objects, and methods to implement
the behaviour of the class and its objects.The preceding class declaration is a minimal one. It contains only those components of a class declaration that are required. You can provide more information about the class, such as the name of its superclass, whether it implements any interfaces, and so on, at the start of the class declaration. For example,
class MyClass extends MySuperClass implements YourInterface {
// field, constructor, and
// method declarations
}
means that MyClass is a subclass of MySuperClass and that it implements the YourInterface interface.You can also add modifiers like public or private at the very beginning—so you can see that the opening line of a class declaration can become quite complicated. The modifiers public and private, which determine what other classes can access
MyClass, are discussed later in this
lesson. The lesson on interfaces and inheritance will explain how and why you
would use the extends and implements keywords in a class declaration. For
the moment you do not need to worry about these extra complications.In general, class declarations can include these components, in order:
1.
Modifiers such as public, private, and a number of others
that you will encounter later.
2.
The class name, with the initial letter capitalized by convention.
3.
The name of the class's parent (superclass), if any, preceded by
the keyword extends. A
class can only extend (subclass) one parent.
4.
A comma-separated list of interfaces implemented by the class, if
any, preceded by the keyword implements.
A class can implement more than one interface.
5.
The class body, surrounded by braces, {}.
Declaring
Member Variables
There are several kinds of variables:
·
Member variables in a class—these are
called fields.
·
Variables in a method or block of
code—these are called local variables.
·
Variables in method declarations—these
are called parameters.
The Bicycle class uses the following lines of code to define its fields:
public int cadence;
public int gear;
public int speed;
Field declarations are composed of three components, in order:
1. Zero or more modifiers, such as public or private.
2. The field's type.
3. The field's name.
The fields of Bicycle are named cadence, gear, and speed and are all of data type integer (int). The public keyword identifies these fields as public members, accessible by
any object that can access the class.
Access Modifiers
The first (left-most) modifier used lets you control what other classes
have access to a member field. For the moment, consider only public and private. Other access modifiers will be
discussed later.
·
public modifier—the field is accessible
from all classes.
·
private modifier—the field is accessible
only within its own class.
In the spirit of encapsulation, it is common to make fields private.
This means that they can only be directly accessed from the
Bicycle class. We still need access to these values, however. This can be
done indirectlyby adding public methods that obtain the field
values for us:
public class Bicycle
{
private int cadence;
private int gear;
private int speed;
public Bicycle(int startCadence, int
startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}
public int getCadence() {
return cadence;
}
public void setCadence(int newValue) {
cadence = newValue;
}
public int getGear() {
return gear;
}
public void setGear(int newValue) {
gear = newValue;
}
public int getSpeed() {
return speed;
}
public void applyBrake(int decrement) {
speed -= decrement;
}
public void speedUp(int increment) {
speed += increment;
}
}
Types
All variables must have a type. You can use primitive types such
as int, float, boolean, etc. Or you can use reference types, such as strings, arrays, or
objects.
Variable Names
All variables, whether they are fields, local variables, or parameters,
follow the same naming rules and conventions that were covered in the Language
Basics lesson, Variables—Naming.
In this lesson, be aware that the same naming rules and conventions are
used for method and class names, except that
·
the first letter of a class name should
be capitalized, and
·
the first (or only) word in a method
name should be a verb.
Defining
Methods
Here is an example of a typical method declaration:
public double
calculateAnswer(double wingSpan, int numberOfEngines,
double length,
double grossTons) {
//do the calculation here
}
The only required elements of a method declaration are the method's
return type, name, a pair of parentheses, (), and a body between
braces, {}.
More generally, method declarations have six components, in order:
1. Modifiers—such as public, private, and others you will
learn about later.
2. The return type—the data type of the value returned by the method,
or void if the method does not return a value.
3. The method name—the rules for field names apply to method names as well,
but the convention is a little different.
4. The parameter list in parenthesis—a comma-delimited list of input
parameters, preceded by their data types, enclosed by parentheses, (). If there are no parameters, you must use empty parentheses.
5. An exception list—to be discussed later.
6. The method body, enclosed between braces—the method's code, including
the declaration of local variables, goes here.
Modifiers, return types, and parameters will be discussed later in this
lesson. Exceptions are discussed in a later lesson.
Definition: Two of the components of a method
declaration comprise the method signature—the method's name and the
parameter types.
The signature of the method declared above is:
calculateAnswer(double,
int, double, double)
Naming a Method
Although a method name can be any legal identifier, code conventions
restrict method names. By convention, method names should be a verb in
lowercase or a multi-word name that begins with a verb in lowercase, followed
by adjectives, nouns, etc. In multi-word names, the first letter of each of the
second and following words should be capitalized. Here are some examples:
run
runFast
getBackground
getFinalData
compareTo
setX
isEmpty
Typically, a method has a unique name within its class. However, a method
might have the same name as other methods due to method overloading.
Overloading Methods
The Java programming language supports overloading methods,
and Java can distinguish between methods with different method
signatures. This means that methods within a class can have the same name
if they have different parameter lists (there are some qualifications to this
that will be discussed in the lesson titled "Interfaces and
Inheritance").
Suppose that you have a class that can use calligraphy to draw various
types of data (strings, integers, and so on) and that contains a method for
drawing each data type. It is cumbersome to use a new name for each method—for
example, drawString, drawInteger, drawFloat, and so on. In the Java programming language, you can use the same name
for all the drawing methods but pass a different argument list to each method.
Thus, the data drawing class might declare four methods named draw, each of which has a different parameter list.
public class
DataArtist {
...
public void draw(String s) {
...
}
public void draw(int i) {
...
}
public void draw(double f) {
...
}
public void draw(int i, double f) {
...
}
}
Overloaded methods are differentiated by the number and the type of the
arguments passed into the method. In the code sample, draw(String s) and draw(int i) are distinct
and unique methods because they require different argument types.
You cannot declare more than one method with the same name and the same number
and type of arguments, because the compiler cannot tell them apart.
The compiler does not consider return type when differentiating methods,
so you cannot declare two methods with the same signature even if they have a
different return type.
Note: Overloaded methods should be used sparingly,
as they can make code much less readable.
Providing Constructors for Your Classes
A class contains
constructors that are invoked to create objects from the class blueprint.
Constructor declarations look like method declarations—except that they use the
name of the class and have no return type. For example, Bicycle has one constructor:public Bicycle(int startCadence, int startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}
To create a new Bicycle object called myBike, a constructor is called by
the new operator:Bicycle myBike = new Bicycle(30, 0, 8);
new Bicycle(30,
0, 8) creates space in memory for the object and
initializes its fields.Although
Bicycle only has one constructor, it could have others, including a
no-argument constructor:public Bicycle() {
gear = 1;
cadence = 10;
speed = 0;
}
Bicycle yourBike
= new Bicycle(); invokes the no-argument
constructor to create a new Bicycle object called yourBike.Both constructors could have been declared in
Bicycle because they have different argument lists. As with methods, the
Java platform differentiates constructors on the basis of the number of
arguments in the list and their types. You cannot write two constructors that
have the same number and type of arguments for the same class, because the
platform would not be able to tell them apart. Doing so causes a compile-time
error.You don't have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors. This default constructor will call the no-argument constructor of the superclass. In this situation, the compiler will complain if the superclass doesn't have a no-argument constructor so you must verify that it does. If your class has no explicit superclass, then it has an implicit superclass of
Object, which does have a no-argument constructor.You can use a superclass constructor yourself. The
MountainBike class at the beginning of this lesson did just that. This will be
discussed later, in the lesson on interfaces and inheritance.You can use access modifiers in a constructor's declaration to control which other classes can call the constructor.
Note: If
another class cannot call a
MyClass constructor, it cannot
directly create MyClass objects.
Passing Information to a Method or a Constructor
The declaration for a
method or a constructor declares the number and the type of the arguments for
that method or constructor. For example, the following is a method that
computes the monthly payments for a home loan, based on the amount of the loan,
the interest rate, the length of the loan (the number of periods), and the
future value of the loan:public double computePayment(
double loanAmt,
double rate,
double futureValue,
int numPeriods) {
double interest = rate / 100.0;
double partial1 = Math.pow((1 + interest),
- numPeriods);
double denominator = (1 - partial1) / interest;
double answer = (-loanAmt / denominator)
- ((futureValue * partial1) / denominator);
return answer;
}
This method has four
parameters: the loan amount, the interest rate, the future value and the number
of periods. The first three are double-precision floating point numbers, and
the fourth is an integer. The parameters are used in the method body and at
runtime will take on the values of the arguments that are passed in.
Note: Parameters refers to the list of variables in a
method declaration. Arguments are the actual values that are passed
in when the method is invoked. When you invoke a method, the arguments used
must match the declaration's parameters in type and order.
Parameter Types
You can use any data
type for a parameter of a method or a constructor. This includes primitive data
types, such as doubles, floats, and integers, as you saw in the computePayment method, and reference data types, such as objects and arrays.Here's an example of a method that accepts an array as an argument. In this example, the method creates a new
Polygon object and initializes it from an array of Point objects (assume that Point is a class that represents an x, y coordinate):public Polygon polygonFrom(Point[] corners) {
// method body goes here
}
Note: The Java
programming language doesn't let you pass methods into methods. But you can
pass an object into a method and then invoke the object's methods.
Arbitrary Number of Arguments
You can use a
construct called varargs to pass an arbitrary number of values
to a method. You use varargs when you don't know how many of a particular type
of argument will be passed to the method. It's a shortcut to creating an array
manually (the previous method could have used varargs rather than an array).To use varargs, you follow the type of the last parameter by an ellipsis (three dots, ...), then a space, and the parameter name. The method can then be called with any number of that parameter, including none.
public Polygon polygonFrom(Point... corners) {
int numberOfSides = corners.length;
double squareOfSide1, lengthOfSide1;
squareOfSide1 = (corners[1].x - corners[0].x)
* (corners[1].x - corners[0].x)
+ (corners[1].y - corners[0].y)
* (corners[1].y - corners[0].y);
lengthOfSide1 = Math.sqrt(squareOfSide1);
// more method body code follows that creates and returns a
// polygon connecting the Points
}
You can see that,
inside the method, corners is treated like an array. The method can be called either with an
array or with a sequence of arguments. The code in the method body will treat
the parameter as an array in either case.You will most commonly see varargs with the printing methods; for example, this
printf method:public PrintStream printf(String format, Object... args)
allows you to print an
arbitrary number of objects. It can be called like this:System.out.printf("%s: %d, %s%n", name, idnum, address);
or like thisSystem.out.printf("%s: %d, %s, %s, %s%n", name, idnum, address, phone, email);
or with yet a different
number of arguments.
Parameter Names
When you declare a
parameter to a method or a constructor, you provide a name for that parameter.
This name is used within the method body to refer to the passed-in argument.The name of a parameter must be unique in its scope. It cannot be the same as the name of another parameter for the same method or constructor, and it cannot be the name of a local variable within the method or constructor.
A parameter can have the same name as one of the class's fields. If this is the case, the parameter is said to shadow the field. Shadowing fields can make your code difficult to read and is conventionally used only within constructors and methods that set a particular field. For example, consider the following
Circle class and its setOrigin method:public class Circle {
private int x, y, radius;
public void setOrigin(int x, int y) {
...
}
}
The Circle class has three fields: x, y, and radius. The setOrigin method has two parameters, each of which has the same name as one
of the fields. Each method parameter shadows the field that shares its name. So
using the simple names x or y within the body of the method refers to the parameter, not to the field. To access the field, you
must use a qualified name. This will be discussed later in this lesson in the
section titled "Using the this Keyword."
Passing Primitive Data Type Arguments
Primitive arguments,
such as an int or a double, are passed into methods by value. This means that any
changes to the values of the parameters exist only within the scope of the
method. When the method returns, the parameters are gone and any changes to
them are lost. Here is an example:public class PassPrimitiveByValue {
public static void main(String[] args) {
int x = 3;
// invoke passMethod() with
// x as argument
passMethod(x);
// print x to see if its
// value has changed
System.out.println("After invoking passMethod, x = " + x);
}
// change parameter in passMethod()
public static void passMethod(int p) {
p = 10;
}
}
When you run this
program, the output is:After invoking passMethod, x = 3
Passing Reference Data Type Arguments
Reference data type
parameters, such as objects, are also passed into methods by value. This means that when
the method returns, the passed-in reference still references the same object as
before. However, the
values of the object's fields can be changed in the method, if they have
the proper access level.For example, consider a method in an arbitrary class that moves
Circle objects:public void moveCircle(Circle circle, int deltaX, int deltaY) {
// code to move origin of
// circle to x+deltaX, y+deltaY
circle.setX(circle.getX() + deltaX);
circle.setY(circle.getY() + deltaY);
// code to assign a new
// reference to circle
circle = new Circle(0, 0);
}
Let the method be
invoked with these arguments:moveCircle(myCircle, 23, 56)
Inside the method, circle initially refers to myCircle. The method changes the x and
y coordinates of the object that circle references (i.e., myCircle) by 23 and 56, respectively.
These changes will persist when the method returns. Then circle is assigned a reference to a new Circle object with x = y = 0. This reassignment has no
permanence, however, because the reference was passed in by value and cannot
change. Within the method, the object pointed to by circle has changed, but, when the method returns, myCircle still references the same Circle object as before the method was called.
Objects
A typical Java program
creates many objects, which as you know, interact by invoking methods. Through
these object interactions, a program can carry out various tasks, such as
implementing a GUI, running an animation, or sending and receiving information
over a network. Once an object has completed the work for which it was created,
its resources are recycled for use by other objects.Here's a small program, called
CreateObjectDemo,
that creates three objects: one Point object and two Rectangle objects. You will need all three source files to compile this
program.
public class CreateObjectDemo {
public static void main(String[] args) {
// Declare and create a point object
// and two rectangle objects.
Point originOne = new Point(23, 94);
Rectangle rectOne = new
Rectangle(originOne, 100, 200);
Rectangle rectTwo =
new Rectangle(50, 100);
// display rectOne's width,
// height, and area
System.out.println("Width of rectOne: "
+ rectOne.width);
System.out.println("Height of rectOne: "
+ rectOne.height);
System.out.println("Area of rectOne: "
+ rectOne.getArea());
// set rectTwo's position
rectTwo.origin = originOne;
// display rectTwo's position
System.out.println("X Position of rectTwo: "
+ rectTwo.origin.x);
System.out.println("Y Position of rectTwo: "
+ rectTwo.origin.y);
// move rectTwo and display
// its new position
rectTwo.move(40, 72);
System.out.println("X Position of rectTwo: "
+ rectTwo.origin.x);
System.out.println("Y Position of rectTwo: "
+ rectTwo.origin.y);
}
}
This program creates,
manipulates, and displays information about various objects. Here's the output:Width of rectOne: 100
Height of rectOne: 200
Area of rectOne: 20000
X Position of rectTwo: 23
Y Position of rectTwo: 94
X Position of rectTwo: 40
Y Position of rectTwo: 72
The following three
sections use the above example to describe the life cycle of an object within a
program. From them, you will learn how to write code that creates and uses
objects in your own programs. You will also learn how the system cleans up
after an object when its life has ended.
Creating Objects
As you know, a class
provides the blueprint for objects; you create an object from a class. Each of
the following statements taken from the CreateObjectDemo program creates an object and assigns
it to a variable:Point originOne = new Point(23, 94);
Rectangle rectOne = new Rectangle(originOne, 100, 200);
Rectangle rectTwo = new Rectangle(50, 100);The first line creates an object of the
Point class, and the second and third lines each create an object of the Rectangle class.Each of these statements has three parts (discussed in detail below):
1.
Declaration: The code set in bold are all variable declarations that
associate a variable name with an object type.
2.
Instantiation: The new keyword is a Java operator that
creates the object.
3.
Initialization: The new operator is followed by a call
to a constructor, which initializes the new object.
Declaring a Variable to Refer to an Object
Previously, you
learned that to declare a variable, you write:type name;This notifies the compiler that you will use name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable.
You can also declare a reference variable on its own line. For example:
Point originOne;
If you declare originOne like this, its value will be undetermined until an object is
actually created and assigned to it. Simply declaring a reference variable does
not create an object. For that, you need to use thenew operator, as described in the next section. You must assign an
object to originOne before you use it in your code. Otherwise, you will get a compiler
error.A variable in this state, which currently references no object, can be illustrated as follows (the variable name,
originOne, plus a reference pointing to
nothing):
Instantiating a Class
The new operator instantiates a class by allocating memory for a new
object and returning a reference to that memory. The new operator also invokes the object constructor.
Note: The
phrase "instantiating a class" means the same thing as "creating
an object." When you create an object, you are creating an
"instance" of a class, therefore "instantiating" a class.
The new operator returns a reference to the object it created. This reference is usually assigned to a variable of the appropriate type, like:
The reference returned by the new operator does not have to be assigned to a variable. It can also be used directly in an expression. For example:Point originOne = new Point(23, 94);
int height = new Rectangle().height;
This statement will be
discussed in the next section.
Initializing an Object
Here's the code for
the Point class:public class Point {
public int x = 0;
public int y = 0;
//constructor
public Point(int a, int b) {
x = a;
y = b;
}
}
This class contains a
single constructor. You can recognize a constructor because its declaration
uses the same name as the class and it has no return type. The constructor in
the Point class takes two integer arguments, as declared by the code (int a, int b). The following statement
provides 23 and 94 as values for those arguments:Point originOne = new Point(23, 94);
The result of
executing this statement can be illustrated in the next figure:
public class Rectangle {
public int width = 0;
public int height = 0;
public Point origin;
// four constructors
public Rectangle() {
origin = new Point(0, 0);
}
public Rectangle(Point p) {
origin = p;
}
public Rectangle(int w, int h) {
origin = new Point(0, 0);
width = w;
height = h;
}
public Rectangle(Point p, int w, int h) {
origin = p;
width = w;
height = h;
}
// a method for moving the rectangle
public void move(int x, int y) {
origin.x = x;
origin.y = y;
}
// a method for computing the area
// of the rectangle
public int getArea() {
return width * height;
}
}
Each constructor lets
you provide initial values for the rectangle's size and width, using both
primitive and reference types. If a class has multiple constructors, they must
have different signatures. The Java compiler differentiates the constructors
based on the number and the type of the arguments. When the Java compiler
encounters the following code, it knows to call the constructor in the Rectangle class that requires aPoint argument followed by two
integer arguments:
Rectangle rectOne = new Rectangle(originOne, 100, 200);
This calls one of Rectangle's constructors that initializes origin to originOne. Also, the constructor sets width to 100 and height to 200. Now there are two references to the same Point object—an object can have multiple references to
it, as shown in the next figure:
Rectangle constructor that requires two integer arguments, which provide the
initial values for width and height. If you inspect the code
within the constructor, you will see that it creates a new Point object whose x and y values are initialized to 0:Rectangle rectTwo = new Rectangle(50, 100);
The Rectangle constructor used in the following statement doesn't take any
arguments, so it's called a no-argument
constructor:Rectangle rect = new Rectangle();
All classes have at
least one constructor. If a class does not explicitly declare any, the Java
compiler automatically provides a no-argument constructor, called the default constructor. This
default constructor calls the class parent's no-argument constructor, or the Object constructor if the class has no other parent. If the parent has no
constructor (Object does have one), the compiler will reject the program.
Using Objects
Once you've created an
object, you probably want to use it for something. You may need to use the
value of one of its fields, change one of its fields, or call one of its
methods to perform an action.
Referencing an Object's Fields
Object fields are
accessed by their name. You must use a name that is unambiguous.You may use a simple name for a field within its own class. For example, we can add a statement within the
Rectangle class that prints the width and height:System.out.println("Width and height are: " + width + ", " + height);
In this case, width and height are simple names.Code that is outside the object's class must use an object reference or expression, followed by the dot (.) operator, followed by a simple field name, as in:
objectReference.fieldName
For example, the code
in the CreateObjectDemo class is outside the code for the Rectangle class. So to refer to the origin, width, and height fields within the Rectangle object named rectOne, theCreateObjectDemo class must use the names rectOne.origin, rectOne.width, and rectOne.height, respectively. The program
uses two of these names to display the width and the height of rectOne:System.out.println("Width of rectOne: "
+ rectOne.width);
System.out.println("Height of rectOne: "
+ rectOne.height);
Attempting to use the
simple names width and height from the code in the CreateObjectDemo class doesn't make sense — those fields exist only within an
object — and results in a compiler error.Later, the program uses similar code to display information about rectTwo. Objects of the same type have their own copy of the same instance fields. Thus, each Rectangle object has fields named origin,width, and height. When you access an instance field through an object reference, you reference that particular object's field. The two objects rectOne and rectTwo in the CreateObjectDemo program have different origin, width, and height fields.
To access a field, you can use a named reference to an object, as in the previous examples, or you can use any expression that returns an object reference. Recall that the new operator returns a reference to an object. So you could use the value returned from new to access a new object's fields:
int height = new Rectangle().height;
This statement creates
a new Rectangle object and immediately gets its height. In essence, the statement
calculates the default height of a Rectangle. Note that after this
statement has been executed, the program no longer has a reference to the
created Rectangle, because the program never
stored the reference anywhere. The object is unreferenced, and its resources
are free to be recycled by the Java Virtual Machine.
Calling an Object's Methods
You also use an object
reference to invoke an object's method. You append the method's simple name to
the object reference, with an intervening dot operator (.). Also, you provide,
within enclosing parentheses, any arguments to the method. If the method does
not require any arguments, use empty parentheses.objectReference.methodName(argumentList);
or:objectReference.methodName();
The Rectangle class has two methods: getArea() to compute the rectangle's area and move() to change the rectangle's origin. Here's the CreateObjectDemo code that invokes these two methods:System.out.println("Area of rectOne: " + rectOne.getArea());
...
rectTwo.move(40, 72);
The first statement
invokes rectOne's getArea() method and displays the results. The second line moves rectTwo because the move() method assigns new values to the object's origin.x and origin.y.As with instance fields, objectReference must be a reference to an object. You can use a variable name, but you also can use any expression that returns an object reference. The new operator returns an object reference, so you can use the value returned from new to invoke a new object's methods:
new Rectangle(100, 50).getArea()
The expression new Rectangle(100, 50) returns an object reference that refers to a Rectangle object. As shown, you can use the dot notation to invoke the new Rectangle's getArea() method to compute the area of the new rectangle.Some methods, such as getArea(), return a value. For methods that return a value, you can use the method invocation in expressions. You can assign the return value to a variable, use it to make decisions, or control a loop. This code assigns the value returned by getArea() to the variable
areaOfRectangle:int areaOfRectangle = new Rectangle(100, 50).getArea();
Remember, invoking a
method on a particular object is the same as sending a message to that object.
In this case, the object that getArea() is invoked on is the rectangle returned by the constructor.
The Garbage Collector
Some object-oriented
languages require that you keep track of all the objects you create and that
you explicitly destroy them when they are no longer needed. Managing memory
explicitly is tedious and error-prone. The Java platform allows you to create
as many objects as you want (limited, of course, by what your system can
handle), and you don't have to worry about destroying them. The Java runtime
environment deletes objects when it determines that they are no longer being
used. This process is called garbage
collection.An object is eligible for garbage collection when there are no more references to that object. References that are held in a variable are usually dropped when the variable goes out of scope. Or, you can explicitly drop an object reference by setting the variable to the special value null. Remember that a program can have multiple references to the same object; all references to an object must be dropped before the object is eligible for garbage collection.
The Java runtime environment has a garbage collector that periodically frees the memory used by objects that are no longer referenced. The garbage collector does its job automatically when it determines that the time is right.
More on
Classes
This section covers more aspects of classes that depend on using object
references and the dot operator that you learned about
in the preceding sections on objects:
·
Returning values from methods.
·
The this keyword.
·
Class vs. instance members.
·
Access control.
Returning a
Value from a Method
A method returns to the code that invoked it when it
·
completes all the statements in the
method,
·
reaches a return statement, or
·
throws an exception (covered later),
whichever occurs first.
You declare a method's return type in its method declaration. Within the
body of the method, you use the return statement to
return the value.
Any method declared void doesn't return a value. It does
not need to contain a return statement, but it may do so. In
such a case, a return statement can be used to branch out of a control flow block and
exit the method and is simply used like this:
return;
If you try to return a value from a method that is declared void, you will get a compiler error.
Any method that is not declared void must contain
a return statement with a corresponding return value, like this:
return returnValue;
The data type of the return value must match the method's declared
return type; you can't return an integer value from a method declared to return
a boolean.
The getArea() method in the Rectangle Rectangle class
that was discussed in the sections on objects returns an integer:
// a method for computing the area of the
rectangle
public int getArea() {
return width * height;
}
This method returns the integer that the expression width*height evaluates to.
The getArea method returns a primitive type. A method can also return a
reference type. For example, in a program to manipulate Bicycle objects, we might have a method like this:
public Bicycle
seeWhosFastest(Bicycle myBike, Bicycle yourBike,
Environment env)
{
Bicycle fastest;
// code to calculate which bike is
// faster, given each bike's gear
// and cadence and given the
// environment (terrain and wind)
return fastest;
}
Returning a Class or Interface
If this section confuses you, skip it and return to it after you have
finished the lesson on interfaces and inheritance.
When a method uses a class name as its return type, such as whosFastest does, the class of the type of the returned object must be either
a subclass of, or the exact class of, the return type. Suppose that you have a
class hierarchy in which ImaginaryNumber is a subclass
of java.lang.Number, which is in turn a subclass of Object, as illustrated in the following figure.
The class hierarchy for ImaginaryNumber
Now suppose that you have a method declared to return a Number:
public Number
returnANumber() {
...
}
The returnANumber method can return an ImaginaryNumber but not an Object. ImaginaryNumber is a Number because it's a subclass of Number. However, an Object is not necessarily a Number — it could be
a String or another type.
You can override a method and define it to return a subclass of the
original method, like this:
public
ImaginaryNumber returnANumber() {
...
}
This technique, called covariant return type, means that the
return type is allowed to vary in the same direction as the subclass.
Note: You also can use interface names as return
types. In this case, the object returned must implement the specified interface.
Using the this Keyword
Within an instance
method or a constructor, this is a reference to the current
object — the object whose
method or constructor is being called. You can refer to any member of the
current object from within an instance method or a constructor by using this.
Using this with
a Field
The most common reason
for using the this keyword is because a field is shadowed by a method or constructor
parameter.For example, the
Point class was written like thispublic class Point {
public int x = 0;
public int y = 0;
//constructor
public Point(int a, int b) {
x = a;
y = b;
}
}
but it could have been
written like this:public class Point {
public int x = 0;
public int y = 0;
//constructor
public Point(int x, int y) {
this.x = x;
this.y = y;
}
}
Each argument to the
constructor shadows one of the object's fields — inside the constructor x is a local copy of the constructor's first argument. To refer to
the Point field x, the constructor must use this.x.
Using this with
a Constructor
From within a
constructor, you can also use the this keyword to call another constructor in the same class. Doing so is
called an explicit constructor
invocation. Here's another Rectangle class, with a different implementation from the one in the Objects section.public class Rectangle {
private int x, y;
private int width, height;
public Rectangle() {
this(0, 0, 0, 0);
}
public Rectangle(int width, int height) {
this(0, 0, width, height);
}
public Rectangle(int x, int y, int width, int height) {
this.x = x;
this.y = y;
this.width = width;
this.height = height;
}
...
}
This class contains a
set of constructors. Each constructor initializes some or all of the
rectangle's member variables. The constructors provide a default value for any
member variable whose initial value is not provided by an argument. For
example, the no-argument constructor calls the four-argument constructor with
four 0 values and the two-argument constructor calls the four-argument
constructor with two 0 values. As before, the compiler determines which
constructor to call, based on the number and the type of arguments.If present, the invocation of another constructor must be the first line in the constructor.
Controlling
Access to Members of a Class
Access level modifiers determine whether other classes can use a
particular field or invoke a particular method. There are two levels of access
control:
·
At the top level—public, or package-private (no explicit modifier).
·
At the member level—public, private, protected, or package-private (no explicit
modifier).
A class may be declared with the modifier public, in which case that
class is visible to all classes everywhere. If a class has no modifier (the
default, also known as package-private), it is visible only within
its own package (packages are named groups of related classes — you will learn
about them in a later lesson.)
At the member level, you can also use the public modifier or no
modifier (package-private) just as with top-level classes, and with the
same meaning. For members, there are two additional access modifiers:private and protected. The private modifier
specifies that the member can only be accessed in its own class. The protected modifier specifies that the member can only be accessed within its
own package (as with package-private) and, in addition, by a
subclass of its class in another package.
The following table shows the access to members permitted by each
modifier.
|
Access Levels
|
||||
|
Modifier
|
Class
|
Package
|
Subclass
|
World
|
|
public
|
Y
|
Y
|
Y
|
Y
|
|
protected
|
Y
|
Y
|
Y
|
N
|
|
no
modifier
|
Y
|
Y
|
N
|
N
|
|
private
|
Y
|
N
|
N
|
N
|
The first data column indicates whether the class itself has access to
the member defined by the access level. As you can see, a class always has
access to its own members. The second column indicates whether classes in the
same package as the class (regardless of their parentage) have access to the
member. The third column indicates whether subclasses of the class declared
outside this package have access to the member. The fourth column indicates
whether all classes have access to the member.
Access levels affect you in two ways. First, when you use classes that
come from another source, such as the classes in the Java platform, access
levels determine which members of those classes your own classes can use.
Second, when you write a class, you need to decide what access level every
member variable and every method in your class should have.
Let's look at a collection of classes and see how access levels affect
visibility. The following figure shows the four classes in this
example and how they are related.
Classes and Packages of the Example Used to Illustrate Access Levels
The following table shows where the members of the Alpha class are
visible for each of the access modifiers that can be applied to them.
|
Visibility
|
||||
|
Modifier
|
Alpha
|
Beta
|
Alphasub
|
Gamma
|
|
public
|
Y
|
Y
|
Y
|
Y
|
|
protected
|
Y
|
Y
|
Y
|
N
|
|
no
modifier
|
Y
|
Y
|
N
|
N
|
|
private
|
Y
|
N
|
N
|
N
|
Tips on Choosing an Access Level:
If other programmers use your class, you want to ensure that errors from
misuse cannot happen. Access levels can help you do this.
·
Use the most restrictive access level
that makes sense for a particular member. Use private unless you have
a good reason not to.
·
Avoid public fields except
for constants. (Many of the examples in the tutorial use public fields. This
may help to illustrate some points concisely, but is not recommended for
production code.) Public fields tend to link you to a particular implementation
and limit your flexibility in changing your code.
Understanding
Instance and Class Members
In this section, we discuss the use of the static keyword to
create fields and methods that belong to the class, rather than to an instance
of the class.
Class Variables
When a number of objects are created from the same class blueprint, they
each have their own distinct copies of instance variables. In the
case of the Bicycle class, the instance variables are cadence, gear, andspeed. Each Bicycle object has its own values for these variables, stored in different
memory locations.
Sometimes, you want to have variables that are common to all objects.
This is accomplished with the static modifier.
Fields that have the static modifier in their declaration are
called static fields or class variables. They are
associated with the class, rather than with any object. Every instance of the
class shares a class variable, which is in one fixed location in memory. Any
object can change the value of a class variable, but class variables can also
be manipulated without creating an instance of the class.
For example, suppose you want to create a number of Bicycle objects and assign each a serial number, beginning with 1 for the
first object. This ID number is unique to each object and is therefore an
instance variable. At the same time, you need a field to keep track of how
many Bicycle objects have been created so that you know what ID to assign to
the next one. Such a field is not related to any individual object, but to the
class as a whole. For this you need a class variable, numberOfBicycles, as follows:
public class Bicycle{
private int cadence;
private int gear;
private int speed;
// add an instance variable for
// the object ID
private int id;
// add a class variable for the
// number of Bicycle objects instantiated
private static int numberOfBicycles
= 0;
...
}
Class variables are referenced by the class name itself, as in
Bicycle.numberOfBicycles
This makes it clear that they are class variables.
Note: You can also refer to static fields with an
object reference like
myBike.numberOfBicycles
but this is discouraged because it does not make it clear that they are
class variables.
You can use the Bicycle constructor to set the id instance variable and increment the numberOfBicycles class variable:
public class Bicycle{
private int cadence;
private int gear;
private int speed;
private int id;
private static int numberOfBicycles = 0;
public Bicycle(int startCadence, int
startSpeed, int startGear){
gear = startGear;
cadence = startCadence;
speed = startSpeed;
// increment number of Bicycles
// and assign ID number
id = ++numberOfBicycles;
}
// new method to return the ID instance
variable
public int getID() {
return id;
}
...
}
Class Methods
The Java programming language supports static methods as well as static
variables. Static methods, which have the static modifier in
their declarations, should be invoked with the class name, without the need for
creating an instance of the class, as in
ClassName.methodName(args)
Note: You can also refer to static methods with an
object reference like
instanceName.methodName(args)
but this is discouraged because it does not make it clear that they are
class methods.
A common use for static methods is to access static fields. For example,
we could add a static method to the Bicycle class to access
the numberOfBicycles static field:
public static
int getNumberOfBicycles() {
return numberOfBicycles;
}
Not all combinations of instance and class variables and methods are
allowed:
·
Instance methods can access instance
variables and instance methods directly.
·
Instance methods can access class
variables and class methods directly.
·
Class methods can access class
variables and class methods directly.
·
Class methods cannot access
instance variables or instance methods directly—they must use an object
reference. Also, class methods cannot use the this keyword as
there is no instance for this to refer to.
Constants
The static modifier, in combination with the final modifier, is
also used to define constants. The final modifier
indicates that the value of this field cannot change.
For example, the following variable declaration defines a constant
named PI, whose value is an approximation of pi (the ratio of the circumference
of a circle to its diameter):
static final double
PI = 3.141592653589793;
Constants defined in this way cannot be reassigned, and it is a
compile-time error if your program tries to do so. By convention, the names of
constant values are spelled in uppercase letters. If the name is composed of
more than one word, the words are separated by an underscore (_).
Note: If a primitive type or a string is defined as
a constant and the value is known at compile time, the compiler replaces the
constant name everywhere in the code with its value. This is called a compile-time
constant. If the value of the constant in the outside world changes (for
example, if it is legislated that pi actually should be 3.975), you will need
to recompile any classes that use this constant to get the current value.
The Bicycle Class
After all the modifications made in this section, the Bicycle class is now:
public class Bicycle{
private int cadence;
private int gear;
private int speed;
private int id;
private static int numberOfBicycles
= 0;
public Bicycle(int startCadence,
int startSpeed,
int startGear){
gear = startGear;
cadence = startCadence;
speed = startSpeed;
id = ++numberOfBicycles;
}
public int getID() {
return id;
}
public static int getNumberOfBicycles() {
return numberOfBicycles;
}
public int getCadence(){
return cadence;
}
public void setCadence(int newValue){
cadence = newValue;
}
public int getGear(){
return gear;
}
public void setGear(int newValue){
gear = newValue;
}
public int getSpeed(){
return speed;
}
public void applyBrake(int decrement){
speed -= decrement;
}
public void speedUp(int increment){
speed += increment;
}
}
Initializing Fields
As you have seen, you
can often provide an initial value for a field in its declaration:public class BedAndBreakfast {
// initialize to 10
public static int capacity = 10;
// initialize to false
private boolean full = false;
}
This works well when
the initialization value is available and the initialization can be put on one
line. However, this form of initialization has limitations because of its
simplicity. If initialization requires some logic (for example, error handling
or a for loop to fill a complex array), simple assignment is inadequate.
Instance variables can be initialized in constructors, where error handling or
other logic can be used. To provide the same capability for class variables,
the Java programming language includes static
initialization blocks.
Note: It is
not necessary to declare fields at the beginning of the class definition,
although this is the most common practice. It is only necessary that they be
declared and initialized before they are used.
Static Initialization Blocks
A static initialization block is a normal block of code enclosed in
braces, { }, and preceded by the static keyword. Here is an example:static {
// whatever code is needed for initialization goes here
}
A class can have any
number of static initialization blocks, and they can appear anywhere in the
class body. The runtime system guarantees that static initialization blocks are
called in the order that they appear in the source code.There is an alternative to static blocks — you can write a private static method:
class Whatever {
public static varType myVar = initializeClassVariable();
private static varType initializeClassVariable() {
// initialization code goes here
}
}
The advantage of
private static methods is that they can be reused later if you need to
reinitialize the class variable.
Initializing Instance Members
Normally, you would
put code to initialize an instance variable in a constructor. There are two
alternatives to using a constructor to initialize instance variables:
initializer blocks and final methods.Initializer blocks for instance variables look just like static initializer blocks, but without the
static keyword:{
// whatever code is needed for initialization goes here
}
The Java compiler
copies initializer blocks into every constructor. Therefore, this approach can
be used to share a block of code between multiple constructors.A final method cannot be overridden in a subclass. This is discussed in the lesson on interfaces and inheritance. Here is an example of using a final method for initializing an instance variable:
class Whatever {
private varType myVar = initializeInstanceVariable();
protected final varType initializeInstanceVariable() {
// initialization code goes here
}
}
This is especially
useful if subclasses might want to reuse the initialization method. The method
is final because calling non-final methods during instance initialization can
cause problems.
Summary of Creating and Using Classes and Objects
A class declaration
names the class and encloses the class body between braces. The class name can
be preceded by modifiers. The class body contains fields, methods, and
constructors for the class. A class uses fields to contain state information
and uses methods to implement behavior. Constructors that initialize a new
instance of a class use the name of the class and look like methods without a
return type.You control access to classes and members in the same way: by using an access modifier such as
public in their declaration.You specify a class variable or a class method by using the
static keyword in the member's declaration. A member that is not declared
as static is implicitly an instance member. Class variables are shared by
all instances of a class and can be accessed through the class name as well as
an instance reference. Instances of a class get their own copy of each instance
variable, which must be accessed through an instance reference.You create an object from a class by using the
new operator and a constructor. The new operator returns a reference
to the object that was created. You can assign the reference to a variable or
use it directly.Instance variables and methods that are accessible to code outside of the class that they are declared in can be referred to by using a qualified name. The qualified name of an instance variable looks like this:
objectReference.variableNameThe qualified name of a method looks like this:
objectReference.methodName(argumentList)or:
objectReference.methodName()The garbage collector automatically cleans up unused objects. An object is unused if the program holds no more references to it. You can explicitly drop a reference by setting the variable holding the reference to
null.
Questions and Exercises: Classes
Questions
1.
Consider the following class:
2. public class IdentifyMyParts {
3. public static int x = 7;
4. public int y = 3;
5. }
a.
What are the class variables?
b.
What are the instance variables?
c.
What is the output from the following code:
d. IdentifyMyParts a = new IdentifyMyParts();
e. IdentifyMyParts b = new IdentifyMyParts();
f. a.y = 5;
g. b.y = 6;
h. a.x = 1;
i. b.x = 2;
j. System.out.println("a.y = " + a.y);
k. System.out.println("b.y = " + b.y);
l. System.out.println("a.x = " + a.x);
m. System.out.println("b.x = " + b.x);
n. System.out.println("IdentifyMyParts.x = " +
o. IdentifyMyParts.x);
Exercises
1.
Write a class whose instances represent a single playing card from
a deck of cards. Playing cards have two distinguishing properties: rank and
suit. Be sure to keep your solution as you will be asked to rewrite it in Enum
Types.
Hint:
You can use the
assert statement to check your assignments. You write:assert (boolean expression to test);
If the boolean expression is false, you will get an error message.
For example,
assert toString(ACE) == "Ace";
should return
true, so there will be no error
message.
If you use the
assert statement, you must run your program with the ea flag:java -ea YourProgram.class
2.
Write a class whose instances represent a full deck of cards. You should also keep
this solution.
3.
3. Write a small program to test your deck and card classes. The
program can be as simple as creating a deck of cards and displaying its cards.
Questions and Exercises: Objects
Questions
1.
What's wrong with the following program?
2. public class SomethingIsWrong {
3. public static void main(String[] args) {
4. Rectangle myRect;
5. myRect.width = 40;
6. myRect.height = 50;
7. System.out.println("myRect's area is "
8. + myRect.area());
9. }
10.}
11.
The following code creates one array and one string object. How
many references to those objects exist after the code executes? Is either
object eligible for garbage collection?
12....
13.String[] students = new String[10];
14.String studentName = "Peter Parker";
15.students[0] = studentName;
16.studentName = null;
17....
18.
How does a program destroy an object that it creates?
Exercises
1.
Fix the program called
SomethingIsWrong shown in Question 1.
2.
Given the following class, called
NumberHolder,
write some code that creates an instance of the class, initializes its two
member variables, and then displays the value of each member variable.3. public class NumberHolder {
4. public int anInt;
5. public float aFloat;
}
Nested Classes
The Java programming
language allows you to define a class within another class. Such a class is
called a nested class and is illustrated here:class OuterClass {
...
class NestedClass {
...
}
}
Terminology: Nested
classes are divided into two categories: static and non-static. Nested classes
that are declared
static are simply called static nested classes.
Non-static nested classes are calledinner classes.class OuterClass {
...
static class StaticNestedClass {
...
}
class InnerClass {
...
}
}
A nested class is a
member of its enclosing class. Non-static nested classes (inner classes) have
access to other members of the enclosing class, even if they are declared
private. Static nested classes do not have access to other members of the
enclosing class. As a member of the OuterClass, a nested class can be
declared private, public, protected, or package private. (Recall that
outer classes can only be declared public or package
private.)
Why Use Nested Classes?
There are several
compelling reasons for using nested classes, among them:- It is a way of logically grouping
classes that are only used in one place.
- It increases encapsulation.
- Nested classes can lead to more
readable and maintainable code.
Increased encapsulation—Consider two top-level classes, A and B, where B needs access to members of A that would otherwise be declared
private. By hiding class B within
class A, A's members can be declared private and B can access them. In
addition, B itself can be hidden from the outside world.More readable, maintainable code—Nesting small classes within top-level classes places the code closer to where it is used.
Static Nested Classes
As with class methods
and variables, a static nested class is associated with its outer class. And
like static class methods, a static nested class cannot refer directly to
instance variables or methods defined in its enclosing class — it can use them
only through an object reference.
Note: A static
nested class interacts with the instance members of its outer class (and other
classes) just like any other top-level class. In effect, a static nested class
is behaviorally a top-level class that has been nested in another top-level
class for packaging convenience.
OuterClass.StaticNestedClass
For example, to create
an object for the static nested class, use this syntax:OuterClass.StaticNestedClass nestedObject =
new OuterClass.StaticNestedClass();
Inner Classes
As with instance
methods and variables, an inner class is associated with an instance of its
enclosing class and has direct access to that object's methods and fields.
Also, because an inner class is associated with an instance, it cannot define
any static members itself.Objects that are instances of an inner class exist within an instance of the outer class. Consider the following classes:
class OuterClass {
...
class InnerClass {
...
}
}
An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing
instance. The next figure illustrates this idea.
An Instance of InnerClass Exists Within an
Instance of OuterClass
To instantiate an
inner class, you must first instantiate the outer class. Then, create the inner
object within the outer object with this syntax:OuterClass.InnerClass innerObject = outerObject.new InnerClass();
Additionally, there
are two special kinds of inner classes: local classes and anonymous classes
(also called anonymous inner classes). Both of these will be discussed briefly
in the next section.
Note: If you
want more information on the taxonomy of the different kinds of classes in the
Java programming language (which can be tricky to describe concisely, clearly,
and correctly), you might want to read Joseph Darcy's blog: Nested, Inner,
Member and Top-Level Classes.
Inner Class
Example
To see an inner class in use, let's first consider an array. In the
following example, we will create an array, fill it with integer values and
then output only values of even indices of the array in ascending order.
The DataStructure class below consists of:
·
The DataStructure outer class,
which includes methods to add an integer onto the array and print out values of
even indices of the array.
·
The InnerEvenIterator inner class, which is similar to a standard Java iterator.
Iterators are used to step through a data structure and typically have methods
to test for the last element, retrieve the current element, and move to the
next element.
·
A main method that
instantiates a DataStructure object (ds) and uses it to fill
the arrayOfInts array with integer values (0, 1, 2, 3, etc.),
then calls a printEven method to print out values of even indices
of arrayOfInts.
public class
DataStructure {
// create an array
private final static int SIZE = 15;
private int[] arrayOfInts = new int[SIZE];
public DataStructure() {
// fill the array with ascending
integer values
for (int i = 0; i < SIZE; i++) {
arrayOfInts[i] = i;
}
}
public void printEven() {
// print out values of even indices of
the array
InnerEvenIterator iterator = this.new
InnerEvenIterator();
while (iterator.hasNext()) {
System.out.println(iterator.getNext() + " ");
}
}
// inner class implements the Iterator
pattern
private class InnerEvenIterator {
// start stepping through the array
from the beginning
private int next = 0;
public boolean hasNext() {
// check if a current element is
the last in the array
return (next <= SIZE - 1);
}
public int getNext() {
// record a value of an even index
of the array
int retValue = arrayOfInts[next];
//get the next even element
next += 2;
return retValue;
}
}
public static void main(String s[]) {
// fill the array with integer values
and print out only
// values of even indices
DataStructure ds = new DataStructure();
ds.printEven();
}
}
The output is:
0 2 4 6 8 10 12 14
Note that the InnerEvenIterator class refers
directly to the arrayOfInts instance
variable of the DataStructure object.
Inner classes can be used to implement helper classes like the one shown
in the example above. If you plan on handling user-interface events, you will
need to know how to use inner classes because the event-handling mechanism
makes extensive use of them.
Local and Anonymous Inner Classes
There are two additional types of inner classes. You can declare an inner
class within the body of a method. Such a class is known as a local
inner class. You can also declare an inner class within the body of a
method without naming it. These classes are known as anonymous inner
classes. You will encounter such classes in advanced Java programming.
Modifiers
You can use the same modifiers for inner classes that you use for other
members of the outer class. For example, you can use the access specifiers
— private, public, and protected — to restrict access to inner classes, just
as you do to other class members.
Summary of Nested Classes
A class defined within
another class is called a nested class. Like other members of a class, a nested
class can be declared static or not. A nonstatic nested class is called an
inner class. An instance of an inner class can exist only within an instance of
its enclosing class and has access to its enclosing class's members even if
they are declared private.The following table shows the types of nested classes:
|
Types of Nested Classes
|
||
|
Type
|
Scope
|
Inner
|
|
static nested class
|
member
|
no
|
|
inner [non-static] class
|
member
|
yes
|
|
local class
|
local
|
yes
|
|
anonymous class
|
only the point where it is defined
|
yes
|
Questions and Exercises: Nested Classes
Questions
2.
Use the Java API documentation for the
Box class (in the javax.swing package) to help you answer the following questions.
a.
What static nested class does
Box define?
b.
What inner class does
Box define?
c.
What is the superclass of
Box's inner class?
d.
Which of
Box's nested classes can you use
from any class?
e.
How do you create an instance of
Box's Filler class?
Exercises
Enum Types
An enum type is a type whose fields consist of a fixed set of constants.
Common examples include compass directions (values of NORTH, SOUTH, EAST, and
WEST) and the days of the week.Because they are constants, the names of an enum type's fields are in uppercase letters.
In the Java programming language, you define an enum type by using the
enum keyword. For example, you would specify a days-of-the-week enum
type as:
public enum Day {
SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
THURSDAY, FRIDAY, SATURDAY
}
You should use enum
types any time you need to represent a fixed set of constants. That includes
natural enum types such as the planets in our solar system and data sets where
you know all possible values at compile time—for example, the choices on a
menu, command line flags, and so on.Here is some code that shows you how to use the
Day enum defined above:
public class EnumTest {
Day day;
public EnumTest(Day day) {
this.day = day;
}
public void tellItLikeItIs() {
switch (day) {
case MONDAY:
System.out.println("Mondays are bad.");
break;
case FRIDAY:
System.out.println("Fridays are better.");
break;
case SATURDAY: case SUNDAY:
System.out.println("Weekends are best.");
break;
default:
System.out.println("Midweek days are so-so.");
break;
}
}
public static void main(String[] args) {
EnumTest firstDay = new EnumTest(Day.MONDAY);
firstDay.tellItLikeItIs();
EnumTest thirdDay = new EnumTest(Day.WEDNESDAY);
thirdDay.tellItLikeItIs();
EnumTest fifthDay = new EnumTest(Day.FRIDAY);
fifthDay.tellItLikeItIs();
EnumTest sixthDay = new EnumTest(Day.SATURDAY);
sixthDay.tellItLikeItIs();
EnumTest seventhDay = new EnumTest(Day.SUNDAY);
seventhDay.tellItLikeItIs();
}
}
The output is:Mondays are bad.
Midweek days are so-so.
Fridays are better.
Weekends are best.
Weekends are best.
Java programming
language enum types are much more powerful than their counterparts in other
languages. The enum declaration defines a class (called an enum type). The enum class body
can include methods and other fields. The compiler automatically adds some
special methods when it creates an enum. For example, they have a static values method that returns an array containing all of the values of the
enum in the order they are declared. This method is commonly used in
combination with the for-each construct to iterate over the values of an enum
type. For example, this code from the Planet class example below iterates over all the planets in the solar
system.for (Planet p : Planet.values()) {
System.out.printf("Your weight on %s is %f%n",
p, p.surfaceWeight(mass));
}
Note: All enums implicitly extend
java.lang.Enum. Since Java
does not support multiple inheritance, an enum cannot extend anything else.Planet is an enum type that represents the planets in the solar system.
They are defined with constant mass and radius properties.Each enum constant is declared with values for the mass and radius parameters. These values are passed to the constructor when the constant is created. Java requires that the constants be defined first, prior to any fields or methods. Also, when there are fields and methods, the list of enum constants must end with a semicolon.
Note: The
constructor for an enum type must be package-private or private access. It
automatically creates the constants that are defined at the beginning of the
enum body. You cannot invoke an enum constructor yourself.
Planet has methods that allow you to retrieve the surface gravity and
weight of an object on each planet. Here is a sample program that takes your
weight on earth (in any unit) and calculates and prints your weight on all of
the planets (in the same unit):
public enum Planet {
MERCURY (3.303e+23, 2.4397e6),
VENUS (4.869e+24, 6.0518e6),
EARTH (5.976e+24, 6.37814e6),
MARS (6.421e+23, 3.3972e6),
JUPITER (1.9e+27, 7.1492e7),
SATURN (5.688e+26, 6.0268e7),
URANUS (8.686e+25, 2.5559e7),
NEPTUNE (1.024e+26, 2.4746e7);
private final double mass; // in kilograms
private final double radius; // in meters
Planet(double mass, double radius) {
this.mass = mass;
this.radius = radius;
}
private double mass() { return mass; }
private double radius() { return radius; }
// universal gravitational constant (m3 kg-1 s-2)
public static final double G = 6.67300E-11;
double surfaceGravity() {
return G * mass / (radius * radius);
}
double surfaceWeight(double otherMass) {
return otherMass * surfaceGravity();
}
public static void main(String[] args) {
if (args.length != 1) {
System.err.println("Usage: java Planet <earth_weight>");
System.exit(-1);
}
double earthWeight = Double.parseDouble(args[0]);
double mass = earthWeight/EARTH.surfaceGravity();
for (Planet p : Planet.values())
System.out.printf("Your weight on %s is %f%n",
p, p.surfaceWeight(mass));
}
}
If you run Planet.class from the command line with an argument of 175, you get this
output:$ java Planet 175
Your weight on MERCURY is 66.107583
Your weight on VENUS is 158.374842
Your weight on EARTH is 175.000000
Your weight on MARS is 66.279007
Your weight on JUPITER is 442.847567
Your weight on SATURN is 186.552719
Your weight on URANUS is 158.397260
Your weight on NEPTUNE is 199.207413
Questions and Exercises: Enum Types
Exercises
1.
Rewrite the class
Card from the exercise in Questions and Exercises: Classes so that it represents the rank and suit of a card with enum types.
2.
Rewrite the
Deck class.
Annotations
Annotations provide data
about a program that is not part of the program itself. They have no direct
effect on the operation of the code they annotate.
Annotations have a number of uses, among them:
·
Information for the compiler — Annotations can be used by the compiler to detect errors or
suppress warnings.
·
Compiler-time and deployment-time
processing — Software tools can process annotation information to generate
code, XML files, and so forth.
·
Runtime processing — Some annotations are available to be examined at runtime.
Annotations can be applied to a program's declarations of classes,
fields, methods, and other program elements.
The annotation appears first, often (by convention) on its own line, and
may include elements with named or unnamed values:
@Author(
name = "Benjamin Franklin",
date = "3/27/2003"
)
class MyClass() { }
or
@SuppressWarnings(value
= "unchecked")
void myMethod() { }
If there is just one element named "value," then the name may
be omitted, as in:
@SuppressWarnings("unchecked")
void myMethod() { }
Also, if an annotation has no elements, the parentheses may be omitted,
as in:
@Override
void mySuperMethod()
{ }
Documentation
Many annotations replace what would otherwise have been comments in
code.
Suppose that a software group has traditionally begun the body of every
class with comments providing important information:
public class
Generation3List extends Generation2List {
// Author: John Doe
// Date: 3/17/2002
// Current revision: 6
// Last modified: 4/12/2004
// By: Jane Doe
// Reviewers: Alice, Bill, Cindy
// class code goes here
}
To add this same metadata with an annotation, you must first define
the annotation type. The syntax for doing this is:
@interface
ClassPreamble {
String author();
String date();
int currentRevision() default 1;
String lastModified() default
"N/A";
String lastModifiedBy() default
"N/A";
// Note use of array
String[] reviewers();
}
The annotation type definition looks somewhat like an interface
definition where the keyword interface is preceded by
the @ character (@ = "AT" as in Annotation Type). Annotation types
are, in fact, a form ofinterface, which will be covered in a later
lesson. For the moment, you do not need to understand interfaces.
The body of the annotation definition above contains annotation
type element declarations, which look a lot like methods. Note that
they may define optional default values.
Once the annotation type has been defined, you can use annotations of
that type, with the values filled in, like this:
@ClassPreamble (
author = "John Doe",
date = "3/17/2002",
currentRevision = 6,
lastModified = "4/12/2004",
lastModifiedBy = "Jane Doe",
// Note array notation
reviewers = {"Alice",
"Bob", "Cindy"}
)
public class
Generation3List extends Generation2List {
// class code goes
here
}
Note: To make the information in @ClassPreamble appear in Javadoc-generated documentation, you must annotate
the @ClassPreamble definition itself with the @Documented annotation:
// import this to use
@Documented
import
java.lang.annotation.*;
@Documented
@interface
ClassPreamble {
// Annotation element definitions
}
Annotations Used by the Compiler
There are three annotation types that are predefined by the language specification
itself: @Deprecated, @Override, and @SuppressWarnings.
@Deprecated—the @Deprecated annotation indicates that the marked element is deprecated and
should no longer be used. The compiler generates a warning whenever a program
uses a method, class, or field with the @Deprecated annotation.
When an element is deprecated, it should also be documented using the
Javadoc @deprecated tag, as shown in the following example. The
use of the "@" symbol in both Javadoc comments and in annotations is
not coincidental — they are related conceptually. Also, note that the Javadoc
tag starts with a lowercase "d" and the annotation starts with an uppercase
"D".
// Javadoc comment follows
/**
* @deprecated
* explanation of why it
* was deprecated
*/
@Deprecated
static void deprecatedMethod() { }
}
@Override—the @Override annotation informs the compiler that the element is meant to
override an element declared in a superclass (overriding methods will be
discussed in the the lesson titled "Interfaces and Inheritance").
// mark method as a superclass method
// that has been overridden
@Override
int overriddenMethod() { }
While it's not required to use this annotation when overriding a method,
it helps to prevent errors. If a method marked with @Override fails to correctly override a method in one of its superclasses,
the compiler generates an error.
@SuppressWarnings—the @SuppressWarnings annotation tells the compiler to suppress specific warnings that
it would otherwise generate. In the example below, a deprecated method is used
and the compiler would normally generate a warning. In this case, however, the
annotation causes the warning to be suppressed.
// use a deprecated method and tell
// compiler not to generate a warning
@SuppressWarnings("deprecation")
void useDeprecatedMethod() {
// deprecation warning
// - suppressed
objectOne.deprecatedMethod();
}
Every compiler warning belongs to a category. The Java Language
Specification lists two categories: "deprecation" and
"unchecked." The "unchecked" warning can occur when
interfacing with legacy code written before the advent of generics (discussed
in the lesson titled "Generics"). To suppress more than one category
of warnings, use the following syntax:
@SuppressWarnings({"unchecked",
"deprecation"})
Annotation Processing
The more advanced uses of annotations include writing an annotation
processor that can read a Java program and take actions based on its
annotations. It might, for example, generate auxiliary source code, relieving
the programmer of having to create boilerplate code that always follows
predictable patterns. To facilitate this task, release 5.0 of the JDK includes
an annotation processing tool, called apt. In release 6 of the
JDK, the functionality of apt is a standard part of the Java
compiler.
To make annotation information available at runtime, the annotation type
itself must be annotated with @Retention(RetentionPolicy.RUNTIME), as follows:
import
java.lang.annotation.*;
@Retention(RetentionPolicy.RUNTIME)
@interface
AnnotationForRuntime {
// Elements that give information
// for runtime processing
}
Questions and Exercises: Annotations
Questions
1.
What is wrong with the following interface?
2. public interface House {
3. @Deprecated
4. void open();
5. void openFrontDoor();
6. void openBackDoor();
7. }
8.
Consider this implementation of the
House interface, shown in Question 1.9. public class MyHouse implements House {
10. public void open() {}
11. public void openFrontDoor() {}
12. public void openBackDoor() {}
13.}
If you compile this program, the compiler complains that
open has been deprecated (in the interface). What can you do to get rid
of that warning?