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.

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 indirectly by 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;
  }
}

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.

All variables, whether they are fields, local variables, or parameters, follow the same naming rules and conventions that were covered in the Naming section (page 44).

In this chapter, be aware that the same naming rules and conventions are used for method and class names, except that:

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 multiword 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.

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 Chapter 5.)

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.

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
}

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 invoked 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 invoked 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 invoked like this:

System.out.printf("%s: %d, %s%n", name, idnum, address);

or like this:

System.out.printf("%s: %d, %s, %s, %s%n", name,
                          idnum, address, phone, email);

or with yet a different number of arguments.

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 chapter in the Using the this Keyword section (page 109).

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

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 invoked.

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 the new 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 by Figure 4.1 (the variable name, originOne, plus a reference pointing to nothing).

Figure 4.1. originOne is null.

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.

The new operator requires a single, postfix argument: invoking a constructor. The name of the constructor provides the name of the class to instantiate.

The new operator returns a reference to the object it created. This reference is usually assigned to a variable of the appropriate type, like:

Point originOne = new Point(23, 94);

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:

int height = new Rectangle().height;

This statement will be discussed in the next section.

Here’s the code for the Point class:^[2]

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 by Figure 4.2.

Figure 4.2. originOne now points to a Point object.

Here’s the code for the Rectangle class,^[3] which contains four constructors:

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 invoke the constructor in the Rectangle class that requires a Point argument followed by two integer arguments:

Rectangle rectOne = new Rectangle(originOne, 100, 200);

This invokes 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 Figure 4.3.

Figure 4.3. originOne now points to a Point object.

The following line of code invokes the 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 invokes 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.

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, the CreateObjectDemo 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.

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.

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.

Subclasses and interfaces will be discussed in Chapter 5. If this section confuses you, skip it and return to it after you have finished Chapter 5.

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 by Figure 4.4.

Figure 4.4. 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.

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 this:

public 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 second 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.

From within a constructor, you can also use the this keyword to invoke 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 (page 97).

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 noargument constructor invokes the four-argument constructor with four 0 values and the two-argument constructor invokes the four-argument constructor with two 0 values. As before, the compiler determines which constructor to invoke, based on the number and the type of arguments.

If present, the invocation of another constructor must be the first line in the constructor.

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, and speed. 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.

myBike.numberOfBicycles

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;
  }
  .....
}

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)

instanceName.methodName(args)

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:

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 (_).

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;
  }
}

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 executed 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.

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 Chapter 5. 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 invoking non-final methods during instance initialization can cause problems. Joshua Bloch describes this in more detail in Effective Java.^[4]

Consider the following class:

public class IdentifyMyParts {
  public static int x = 7;
  public int y = 3;
}

IdentifyMyParts a = new IdentifyMyParts();
IdentifyMyParts b = new IdentifyMyParts();
a.y = 5;
b.y = 6;
IdentifyMyParts.x = 1;
b.x = 2;
System.out.println("a.y = " + a.y);
System.out.println("b.y = " + b.y);
System.out.println("IdentifyMyParts.x = " + a.x);
System.out.println("b.x = " + b.x);

You can find answers to these Questions and Exercises at:

tutorial/java/javaOO/QandE/creating-answers.html

You can find answers to these Questions and Exercises at:

tutorial/java/javaOO/QandE/objects-answers.html

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.

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.

You can find answers to these Questions and Exercises at:

tutorial/java/javaOO/QandE/nested-answers.html

You can find answers to these Questions and Exercises at:

tutorial/java/javaOO/QandE/enum-answers.html

You can find answers to these Questions and Exercises at:

tutorial/java/javaOO/QandE/annotations-answers.html