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Nested Classes
Anonymous Inner Classes
Mixing Static and Non-static
Static Import
Enums as Classes

Section 5.7

Interfaces, Nested Classes, and Other Details

THIS SECTION simply pulls together a few more miscellaneous features of object oriented programming in Java. Read it now, or just look through it and refer back to it later when you need this material. (You will need to know about the first topic, interfaces, almost as soon as we begin GUI programming.)

5.7.1  Interfaces

Some object-oriented programming languages, such as C++, allow a class to extend two or more superclasses. This is called multiple inheritance. In the illustration below, for example, class E is shown as having both class A and class B as direct superclasses, while class F has three direct superclasses.

Such multiple inheritance is not allowed in Java. The designers of Java wanted to keep the language reasonably simple, and felt that the benefits of multiple inheritance were not worth the cost in increased complexity. However, Java does have a feature that can be used to accomplish many of the same goals as multiple inheritance: interfaces.

We've encountered the term "interface" before, in connection with black boxes in general and subroutines in particular. The interface of a subroutine consists of the name of the subroutine, its return type, and the number and types of its parameters. This is the information you need to know if you want to call the subroutine. A subroutine also has an implementation: the block of code which defines it and which is executed when the subroutine is called.

In Java, interface is a reserved word with an additional, technical meaning. An "interface" in this sense consists of a set of instance method interfaces, without any associated implementations. (Actually, a Java interface can contain other things as well, but we won't discuss them here.) A class can implement an interface by providing an implementation for each of the methods specified by the interface. Here is an example of a very simple Java interface:

public interface Drawable {
   public void draw(Graphics g);

This looks much like a class definition, except that the implementation of the draw() method is omitted. A class that implements the interface Drawable must provide an implementation for this method. Of course, the class can also include other methods and variables. For example,

public class Line implements Drawable {
    public void draw(Graphics g) {
        . . . // do something -- presumably, draw a line
    . . . // other methods and variables

Note that to implement an interface, a class must do more than simply provide an implementation for each method in the interface; it must also state that it implements the interface, using the reserved word implements as in this example: "public class Line implements Drawable". Any class that implements the Drawable interface defines a draw() instance method. Any object created from such a class includes a draw() method. We say that an object implements an interface if it belongs to a class that implements the interface. For example, any object of type Line implements the Drawable interface.

While a class can extend only one other class, it can implement any number of interfaces. In fact, a class can both extend one other class and implement one or more interfaces. So, we can have things like

class FilledCircle extends Circle 
                        implements Drawable, Fillable {
   . . .

The point of all this is that, although interfaces are not classes, they are something very similar. An interface is very much like an abstract class, that is, a class that can never be used for constructing objects, but can be used as a basis for making subclasses. The subroutines in an interface are abstract methods, which must be implemented in any concrete class that implements the interface. And as with abstract classes, even though you can't construct an object from an interface, you can declare a variable whose type is given by the interface. For example, if Drawable is an interface, and if Line and FilledCircle are classes that implement Drawable, then you could say:

Drawable figure;  // Declare a variable of type Drawable.  It can
                  //    refer to any object that implements the
                  //    Drawable interface.
figure = new Line();  // figure now refers to an object of class Line
figure.draw(g);   // calls draw() method from class Line

figure = new FilledCircle();   // Now, figure refers to an object
                               //   of class FilledCircle.
figure.draw(g);   // calls draw() method from class FilledCircle

A variable of type Drawable can refer to any object of any class that implements the Drawable interface. A statement like figure.draw(g), above, is legal because figure is of type Drawable, and any Drawable object has a draw() method. So, whatever object figure refers to, that object must have a draw() method.

Note that a type is something that can be used to declare variables. A type can also be used to specify the type of a parameter in a subroutine, or the return type of a function. In Java, a type can be either a class, an interface, or one of the eight built-in primitive types. These are the only possibilities. Of these, however, only classes can be used to construct new objects.

You are not likely to need to write your own interfaces until you get to the point of writing fairly complex programs. However, there are a few interfaces that are used in important ways in Java's standard packages. You'll learn about some of these standard interfaces in the next few chapters.

5.7.2  Nested Classes

A class seems like it should be a pretty important thing. A class is a high-level building block of a program, representing a potentially complex idea and its associated data and behaviors. I've always felt a bit silly writing tiny little classes that exist only to group a few scraps of data together. However, such trivial classes are often useful and even essential. Fortunately, in Java, I can ease the embarrassment, because one class can be nested inside another class. My trivial little class doesn't have to stand on its own. It becomes part of a larger more respectable class. This is particularly useful when you want to create a little class specifically to support the work of a larger class. And, more seriously, there are other good reasons for nesting the definition of one class inside another class.

In Java, a nested class is any class whose definition is inside the definition of another class. Nested classes can be either named or anonymous. I will come back to the topic of anonymous classes later in this section. A named nested class, like most other things that occur in classes, can be either static or non-static.

The definition of a static nested class looks just like the definition of any other class, except that it is nested inside another class and it has the modifier static as part of its declaration. A static nested class is part of the static structure of the containing class. It can be used inside that class to create objects in the usual way. If it has not been declared private, then it can also be used outside the containing class, but when it is used outside the class, its name must indicate its membership in the containing class. This is similar to other static components of a class: A static nested class is part of the class itself in the same way that static member variables are parts of the class itself.

For example, suppose a class named WireFrameModel represents a set of lines in three-dimensional space. (Such models are used to represent three-dimensional objects in graphics programs.) Suppose that the WireFrameModel class contains a static nested class, Line, that represents a single line. Then, outside of the class WireFrameModel, the Line class would be referred to as WireFrameModel.Line. Of course, this just follows the normal naming convention for static members of a class. The definition of the WireFrameModel class with its nested Line class would look, in outline, like this:

public class WireFrameModel {

   . . . // other members of the WireFrameModel class
   static public class Line {
         // Represents a line from the point (x1,y1,z1)
         // to the point (x2,y2,z2) in 3-dimensional space.
      double x1, y1, z1;
      double x2, y2, z2;
   } // end class Line
   . . . // other members of the WireFrameModel class
} // end WireFrameModel

Inside the WireFrameModel class, a Line object would be created with the constructor "new Line()". Outside the class, "new WireFrameModel.Line()" would be used.

A static nested class has full access to the static members of the containing class, even to the private members. Similarly, the containing class has full access to the members of the nested class. This can be another motivation for declaring a nested class, since it lets you give one class access to the private members of another class without making those members generally available to other classes.

When you compile the above class definition, two class files will be created. Even though the definition of Line is nested inside WireFrameModel, the compiled Line class is stored in a separate file. The name of the class file for Line will be WireFrameModel$Line.class.

Non-static nested classes are referred to as inner classes. Inner classes are not, in practice, very different from static nested classes, but a non-static nested class is actually associated with an object rather than to the class in which it is nested. This can take some getting used to.

Any non-static member of a class is not really part of the class itself (although its source code is contained in the class definition). This is true for inner classes, just as it is for any other non-static part of a class. The non-static members of a class specify what will be contained in objects that are created from that class. The same is true -- at least logically -- for inner classes. It's as if each object that belongs to the containing class has its own copy of the nested class. This copy has access to all the instance methods and instance variables of the object, even to those that are declared private. The two copies of the inner class in two different objects differ because the instance variables and methods they refer to are in different objects. In fact, the rule for deciding whether a nested class should be static or non-static is simple: If the nested class needs to use any instance variable or instance method from the containing class, make the nested class non-static. Otherwise, it might as well be static.

From outside the containing class, a non-static nested class has to be referred to using a name of the form variableName.NestedClassName, where variableName is a variable that refers to the object that contains the class. This is actually rather rare, however. A non-static nested class is generally used only inside the class in which it is nested, and there it can be referred to by its simple name.

In order to create an object that belongs to an inner class, you must first have an object that belongs to the containing class. (When working inside the class, the object "this" is used implicitly.) The inner class object is permanently associated with the containing class object, and it has complete access to the members of the containing class object. Looking at an example will help, and will hopefully convince you that inner classes are really very natural. Consider a class that represents poker games. This class might include a nested class to represent the players of the game. This structure of the PokerGame class could be:

public class PokerGame {  // Represents a game of poker.
    private class Player {  // Represents one of the players in this game.
    } // end class Player
    private Deck deck;      // A deck of cards for playing the game.
    private int pot;        // The amount of money that has been bet.

} // end class PokerGame

If game is a variable of type PokerGame, then, conceptually, game contains its own copy of the Player class. In an instance method of a PokerGame object, a new Player object would be created by saying "new Player()", just as for any other class. (A Player object could be created outside the PokerGame class with an expression such as "game.new Player()". Again, however, this is very rare.) The Player object will have access to the deck and pot instance variables in the PokerGame object. Each PokerGame object has its own deck and pot and Players. Players of that poker game use the deck and pot for that game; players of another poker game use the other game's deck and pot. That's the effect of making the Player class non-static. This is the most natural way for players to behave. A Player object represents a player of one particular poker game. If Player were a static nested class, on the other hand, it would represent the general idea of a poker player, independent of a particular poker game.

5.7.3  Anonymous Inner Classes

In some cases, you might find yourself writing an inner class and then using that class in just a single line of your program. Is it worth creating such a class? Indeed, it can be, but for cases like this you have the option of using an anonymous inner class. An anonymous class is created with a variation of the new operator that has the form

          new  superclass-or-interface ( parameter-list ) {

This constructor defines a new class, without giving it a name, and it simultaneously creates an object that belongs to that class. This form of the new operator can be used in any statement where a regular "new" could be used. The intention of this expression is to create: "a new object belonging to a class that is the same as superclass-or-interface but with these methods-and-variables added." The effect is to create a uniquely customized object, just at the point in the program where you need it. Note that it is possible to base an anonymous class on an interface, rather than a class. In this case, the anonymous class must implement the interface by defining all the methods that are declared in the interface. If an interface is used as a base, the parameter-list must be empty. Otherwise, it can contain parameters for a constructor in the superclass.

Anonymous classes are often used for handling events in graphical user interfaces, and we will encounter them several times in the chapters on GUI programming. For now, we will look at one not-very-plausible example. Consider the Drawable interface, which is defined earlier in this section. Suppose that we want a Drawable object that draws a filled, red, 100-pixel square. Rather than defining a new, separate class and then using that class to create the object, we can use an anonymous class to create the object in one statement:

Drawable redSquare = new Drawable() {
       void draw(Graphics g) {

The semicolon at the end of this statement is not part of the class definition. It's the semicolon that is required at the end of every declaration statement.

When a Java class is compiled, each anonymous nested class will produce a separate class file. If the name of the main class is MainClass, for example, then the names of the class files for the anonymous nested classes will be MainClass$1.class, MainClass$2.class, MainClass$3.class, and so on.

5.7.4  Mixing Static and Non-static

Classes, as I've said, have two very distinct purposes. A class can be used to group together a set of static member variables and static member subroutines. Or it can be used as a factory for making objects. The non-static variables and subroutines in the class definition specify the instance variables and methods of the objects. In most cases, a class performs one or the other of these roles, not both.

Sometimes, however, static and non-static members are mixed in a single class. In this case, the class plays a dual role. Sometimes, these roles are completely separate. It is also possible for the static and non-static parts of a class to interact. This happens when instance methods use static member variables or call static member subroutines. An instance method belongs to an object, not to the class itself, and there can be many objects with their own versions of the instance method. But there is only one copy of a static member variable. So, effectively, we have many objects sharing that one variable.

Suppose, for example, that we want to write a PairOfDice class that uses the Random class mentioned in Section 5.3 for rolling the dice. To do this, a PairOfDice object needs access to an object of type Random. But there is no need for each PairOfDice object to have a separate Random object. (In fact, it would not even be a good idea: Because of the way random number generators work, a program should, in general, use only one source of random numbers.) A nice solution is to have a single Random variable as a static member of the PairOfDice class, so that it can be shared by all PairOfDice objects. For example:

import java.util.Random;
public class PairOfDice {
    private static Random randGen = new Random();
    public int die1;   // Number showing on the first die.
    public int die2;   // Number showing on the second die.
    public PairOfDice() {
            // Constructor.  Creates a pair of dice that
            // initially shows random values.
    public void roll() {
            // Roll the dice by setting each of the dice to be
            // a random number between 1 and 6.
         die1 = randGen.nextInt(6) + 1;
         die2 = randGen.nextInt(6) + 1;
} // end class PairOfDice

As another example, let's rewrite the Student class that was used in Section 5.2. I've added an ID for each student and a static member called nextUniqueID. Although there is an ID variable in each student object, there is only one nextUniqueID variable.

public class Student {

   private String name;  // Student's name.
   private int ID;  // Unique ID number for this student.
   public double test1, test2, test3;   // Grades on three tests.
   private static int nextUniqueID = 0;
             // keep track of next available unique ID number
   Student(String theName) {
        // Constructor for Student objects; provides a name for the Student,
        // and assigns the student a unique ID number.
      name = theName;
      ID = nextUniqueID;
   public String getName() {
        // Accessor method for reading value of private
        // instance variable, name.
      return name;
   public int getID() {
        // Accessor method for reading value of ID.
      return ID;
   public double getAverage() {  
        // Compute average test grade.
      return (test1 + test2 + test3) / 3;
}  // end of class Student

The initialization "nextUniqueID = 0" is done only once, when the class is first loaded. Whenever a Student object is constructed and the constructor says "nextUniqueID++;", it's always the same static member variable that is being incremented. When the very first Student object is created, nextUniqueID becomes 1. When the second object is created, nextUniqueID becomes 2. After the third object, it becomes 3. And so on. The constructor stores the new value of nextUniqueID in the ID variable of the object that is being created. Of course, ID is an instance variable, so every object has its own individual ID variable. The class is constructed so that each student will automatically get a different value for its ID variable. Furthermore, the ID variable is private, so there is no way for this variable to be tampered with after the object has been created. You are guaranteed, just by the way the class is designed, that every student object will have its own permanent, unique identification number. Which is kind of cool if you think about it.

5.7.5  Static Import

The import directive makes it possible to refer to a class such as java.awt.Color using its simple name, Color. All you have to do is say import java.awt.Color or import java.awt.*. But you still have to use compound names to refer to static member variables such as System.out and to static methods such as Math.sqrt.

Java 5.0 introduced a new form of the import directive that can be used to import static members of a class in the same way that the ordinary import directive imports classes from a package. The new form of the directive is called a static import, and it has syntax

import static package-name.class-name.static-member-name;

to import one static member name from a class, or

import static package-name.class-name.*;

to import all the public static members from a class. For example, if you preface a class definition with

import static java.lang.System.out;

then you can use the simple name out instead of the compound name System.out. This means you can use out.println instead of System.out.println. If you are going to work extensively with the Math class, you can preface your class definition with

import static java.lang.Math.*;

This would allow you to say sqrt instead of Math.sqrt, log instead of Math.log, PI instead of Math.PI, and so on.

Note that the static import directive requires a package-name, even for classes in the standard package java.lang. One consequence of this is that you can't do a static import from a class in the default package. In particular, it is not possible to do a static import from my TextIO class -- if you wanted to do that, you would have to move TextIO into a package.

5.7.6  Enums as Classes

Enumerated types were introduced in Subsection 2.3.3. Now that we have covered more material on classes and objects, we can revisit the topic (although still not covering enumerated types in their full complexity).

Enumerated types are actually classes, and each enumerated type constant is a public, final, static member variable in that class (even though they are not declared with these modifiers). The value of the variable is an object belonging to the enumerated type class. There is one such object for each enumerated type constant, and these are the only objects of the class that can ever be created. It is really these objects that represent the possible values of the enumerated type. The enumerated type constants are actually variables that refer to these objects.

When an enumerated type is defined inside another class, it is a nested class inside the enclosing class. In fact, it is a static nested class, whether you declare it to be static or not. But it can also be declared as a non-nested class, in a file of its own. For example, we could define the following enumerated type in a file named Suit.java:

public enum Suit {

This enumerated type represents the four possible suits for a playing card, and it could have been used in the example Card.java from Subsection 5.4.2.

Furthermore, in addition to its list of values, an enumerated type can contain some of the other things that a regular class can contain, including methods and additional member variables. Just add a semicolon (;) at the end of the list of values, and then add definitions of the methods and variables in the usual way. For example, we might make an enumerated type to represent the possible values of a playing card. It might be useful to have a method that returns the corresponding value in the game of Blackjack. As another example, suppose that when we print out one of the values, we'd like to see something different from the default string representation (the identifier that names the constant). In that case, we can override the toString() method in the class to print out a different string representation. This would give something like:

public enum CardValue {

    * Return the value of this CardValue in the game of Blackjack.
    * Note that the value returned for an ace is 1.
   public int blackJackValue() {
      if (this == JACK || this == QUEEN || this == KING)
         return 10;
         return 1 + ordinal();
    * Return a String representation of this CardValue, using numbers
    * for the numerical cards and names for the ace and face cards.
   public String toString() {
      switch (this) {       // "this" is one of the enumerated type values
      case ACE:             // ordinal number of ACE
         return "Ace";
      case JACK:            // ordinal number of JACK
         return "Jack";
      case QUEEN:           // ordinal number of QUEEN
         return "Queen";
      case KING:            // ordinal number of KING
         return "King";
      default:              // it's a numeric card value
         int numericValue = 1 + ordinal();
         return "" + numericValue;     

} // end CardValue

The methods blackjackValue() and toString() are instance methods in CardValue. Since CardValue.JACK is an object belonging to that class, you can call CardValue.JACK.blackjackValue(). Suppose that cardVal is declared to be a variable of type CardValue, so that it can refer to any of the values in the enumerated type. We can call cardVal.blackjackValue() to find the Blackjack value of the CardValue object to which cardVal refers, and System.out.println(cardVal) will implicitly call the method cardVal.toString() to obtain the print representation of that CardValue. (One other thing to keep in mind is that since CardValue is a class, the value of cardVal can be null, which means it does not refer to any object.)

Remember that ACE, TWO, ..., KING are the only possible objects of type CardValue, so in an instance method in that class, this will refer to one of those values. Recall that the instance method ordinal() is defined in any enumerated type and gives the position of the enumerated type value in the list of possible values, with counting starting from zero.

(If you find it annoying to use the class name as part of the name of every enumerated type constant, you can use static import to make the simple names of the constants directly available -- but only if you put the enumerated type into a package. For example, if the enumerated type CardValue is defined in a package named cardgames, then you could place

import static cardgames.CardValue.*;

at the beginning of a source code file. This would allow you, for example, to use the name JACK in that file instead of CardValue.JACK.)

End of Chapter 5

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