Inner Classes | Core Java(TM) 2, Volume I--Fundamentals (7th Edition) (Core Series) (Core Series)

An inner class is a class that is defined inside another class. Why would you want to do that? There are three reasons:

Inner class methods can access the data from the scope in which they are defined including data that would otherwise be private.
Inner classes can be hidden from other classes in the same package.
Anonymous inner classes are handy when you want to define callbacks without writing a lot of code.

We will break up this rather complex topic into several steps.

Starting on page 227, you will see a simple inner class that accesses an instance field of its outer class.
On page 230, we cover the special syntax rules for inner classes.
Starting on page 230, we peek inside inner classes to see how they are translated into regular classes. Squeamish readers may want to skip that section.
Starting on page 232, we discuss local inner classes that can access local variables of the enclosing scope.
Starting on page 234, we introduce anonymous inner classes and show how they are commonly used to implement callbacks.
Finally, starting on page 237, you will see how static inner classes can be used for nested helper classes.

C++ NOTE

C++ has nested classes. A nested class is contained inside the scope of the enclosing class. Here is a typical example: a linked list class defines a class to hold the links, and a class to define an iterator position.

 class LinkedList { public:    class Iterator // a nested class    {    public:       void insert(int x);       int erase();       . . .    };    . . . private:    class Link // a nested class    {    public:       Link* next;       int data;    };    . . . };

The nesting is a relationship between classes, not objects. A LinkedList object does not have subobjects of type Iterator or Link.

There are two benefits: name control and access control. Because the name Iterator is nested inside the LinkedList class, it is externally known as LinkedList::Iterator and cannot conflict with another class called Iterator. In Java, this benefit is not as important because Java packages give the same kind of name control. Note that the Link class is in the private part of the LinkedList class. It is completely hidden from all other code. For that reason, it is safe to make its data fields public. They can be accessed by the methods of the LinkedList class (which has a legitimate need to access them), and they are not visible elsewhere. In Java, this kind of control was not possible until inner classes were introduced.

However, the Java inner classes have an additional feature that makes them richer and more useful than nested classes in C++. An object that comes from an inner class has an implicit reference to the outer class object that instantiated it. Through this pointer, it gains access to the total state of the outer object. You will see the details of the Java mechanism later in this chapter.

In Java, static inner classes do not have this added pointer. They are the Java analog to nested classes in C++.

Use of an Inner Class to Access Object State

The syntax for inner classes is rather complex. For that reason, we use a simple but somewhat artificial example to demonstrate the use of inner classes. We refactor the TimerTest example and extract a TalkingClock class. A talking clock is constructed with two parameters: the interval between announcements and a flag to turn beeps on or off.

 class TalkingClock {    public TalkingClock(int interval, boolean beep) { . . . }    public void start() { . . . }    private int interval;    private boolean beep;    private class TimePrinter implements ActionListener       // an inner class    {       . . .    } }

Note that the TimePrinter class is now located inside the TalkingClock class. This does not mean that every TalkingClock has a TimePrinter instance field. As you will see, the TimePrinter objects are constructed by methods of the TalkingClock class.

The TimePrinter class is a private inner class inside TalkingClock. This is a safety mechanism. Only TalkingClock methods can generate TimePrinter objects.

Only inner classes can be private. Regular classes always have either package or public visibility.

Here is the TimePrinter class in greater detail. Note that the actionPerformed method checks the beep flag before emitting a beep.

 private class TimePrinter implements ActionListener {    public void actionPerformed(ActionEvent event)    {       Date now = new Date();       System.out.println("At the tone, the time is " + now);       if (beep) Toolkit.getDefaultToolkit().beep();    } }

Something surprising is going on. The TimePrinter class has no instance field or variable named beep. Instead, beep refers to the field of the TalkingClock object that created this TimePrinter. This is quite innovative. Traditionally, a method could refer to the data fields of the object invoking the method. An inner class method gets to access both its own data fields and those of the outer object creating it.

For this to work, an object of an inner class always gets an implicit reference to the object that created it. (See Figure 6-3.)

Figure 6-3. An inner class object has a reference to an outer class object

This reference is invisible in the definition of the inner class. However, to illuminate the concept, let us call the reference to the outer object outer. Then, the actionPerformed method is equivalent to the following:

 public void actionPerformed(ActionEvent event) {      Date now = new Date();    System.out.println("At the tone, the time is " + now);    if (outer.beep) Toolkit.getDefaultToolkit().beep(); }

The outer class reference is set in the constructor. Because the TalkingClock defines no constructors, the compiler synthesizes a constructor, generating code like this:

public TimePrinter(TalkingClock clock) // automatically generated code
{
outer = clock;
}

Again, please note, outer is not a Java keyword. We just use it to illustrate the mechanism involved in an inner class.

NOTE

If an inner class has constructors, the compiler modifies them, adding a parameter for the outer class reference.

When a TimePrinter object is constructed in the start method, the compiler passes the this reference to the current talking clock into the constructor:

 ActionListener listener = new TimePrinter(this); // parameter automatically added

Example 6-4 shows the complete program that tests the inner class. Have another look at the access control. Had the TimePrinter class been a regular class, then it would have needed to access the beep flag through a public method of the TalkingClock class. Using an inner class is an improvement. There is no need to provide accessors that are of interest only to one other class.

Example 6-4. InnerClassTest.java

  1. import java.awt.*;  2. import java.awt.event.*;  3. import java.util.*;  4. import javax.swing.*;  5. import javax.swing.Timer;  6.  7. public class InnerClassTest  8. {  9.    public static void main(String[] args) 10.    { 11.       TalkingClock clock = new TalkingClock(1000, true); 12.       clock.start(); 13. 14.       // keep program running until user selects "Ok" 15.       JOptionPane.showMessageDialog(null, "Quit program?"); 16.       System.exit(0); 17.    } 18. } 19. 20. /** 21.    A clock that prints the time in regular intervals. 22. */ 23. class TalkingClock 24. { 25.    /** 26.       Constructs a talking clock 27.       @param interval the interval between messages (in milliseconds) 28.       @param beep true if the clock should beep 29.    */ 30.    public TalkingClock(int interval, boolean beep) 31.    { 32.       this.interval = interval; 33.       this.beep = beep; 34.    } 35. 36.    /** 37.       Starts the clock. 38.    */ 39.    public void start() 40.    { 41.       ActionListener listener = new TimePrinter(); 42.       Timer t = new Timer(interval, listener); 43.       t.start(); 44.    } 45. 46.    private int interval; 47.    private boolean beep; 48. 49.    private class TimePrinter implements ActionListener 50.    { 51.       public void actionPerformed(ActionEvent event) 52.       { 53.          Date now = new Date(); 54.          System.out.println("At the tone, the time is " + now); 55.          if (beep) Toolkit.getDefaultToolkit().beep(); 56.       } 57.    } 58. }

Special Syntax Rules for Inner Classes

In the preceding section, we explained the outer class reference of an inner class by calling it outer. Actually, the proper syntax for the outer reference is a bit more complex. The expression

OuterClass.this

denotes the outer class reference. For example, you can write the actionPerformed method of the TimePrinter inner class as

 public void actionPerformed(ActionEvent event) {    . . .    if (TalkingClock.this.beep) Toolkit.getDefaultToolkit().beep(); }

Conversely, you can write the inner object constructor more explicitly, using the syntax

outerObject.new InnerClass(construction parameters)

For example,

 ActionListener listener = this.new TimePrinter();

Here, the outer class reference of the newly constructed TimePrinter object is set to the this reference of the method that creates the inner class object. This is the most common case. As always, the this. qualifier is redundant. However, it is also possible to set the outer class reference to another object by explicitly naming it. For example, if TimePrinter were a public inner class, you could construct a TimePrinter for any talking clock:

 TalkingClock jabberer = new TalkingClock(1000, true); TalkingClock.TimePrinter listener = jabberer.new TimePrinter();

Note that you refer to an inner class as

OuterClass.InnerClass

when it occurs outside the scope of the outer class. For example, if TimePrinter had been a public class, you could have referred to it as TalkingClock.TimePrinter elsewhere in your program.

Are Inner Classes Useful? Actually Necessary? Secure?

When inner classes were added to the Java language in JDK 1.1, many programmers considered them a major new feature that was out of character with the Java philosophy of being simpler than C++. The inner class syntax is undeniably complex. (It gets more complex as we study anonymous inner classes later in this chapter.) It is not obvious how inner classes interact with other features of the language, such as access control and security.

By adding a feature that was elegant and interesting rather than needed, has Java started down the road to ruin that has afflicted so many other languages?

While we won't try to answer this question completely, it is worth noting that inner classes are a phenomenon of the compiler, not the virtual machine. Inner classes are translated into regular class files with $ (dollar signs) delimiting outer and inner class names, and the virtual machine does not have any special knowledge about them.

For example, the TimePrinter class inside the TalkingClock class is translated to a class file TalkingClock$TimePrinter.class. To see this at work, try the following experiment: run the ReflectionTest program of Chapter 5, and give it the class TalkingClock$TimePrinter to reflect upon. You will get the following printout:

 class TalkingClock$TimePrinter {    private TalkingClock$TimePrinter(TalkingClock);    TalkingClock$TimePrinter(TalkingClock, TalkingClock$1);    public void actionPerformed(java.awt.event.ActionEvent);    final TalkingClock this$0; }

NOTE

If you use UNIX, remember to escape the $ character if you supply the class name on the command line. That is, run the ReflectionTest program as

 java ReflectionTest 'TalkingClock$TimePrinter'

You can plainly see that the compiler has generated an additional instance field, this$0, for the reference to the outer class. (The name this$0 is synthesized by the compiler you cannot refer to it in your code.) You can also see the added parameter for the constructor. Actually, the construction sequence is somewhat mysterious. A private constructor sets the this$0 field, and a package-visible constructor has a second parameter of type TalkingClock$1, a package-visible class with no fields or methods. That class is never instantiated. The TalkingClock class calls

 new TalkingClock$TimePrinter(this, null)

If the compiler can automatically do this transformation, couldn't you simply program the same mechanism by hand? Let's try it. We would make TimePrinter a regular class, outside the TalkingClock class. When constructing a TimePrinter object, we pass it the this reference of the object that is creating it.

 class TalkingClock {    . . .    public void start()    {       ActionListener listener = new TimePrinter(this);       Timer t = new Timer(interval, listener);       t.start();    } } class TimePrinter implements ActionListener {    public TimePrinter(TalkingClock clock)    {        outer = clock;    }    . . .    private TalkingClock outer;  }

Now let us look at the actionPerformed method. It needs to access outer.beep.

    if (outer.beep) . . . // ERROR

Here we run into a problem. The inner class can access the private data of the outer class, but our external TimePrinter class cannot.

Thus, inner classes are genuinely more powerful than regular classes because they have more access privileges.

You may well wonder how inner classes manage to acquire those added access privileges, because inner classes are translated to regular classes with funny names the virtual machine knows nothing at all about them. To solve this mystery, let's again use the ReflectionTest program to spy on the TalkingClock class:

 class TalkingClock {    public TalkingClock(int, boolean);    static boolean access$100(TalkingClock);    public void start();    private int interval;    private boolean beep; }

Notice the static access$100 method that the compiler added to the outer class. It returns the beep field of the object that is passed as a parameter.

The inner class methods call that method. The statement

 if (beep)

in the actionPerformed method of the TimePrinter class effectively makes the following call:

 if (access$100(outer));

Is this a security risk? You bet it is. It is an easy matter for someone else to invoke the access$100 method to read the private beep field. Of course, access$100 is not a legal name for a Java method. However, hackers who are familiar with the structure of class files can easily produce a class file with virtual machine instructions to call that method, for example, by using a hex editor. Because the secret access methods have package visibility, the attack code would need to be placed inside the same package as the class under attack.

To summarize, if an inner class accesses a private data field, then it is possible to access that data field through other classes that are added to the package of the outer class, but to do so requires skill and determination. A programmer cannot accidentally obtain access but must intentionally build or modify a class file for that purpose.

Local Inner Classes

If you look carefully at the code of the TalkingClock example, you will find that you need the name of the type TimePrinter only once: when you create an object of that type in the start method.

When you have a situation like this, you can define the class locally in a single method.

 public void start() {    class TimePrinter implements ActionListener    {       public void actionPerformed(ActionEvent event)       {           Date now = new Date();           System.out.println("At the tone, the time is " + now);           if (beep) Toolkit.getDefaultToolkit().beep();       }    }    ActionListener listener = new TimePrinter();    Timer t = new Timer(1000, listener);    t.start(); }

Local classes are never declared with an access specifier (that is, public or private). Their scope is always restricted to the block in which they are declared.

Local classes have a great advantage: they are completely hidden from the outside world not even other code in the TalkingClock class can access them. No method except start has any knowledge of the TimePrinter class.

Local classes have another advantage over other inner classes. Not only can they access the fields of their outer classes, they can even access local variables! However, those local variables must be declared final. Here is a typical example. Let's move the interval and beep parameters from the TalkingClock constructor to the start method.

 public void start(int interval, final boolean beep) {    class TimePrinter implements ActionListener    {       public void actionPerformed(ActionEvent event)       {           Date now = new Date();           System.out.println("At the tone, the time is " + now);           if (beep) Toolkit.getDefaultToolkit().beep();       }   }    ActionListener listener = new TimePrinter();    Timer t = new Timer(1000, listener);    t.start(); }

Note that the TimePrinter class no longer needs to store a beep instance variable. It simply refers to the parameter variable of the method that contains the class definition.

Maybe this should not be so surprising. The line

 if (beep) . . .

is, after all, ultimately inside the start method, so why shouldn't it have access to the value of the beep variable?

To see why there is a subtle issue here, let's consider the flow of control more closely.

The start method is called.
The object variable listener is initialized by a call to the constructor of the inner class TimePrinter.
The listener reference is passed to the Timer constructor, the timer is started, and the start method exits. At this point, the beep parameter variable of the start method no longer exists.
A second later, the actionPerformed method executes if (beep) . . ..

For the code in the actionPerformed method to work, the TimePrinter class must have copied the beep field, as a local variable of the start method, before the beep field went away. That is indeed exactly what happens. In our example, the compiler synthesizes the name TalkingClock$1TimePrinter for the local inner class. If you use the ReflectionTest program again to spy on the TalkingClock$1TimePrinter class, you get the following output:

 class TalkingClock$1TimePrinter {    TalkingClock$1TimePrinter(TalkingClock, boolean);    public void actionPerformed(java.awt.event.ActionEvent);    final boolean val$beep;    final TalkingClock this$0; }

Note the boolean parameter to the constructor and the val$beep instance variable. When an object is created, the value beep is passed into the constructor and stored in the val$beep field. This sounds like an extraordinary amount of trouble for the implementors of the compiler. The compiler must detect access of local variables, make matching data fields for each one of them, and copy the local variables into the constructor so that the data fields can be initialized as copies of them.

From the programmer's point of view, however, local variable access is quite pleasant. It makes your inner classes simpler by reducing the instance fields that you need to program explicitly.

As we already mentioned, the methods of a local class can refer only to local variables that are declared final. For that reason, the beep parameter was declared final in our example. A local variable that is declared final cannot be modified after it has been initialized. Thus, it is guaranteed that the local variable and the copy that is made inside the local class always have the same value.

NOTE

You have seen final variables used for constants, such as

 public static final double SPEED_LIMIT = 55;

The final keyword can be applied to local variables, instance variables, and static variables. In all cases it means the same thing: You can assign to this variable once after it has been created. Afterwards, you cannot change the value it is final.

However, you don't have to initialize a final variable when you define it. For example, the final parameter variable beep is initialized once after its creation, when the start method is called. (If the method is called multiple times, each call has its own newly created beep parameter.) The val$beep instance variable that you can see in the TalkingClock$1TimePrinter inner class is set once, in the inner class constructor. A final variable that isn't initialized when it is defined is often called a blank final variable.

Anonymous Inner Classes

When using local inner classes, you can often go a step further. If you want to make only a single object of this class, you don't even need to give the class a name. Such a class is called an anonymous inner class.

 public void start(int interval, final boolean beep) {    ActionListener listener = new       ActionListener()       {          public void actionPerformed(ActionEvent event)          {              Date now = new Date();              System.out.println("At the tone, the time is " + now);              if (beep) Toolkit.getDefaultToolkit().beep();          }       };    Timer t = new Timer(1000, listener);    t.start(); }

This syntax is very cryptic indeed. What it means is this:

Create a new object of a class that implements the ActionListener interface, where the required method actionPerformed is the one defined inside the braces { }.

Any parameters used to construct the object are given inside the parentheses ( ) following the supertype name. In general, the syntax is

new SuperType(construction parameters)
{
inner class methods and data
}

Here, SuperType can be an interface, such as ActionListener; then, the inner class implements that interface. Or SuperType can be a class; then, the inner class extends that class.

An anonymous inner class cannot have constructors because the name of a constructor must be the same as the name of a class, and the class has no name. Instead, the construction parameters are given to the superclass constructor. In particular, whenever an inner class implements an interface, it cannot have any construction parameters. Nevertheless, you must supply a set of parentheses as in

new InterfaceType() { methods and data }

You have to look carefully to see the difference between the construction of a new object of a class and the construction of an object of an anonymous inner class extending that class. If the closing parenthesis of the construction parameter list is followed by an opening brace, then an anonymous inner class is being defined.

 Person queen = new Person("Mary");    // a Person object Person count = new Person("Dracula") { . . .};    // an object of an inner class extending Person

Are anonymous inner classes a great idea or are they a great way of writing obfuscated code? Probably a bit of both. When the code for an inner class is short, just a few lines of simple code, then anonymous inner classes can save typing time, but it is exactly timesaving features like this that lead you down the slippery slope to "Obfuscated Java Code Contests."

It is a shame that the designers of Java did not try to improve the syntax of anonymous inner classes, because generally, Java syntax is a great improvement over C++. The designers of the inner class feature could have helped the human reader with a syntax such as

 Person count = new class extends Person("Dracula") { . . . };    // not the actual Java syntax

But they didn't. Because many programmers find code with too many anonymous inner classes hard to read, we recommend restraint when using them.

Example 6-5 contains the complete source code for the talking clock program with an anonymous inner class. If you compare this program with Example 6-4, you will find that in this case the solution with the anonymous inner class is quite a bit shorter, and, hopefully, with a bit of practice, as easy to comprehend.

Example 6-5. AnonymousInnerClassTest.java

  1. import java.awt.*;  2. import java.awt.event.*;  3. import java.util.*;  4. import javax.swing.*;  5. import javax.swing.Timer;  6.  7. public class AnonymousInnerClassTest  8. {  9.    public static void main(String[] args) 10.    { 11.       TalkingClock clock = new TalkingClock(); 12.       clock.start(1000, true); 13. 14.       // keep program running until user selects "Ok" 15.       JOptionPane.showMessageDialog(null, "Quit program?"); 16.       System.exit(0); 17.    } 18. } 19. 20. /** 21.    A clock that prints the time in regular intervals. 22. */ 23. class TalkingClock 24. { 25.    /** 26.       Starts the clock. 27.       @param interval the interval between messages (in milliseconds) 28.       @param beep true if the clock should beep 29.    */ 30.    public void start(int interval, final boolean beep) 31.    { 32.       ActionListener listener = new 33.          ActionListener() 34.          { 35.             public void actionPerformed(ActionEvent event) 36.             { 37.                Date now = new Date(); 38.                System.out.println("At the tone, the time is " + now); 39.                if (beep) Toolkit.getDefaultToolkit().beep(); 40.             } 41.          }; 42.       Timer t = new Timer(interval, listener); 43.       t.start(); 44.    } 45. }

Static Inner Classes

Occasionally, you want to use an inner class simply to hide one class inside another, but you don't need the inner class to have a reference to the outer class object. You can suppress the generation of that reference by declaring the inner class static.

Here is a typical example of where you would want to do this. Consider the task of computing the minimum and maximum value in an array. Of course, you write one method to compute the minimum and another method to compute the maximum. When you call both methods, then the array is traversed twice. It would be more efficient to traverse the array only once, computing both the minimum and the maximum simultaneously.

 double min = Double.MAX_VALUE; double max = Double.MIN_VALUE; for (double v : values) {    if (min > v) min = v;    if (max < v) max = v; }

However, the method must return two numbers. We can achieve that by defining a class Pair that holds two values:

 class Pair {    public Pair(double f, double s)    {       first = f;       second = s;    }    public double getFirst() { return first; }    public double getSecond() {  return second; }    private double first;    private double second; }

The minmax function can then return an object of type Pair.

 class ArrayAlg {    public static Pair minmax(double[] values)    {       . . .       return new Pair(min, max);    } }

The caller of the function uses the getFirst and getSecond methods to retrieve the answers:

 Pair p = ArrayAlg.minmax(d); System.out.println("min = " + p.getFirst()); System.out.println("max = " + p.getSecond());

Of course, the name Pair is an exceedingly common name, and in a large project, it is quite possible that some other programmer had the same bright idea, except that the other programmer made a Pair class that contains a pair of strings. We can solve this potential name clash by making Pair a public inner class inside ArrayAlg. Then the class will be known to the public as ArrayAlg.Pair:

 ArrayAlg.Pair p = ArrayAlg.minmax(d);

However, unlike the inner classes that we used in previous examples, we do not want to have a reference to any other object inside a Pair object. That reference can be suppressed by declaring the inner class static:

 class ArrayAlg {    public static class Pair    {       . . .    }    . . . }

Of course, only inner classes can be declared static. A static inner class is exactly like any other inner class, except that an object of a static inner class does not have a reference to the outer class object that generated it. In our example, we must use a static inner class because the inner class object is constructed inside a static method:

 public static Pair minmax(double[] d) {     . . .     return new Pair(min, max); }

Had the Pair class not been declared as static, the compiler would have complained that there was no implicit object of type ArrayAlg available to initialize the inner class object.

NOTE

You use a static inner class whenever the inner class does not need to access an outer class object. Some programmers use the term nested class to describe static inner classes.

NOTE

Inner classes that are declared inside an interface are automatically static and public.

Example 6-6 contains the complete source code of the ArrayAlg class and the nested Pair class.

Example 6-6. StaticInnerClassTest.java

  1. public class StaticInnerClassTest  2. {  3.    public static void main(String[] args)  4.    {  5.       double[] d = new double[20];  6.       for (int i = 0; i < d.length; i++)  7.          d[i] = 100 * Math.random();  8.       ArrayAlg.Pair p = ArrayAlg.minmax(d);  9.       System.out.println("min = " + p.getFirst()); 10.       System.out.println("max = " + p.getSecond()); 11.    } 12. } 13. 14. class ArrayAlg 15. { 16.    /** 17.       A pair of floating-point numbers 18.    */ 19.    public static class Pair 20.    { 21.       /** 22.           Constructs a pair from two floating-point numbers 23.           @param f the first number 24.           @param s the second number 25.       */ 26.       public Pair(double f, double s) 27.       { 28.          first = f; 29.          second = s; 30.       } 31. 32.       /** 33.          Returns the first number of the pair 34.          @return the first number 35.       */ 36.       public double getFirst() 37.      { 38.         return first; 39.      } 40. 41.      /** 42.         Returns the second number of the pair 43.         @return the second number 44.      */ 45.      public double getSecond() 46.      { 47.         return second; 48.      } 49. 50.      private double first; 51.      private double second; 52.   } 53. 54.   /** 55.      Computes both the minimum and the maximum of an array 56.      @param values an array of floating-point numbers 57.      @return a pair whose first element is the minimum and whose 58.      second element is the maximum 59.   */ 60.   public static Pair minmax(double[] values) 61.   { 62.      double min = Double.MAX_VALUE; 63.      double max = Double.MIN_VALUE; 64.      for (double v : values) 65.      { 66.         if (min > v) min = v; 67.         if (max < v) max = v; 68.      } 69.      return new Pair(min, max); 70.   } 71.}