Home > Articles > Programming > Java

  • Print
  • + Share This
This chapter is from the book

6.4. Inner Classes

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 the 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.

  1. Starting on page 307, you will see a simple inner class that accesses an instance field of its outer class.
  2. On page 311, we cover the special syntax rules for inner classes.
  3. Starting on page 312, we peek inside inner classes to see how they are translated into regular classes. Squeamish readers may want to skip that section.
  4. Starting on page 315, we discuss local inner classes that can access local variables of the enclosing scope.
  5. Starting on page 318, we introduce anonymous inner classes and show how they are commonly used to implement callbacks.
  6. Finally, starting on page 322, you will see how static inner classes can be used for nested helper classes.

6.4.1. Use of an Inner Class to Access Object State

The syntax for inner classes is rather complex. For that reason, we present 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.

public class TalkingClock
{
   private int interval;
   private boolean beep;

   public TalkingClock(int interval, boolean beep) { . . . }
   public void start() { . . . }

   public 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.

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

public 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

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. The compiler modifies all inner class constructors, adding a parameter for the outer class reference. The TimePrinter class defines no constructors; therefore, the compiler synthesizes a no-argument 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.

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

Listing 6.6 shows the complete program that tests the inner class. Have another look at the access control. Had the TimePrinter class been a regular class, 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.

Listing 6.6. innerClass/InnerClassTest.java

 1  package innerClass;
 2
 3  import java.awt.*;
 4  import java.awt.event.*;
 5  import java.util.*;
 6  import javax.swing.*;
 7  import javax.swing.Timer;
 8  /**
 9   * This program demonstrates the use of inner classes.
10   * @version 1.10 2004-02-27
11   * @author Cay Horstmann
12   */
13  public class InnerClassTest
14  {
15     public static void main(String[] args)
16     {
17        TalkingClock clock = new TalkingClock(1000, true);
18        clock.start();
19
20        // keep program running until user selects "Ok"
21        JOptionPane.showMessageDialog(null, "Quit program?");
22        System.exit(0);
23     }
24  }
25
26  /**
27   * A clock that prints the time in regular intervals.
28   */
29  class TalkingClock
30  {
31     private int interval;
32     private boolean beep;
33
34     /**
35      * Constructs a talking clock
36      * @param interval the interval between messages (in milliseconds)
37      * @param beep true if the clock should beep
38      */
39     public TalkingClock(int interval, boolean beep)
40     {
41        this.interval = interval;
42        this.beep = beep;
43     }
44
45     /**
46      * Starts the clock.
47      */
48     public void start()
49     {
50        ActionListener listener = new TimePrinter();
51        Timer t = new Timer(interval, listener);
52        t.start();
53     }
54     public class TimePrinter implements ActionListener
55     {
56        public void actionPerformed(ActionEvent event)
57        {
58           Date now = new Date();
59           System.out.println("At the tone, the time is " + now);
60           if (beep) Toolkit.getDefaultToolkit().beep();
61        }
62     }
63  }

6.4.2. 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, since TimePrinter is a public inner class, you can 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.

6.4.3. Are Inner Classes Useful? Actually Necessary? Secure?

When inner classes were added to the Java language in Java 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 which 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. Alternatively, simply use the javap utility:

javap -private ClassName

You will get the following printout:

public class TalkingClock$TimePrinter
{
   public TalkingClock$TimePrinter(TalkingClock);

   public void actionPerformed(java.awt.event.ActionEvent);

   final TalkingClock this$0;
}

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 TalkingClock parameter for the constructor.

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
{
   private TalkingClock outer;
   . . .
   public TimePrinter(TalkingClock clock)
   {
       outer = clock;
   }
}

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, if they 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
{
   private int interval;
   private boolean beep;

   public TalkingClock(int, boolean);

   static boolean access$0(TalkingClock);
   public void start();
}

Notice the static access$0 method that the compiler added to the outer class. It returns the beep field of the object that is passed as a parameter. (The method name might be slightly different, such as access$000, depending on your compiler.)

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$0(outer));

Is this a security risk? You bet it is. It is an easy matter for someone else to invoke the access$0 method to read the private beep field. Of course, access$0 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. Since 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 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.

6.4.4. 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.

In 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(interval, 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 one 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.

6.4.5. Accessing final Variables from Outer Methods

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(interval, listener);
   t.start();
}

Note that the TalkingClock class no longer needs to store a beep instance field. It simply refers to the beep parameter variable of the start method.

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.

  1. The start method is called.
  2. The object variable listener is initialized by a call to the constructor of the inner class TimePrinter.
  3. 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.
  4. 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 parameter value 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 will 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. The compiler detects access of local variables, makes matching instance fields for each one of them, and copies the local variables into the constructor so that the instance fields can be initialized.

From the programmer’s point of view, 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 will always have the same value.

The final restriction is somewhat inconvenient. Suppose, for example, that you want to update a counter in the enclosing scope. Here, we want to count how often the compareTo method is called during sorting:

int counter = 0;
Date[] dates = new Date[100];
for (int i = 0; i < dates.length; i++)
   dates[i] = new Date()
      {
         public int compareTo(Date other)
         {
            counter++; // ERROR
            return super.compareTo(other);
         }
      };
Arrays.sort(dates);
System.out.println(counter + " comparisons.");

You can’t declare counter as final because you clearly need to update it. You can’t replace it with an Integer because Integer objects are immutable. The remedy is to use an array of length 1:

final int[] counter = new int[1];
for (int i = 0; i < dates.length; i++)
   dates[i] = new Date()
      {
         public int compareTo(Date other)
         {
            counter[0]++;
            return super.compareTo(other);
         }
      };

(The array variable is still declared as final, but that merely means that you can’t have it refer to a different array. You are free to mutate the array elements.)

When inner classes were first invented, the prototype compiler automatically made this transformation for all local variables that were modified in the inner class. However, some programmers were fearful of having the compiler produce heap objects behind their backs, and the final restriction was adopted instead. It is possible that a future version of the Java language will revise this decision.

6.4.6. 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(interval, 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 { }.

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. SuperType can also 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.

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

If the closing parenthesis of the construction parameter list is followed by an opening brace, then an anonymous inner class is being defined.

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 such time-saving features that lead you down the slippery slope to “Obfuscated Java Code Contests.”

Listing 6.7 contains the complete source code for the talking clock program with an anonymous inner class. If you compare this program with Listing 6.6, 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.

Listing 6.7. anonymousInnerClass/AnonymousInnerClassTest.java

 1  package anonymousInnerClass;
 2
 3  import java.awt.*;
 4  import java.awt.event.*;
 5  import java.util.*;
 6  import javax.swing.*;
 7  import javax.swing.Timer;
 8
 9  /**
10   * This program demonstrates anonymous inner classes.
11   * @version 1.10 2004-02-27
12   * @author Cay Horstmann
13   */
14  public class AnonymousInnerClassTest
15  {
16     public static void main(String[] args)
17     {
18        TalkingClock clock = new TalkingClock();
19        clock.start(1000, true);
20
21        // keep program running until user selects "Ok"
22        JOptionPane.showMessageDialog(null, "Quit program?");
23        System.exit(0);
24     }
25  }
26
27  /**
28   * A clock that prints the time in regular intervals.
29   */
30  class TalkingClock
31  {
32     /**
33      * Starts the clock.
34      * @param interval the interval between messages (in milliseconds)
35      * @param beep true if the clock should beep
36      */
37     public void start(int interval, final boolean beep)
38     {
39        ActionListener listener = new ActionListener()
40           {
41              public void actionPerformed(ActionEvent event)
42              {
43                 Date now = new Date();
44                 System.out.println("At the tone, the time is " + now);
45                 if (beep) Toolkit.getDefaultToolkit().beep();
46              }
47           };
48        Timer t = new Timer(interval, listener);
49        t.start();
50     }
51  }

6.4.7. Static Inner Classes

Occasionally, you may 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, 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
{
  private double first;
  private double second;

  public Pair(double f, double s)
  {
     first = f;
     second = s;
  }

  public double getFirst() { return first; }
  public double getSecond() {  return second; }
}

The minmax method 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 method 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—but 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.

Listing 6.8 contains the complete source code of the ArrayAlg class and the nested Pair class.

Listing 6.8. staticInnerClass/StaticInnerClassTest.java

 1  package staticInnerClass;
 2
 3  /**
 4   * This program demonstrates the use of static inner classes.
 5   * @version 1.01 2004-02-27
 6   * @author Cay Horstmann
 7   */
 8  public class StaticInnerClassTest
 9  {
10     public static void main(String[] args)
11     {
12        double[] d = new double[20];
13        for (int i = 0; i < d.length; i++)
14           d[i] = 100 * Math.random();
15        ArrayAlg.Pair p = ArrayAlg.minmax(d);
16        System.out.println("min = " + p.getFirst());
17        System.out.println("max = " + p.getSecond());
18     }
19  }
20  class ArrayAlg
21  {
22     /**
23      * A pair of floating-point numbers
24      */
25     public static class Pair
26     {
27        private double first;
28        private double second;
29
30        /**
31         * Constructs a pair from two floating-point numbers
32         * @param f the first number
33         * @param s the second number
34         */
35        public Pair(double f, double s)
36        {
37           first = f;
38           second = s;
39        }
40
41        /**
42         * Returns the first number of the pair
43         * @return the first number
44         */
45        public double getFirst()
46        {
47           return first;
48        }
49
50        /**
51         * Returns the second number of the pair
52         * @return the second number
53         */
54        public double getSecond()
55        {
56           return second;
57        }
58     }
59
60     /**
61      * Computes both the minimum and the maximum of an array
62      * @param values an array of floating-point numbers
63      * @return a pair whose first element is the minimum and whose second element
64      * is the maximum
65      */

66     public static Pair minmax(double[] values)
67     {
68        double min = Double.MAX_VALUE;
69        double max = Double.MIN_VALUE;
70        for (double v : values)
71        {
72           if (min > v) min = v;
73           if (max < v) max = v;
74        }
75        return new Pair(min, max);
76     }
77  }
  • + Share This
  • 🔖 Save To Your Account