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Inside Java Interfaces and Inner Classes

After mastering the basics of Java, explore some commonly-used advanced techniques such as interfaces and inner classes to complete your Java tool chest.
This chapter is from the book
  • Interfaces
  • Object Cloning
  • Inner Classes
  • Proxies

You have now seen all the basic tools for object-oriented programming in Java. This chapter shows you two advanced techniques that are very commonly used. Despite their less obvious nature, you will need to master them to complete your Java tool chest.

The first, called an interface, is a way of describing what classes should do, without specifying how they should do it. A class can implement one or more interfaces. You can then use objects of these implementing classes anytime that conformance to the interface is required. After we cover interfaces, we take up cloning an object (or deep copying, as it is sometimes called). A clone of an object is a new object that has the same state as the original but a different identity. In particular, you can modify the clone without affecting the original. Finally, we move on to the mechanism of inner classes. Inner classes are technically somewhat complex—they are defined inside other classes, and their methods can access the fields of the surrounding class. Inner classes are useful when you design collections of cooperating classes. In particular, inner classes are important to write concise, professional-looking code to handle graphical user interface events.

This chapter concludes with a discussion of proxies, objects that implement arbitrary interfaces. A proxy is a very specialized construct that is useful for building system-level tools. You can safely skip that section on first reading.


In the Java programming language, an interface is not a class but a set of requirements for classes that want to conform to the interface.

Typically, the supplier of some service states: "If your class conforms to a particular interface, then I'll perform the service." Let's look at a concrete example. The sort method of the Arrays class promises to sort an array of objects, but under one condition: The objects must belong to classes that implement the Comparable interface.

Here is what the Comparable interface looks like:

public interface Comparable 
   int compareTo(Object other);

This means that any class that implements the Comparable interface is required to have a compareTo method, and the method must take an Object parameter and return an integer.

All methods of an interface are automatically public. For that reason, it is not necessary to supply the keyword public when declaring a method in an interface.

Of course, there is an additional requirement that the interface cannot spell out: When calling x.compareTo(y), the compareTo method must actually be able to compare two objects and return an indication whether x or y is larger. The method is supposed to return a negative number if x is smaller than y, zero if they are equal, and a positive number otherwise.

This particular interface has a single method. Some interfaces have more than one method. As you will see later, interfaces can also define constants. What is more important, however, is what interfaces cannot supply. Interfaces never have instance fields, and the methods are never implemented in the interface. Supplying instance fields and method implementations is the job of the classes that implement the interface. You can think of an interface as being similar to an abstract class with no instance fields. However, there are some differences between these two concepts—we will look at them later in some detail.

Now suppose we want to use the sort method of the Arrays class to sort an array of Employee objects. Then the Employee class must implement the Comparable interface.

To make a class implement an interface, you have to carry out two steps:

  1. You declare that your class intends to implement the given interface.

  2. You supply definitions for all methods in the interface.

To declare that a class implements an interface, use the implements keyword:

class Employee implements Comparable

Of course, now the Employee class needs to supply the compareTo method. Let's suppose that we want to compare employees by their salary. Here is a compareTo method that returns -1 if the first employee's salary is less than the second employee's salary, 0 if they are equal, and 1 otherwise.

public int compareTo(Object otherObject)
   Employee other = (Employee)otherObject;
   if (salary < other.salary) return -1;
   if (salary > other.salary) return 1;
   return 0;


In the interface declaration, the compareTo method was not declared public because all methods in an interface are automatically public. However, when implementing the interface, you must declare the method as public. Otherwise, the compiler assumes that the method has package visibility—the default for a class. Then the compiler complains that you try to supply a weaker access privilege.


The compareTo method of the Comparable interface returns an integer. If the objects are not equal, it does not matter what negative or positive value you return. This flexibility can be useful when comparing integer fields. For example, suppose each employee has a unique integer id, and you want to sort by employee ID number. Then you can simply return id - other.id. That value will be some negative value if the first ID number is less than the other, 0 if they are the same ID, and some positive value otherwise. However, there is one caveat: The range of the integers must be small enough that the subtraction does not overflow. If you know that the IDs are not negative or that their absolute value is at most(Integer.MAX_VALUE - 1) / 2, you are safe.

Of course, the subtraction trick doesn't work for floating-point numbers. The difference salary - other.salary can round to 0 if the salaries are close together but not identical.

Now you saw what a class must do to avail itself of the sorting service—it must implement a compareTo method. That's eminently reasonable. There needs to be some way for the sort method to compare objects. But why can't the Employee class simply provide a compareTo method without implementing the Comparable interface?

The reason for interfaces is that the Java language is strongly typed. When making a method call, the compiler needs to be able to check that the method actually exists. Somewhere in the sort method, there will be statements like this:

if (a[i].compareTo(a[j]) > 0)
   // rearrange a[i] and a[j]
   . . .

The compiler must know that a[i] actually has a compareTo method. If a is an array of Comparable objects, then the existence of the method is assured, because every class that implements the Comparable interface must supply the method.


You would expect that the sort method in the Arrays class is defined to accept a Comparable[] array, so that the compiler can complain if anyone ever calls sort with an array whose element type doesn't implement the Comparable interface. Sadly, that is not the case. Instead, the sort method accepts an Object[] array and uses a clumsy cast:

// from the standard library--not recommended
if (((Comparable)a[i]).compareTo((Comparable)a[j]) > 0)
   // rearrange a[i] and a[j]
   . . .

If a[i] does not belong to a class that implements the Comparable interface, then the virtual machine throws an exception. (Note that the second cast to Comparable is not necessary because the explicit parameter of the compareTo method has type Object, not Comparable.)

See Example 6–1 for the full code for sorting of an employee array.

Example 6–1: EmployeeSortTest.java

 1.   import java.util.*;
 3.   public class EmployeeSortTest
 4.   {  public static void main(String[] args)
 5.      {  Employee[] staff = new Employee[3];
 7.         staff[0] = new Employee("Harry Hacker", 35000);
 8.         staff[1] = new Employee("Carl Cracker", 75000);
 9.         staff[2] = new Employee("Tony Tester", 38000);
11.         Arrays.sort(staff);
13.         // print out information about all Employee objects
14.         for (int i = 0; i < staff.length; i++)
15.         {  Employee e = staff[i];
16.            System.out.println("name=" + e.getName()
17.               + ",salary=" + e.getSalary());
18.         }
19.      }
20.   }
22.   class Employee implements Comparable
23.   {  public Employee(String n, double s)
24.      {  name = n;
25.         salary = s;
26.      }
28.      public String getName()
29.      {  return name;
30.      }
32.      public double getSalary()
33.      {  return salary;
34.      }
36.      public void raiseSalary(double byPercent)
37.      {  double raise = salary * byPercent / 100;
38.         salary += raise;
39.      }
41.      /**
42.         Compares employees by salary
43.         @param otherObject another Employee object
44.         @return a negative value if this employee has a lower
45.         salary than otherObject, 0 if the salaries are the same,
46.         a positive value otherwise
47.      */
48.      public int compareTo(Object otherObject)
49.      {  Employee other = (Employee)otherObject;
50.         if (salary < other.salary) return -1;
51.         if (salary > other.salary) return 1;
52.         return 0;
53.      }
55.      private String name;
56.      private double salary;
57.   }

java.lang.Comparable 1.0

int compareTo(Object otherObject) compares this object with otherObject and returns a negative integer if this object is less than otherObject, zero if they are equal, and a positive integer otherwise.


According to the language standard: "The implementor must ensure sgn(x.compareTo(y)) = -sgn(y.compareTo(x)) for all x and y. (This implies that x.compareTo(y) must throw an exception if y.compareTo(x) throws an exception.)" Here, "sgn" is the sign of a number: sgn(n) is -1 if n is negative, 0 if n equals 0, and 1 if n is positive. In plain English, if you flip the parameters of compareTo, the sign (but not necessarily the actual value) of the result must also flip. That's not a problem, but the implication about exceptions is tricky. Suppose Manager has its own comparison method that compares two managers. It might start like this:

public int compareTo(Object otherObject)
   Manager other = (Manager)otherObject;
   . . .


That violates the "antisymmetry" rule. If x is an Employee and y is a Manager, then the call x.compareTo(y) doesn't throw an exception—it simply compares x and y as employees. But the reverse, y.compareTo(x)throws a ClassCastException.

The same issue comes up when programming an equals method. However, in that case, you simply test if the two classes are identical, and if they aren't, you know that you should return false. However, if x and y aren't of the same class, it is not clear whether x.compareTo(y) should return a negative or a positive value. Maybe managers think that they should compare larger than any employee, no matter what the salary. But then they need to explicitly implement that check.

If you don't trust the implementors of your subclasses to grasp this subtlety, you can declare compareTo as a final method. Then the problem never arises because subclasses can't supply their own version. Conversely, if you implement a compareTo method of a subclass, you need to provide a thorough test. Here is an example:

if (otherObject instanceof Manager)
   Manager other = (Manager)otherObject;
   . . .
else if (otherObject instanceof Employee)
   return 1; // managers are always better :-(
   return -((Comparable)otherObject).compareTo(this);

java.util.Arrays 1.2

static void sort(Object[] a) sorts the elements in the array a, using a tuned mergesort algorithm. All elements in the array must belong to classes that implement the Comparable interface, and they must all be comparable to each other.

Properties of Interfaces

Interfaces are not classes. In particular, you can never use the new operator to instantiate an interface:

x = new Comparable(. . .); // ERROR

However, even though you can't construct interface objects, you can still declare sinterface variables.

Comparable x; // OK

An interface variable must refer to an object of a class that implements the interface:

x = new Employee(. . .); 
   // OK provided Employee implements Comparable

Next, just as you use instanceof to check if an object is of a specific class, you can use instanceof to check if an object implements an interface:

if (anObject instanceof Comparable) { . . . }

Just as you can build hierarchies of classes, you can extend interfaces. This allows for multiple chains of interfaces that go from a greater degree of generality to a greater degree of specialization. For example, suppose you had an interface called Moveable.

public interface Moveable
   void move(double x, double y);

Then, you could imagine an interface called Powered that extends it:

public interface Powered extends Moveable
   double milesPerGallon();

Although you cannot put instance fields or static methods in an interface, you can supply constants in them. For example:

public interface Powered extends Moveable
   double milesPerGallon();
   double SPEED_LIMIT = 95; // a public static final constant

Just as methods in an interface are automatically public, fields are always public static final.


It is legal to tag interface methods as public, and fields as public static final. Some programmers do that, either out of habit or for greater clarity. However, the Java Language Specification recommends not to supply the redundant keywords, and we follow that recommendation.

Some interfaces define just constants and no methods. For example, the standard library contains an interface SwingConstants that defines constants NORTH, SOUTH, HORIZONTAL, and so on. Any class that chooses to implement the SwingConstants interface automatically inherits these constants. Its methods can simply refer to NORTH rather than the more cumbersome SwingConstants.NORTH.

While each class can only have one superclass, classes can implement multiple interfaces. This gives you the maximum amount of flexibility in defining a class's behavior. For example, the Java programming language has an important interface built into it, called Cloneable. (We will discuss this interface in detail in the next section.) If your class implements Cloneable, the clone method in the Object class will make an exact copy of your class's objects. Suppose, therefore, you want cloneability and comparability. Then you simply implement both interfaces.

class Employee implements Cloneable, Comparable

Use commas to separate the interfaces that describe the characteristics that you want to supply.

Interfaces and Abstract Classes

If you read the section about abstract classes in Chapter 5, you may wonder why the designers of the Java programming language bothered with introducing the concept of interfaces. Why can't Comparable simply be an abstract class:

abstract class Comparable // why not?
   public abstract int compareTo(Object other);

Then the Employee class would simply extend this abstract class and supply the compareTo method:

class Employee extends Comparable // why not?
   public int compareTo(Object other) { . . . }

There is, unfortunately, a major problem with using an abstract base class to express a generic property. A class can only extend a single class. Suppose that the Employee class already extends a different class, say Person. Then it can't extend a second class.

class Employee extends Person, Comparable // ERROR

But each class can implement as many interfaces as it likes:

class Employee extends Person implements Comparable // OK

Other programming languages, in particular C++, allow a class to have more than one superclass. This feature is called multiple inheritance. The designers of Java chose not to support multiple inheritance because it makes the language either very complex (as in C++) or less efficient (as in Eiffel).

Instead, interfaces give most of the benefits of multiple inheritance while avoiding the complexities and inefficiencies.


C++ has multiple inheritance and all the complications that come with it, such as virtual base classes, dominance rules, and transverse pointer casts. Few C++ programmers use multiple inheritance, and some say it should never be used. Other programmers recommend using multiple inheritance only for "mix-in" style inheritance. In the mix-in style, a primary base class describes the parent object, and additional base classes (the so-called mix-ins) may supply auxiliary characteristics. That style is similar to a Java class with a single base class and additional interfaces. However, in C++, mix-ins can add default behavior, whereas Java interfaces cannot.


Microsoft has long been a proponent of using interfaces instead of using multiple inheritance. In fact, the Java notion of an interface is essentially equivalent to how Microsoft's COM technology uses interfaces. As a result of this unlikely convergence of minds, it is easy to supply tools based on the Java programming language to build COM objects (such as ActiveX controls). This is done (pretty much transparently to the coder) in, for example, Microsoft's J++ product and is also the basis for Sun's JavaBeans-to-ActiveX bridge.

Interfaces and Callbacks

A common pattern in programming is the callback pattern. In this pattern, you want to specify the action that should occur whenever a particular event happens. For example, you may want a particular action to occur when a button is clicked or a menu item is selected. However, since you have not yet seen how to implement user interfaces, we will consider a similar but simpler situation.

The javax.swing class contains a Timer class that is useful if you want to be notified whenever a time interval has elapsed. For example, if a part of your program contains a clock, then you can ask to be notified every second so that you can update the clock face.

When you construct a timer, you set the time interval, and you tell it what it should do whenever the time interval has elapsed.

How do you tell the timer what it should do? In many programming languages, you supply the name of a function that the timer should call periodically. However, the classes in the Java standard library take an object-oriented approach. You pass an object of some class. The timer then calls one of the methods on that object. Passing an object is more flexible than passing a function because the object can carry additional information.

Of course, the timer needs to know what method to call. The timer requires that you specify an object of a class that implements the ActionListener interface of the java.awt.event package. Here is that interface:

public interface ActionListener
   void actionPerformed(ActionEvent event);

The timer calls the actionPerformed method when the time interval has expired.


As you saw in Chapter 5, Java does have the equivalent of function pointers, namely, Method objects. However, they are difficult to use, slower, and cannot be checked for type safety at compile time. Whenever you would use a function pointer in C++, you should consider using an interface in Java.

Suppose you want to print a message "At the tone, the time is . . .," followed by a beep, once every ten seconds. You need to define a class that implements the ActionListener interface. Then place whatever statements you want to have executed inside the actionPerformed method.

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

Note the ActionEvent parameter of the actionPerformed method. This parameter gives information about the event, such as the source object that generated it—see Chapter 8 for more information. However, detail information about the event is not important in this program, and you can safely ignore the parameter.

Next, you construct an object of this class and pass it to the Timer constructor.

ActionListener listener = new TimePrinter();
Timer t = new Timer(10000, listener);

The first parameter of the Timer constructor is the time interval that must elapse between notifications, measured in milliseconds. We want to be notified every ten seconds. The second parameter is the listener object.

Finally, you start the timer.


Every ten seconds, a message like

At the tone, the time is Thu Apr 13 23:29:08 PDT 2000

is displayed, followed by a beep.

Example 6–2 puts the timer and its action listener to work. After the timer is started, the program puts up a message dialog and waits for the user to click the Ok button to stop. While the program waits for the user, the current time is displayed in ten second intervals.

Be patient when running the program. The "Quit program?" dialog box appears right away, but the first timer message is displayed after ten seconds.

Note that the program imports the javax.swing.Timer class by name, in addition to importing javax.swing.* and java.util.*. This breaks the ambiguity between javax.swing.Timer and java.util.Timer, an unrelated class for scheduling background tasks.

Example 6–2: TimerTest.java

 1.   import java.awt.*;
 2.   import java.awt.event.*;
 3.   import java.util.*;
 4.   import javax.swing.*;
 5.   import javax.swing.Timer; 
 6.   // to resolve conflict with java.util.Timer
 8.   public class TimerTest
 9.   {  
10.      public static void main(String[] args)
11.      {  
12.         ActionListener listener = new TimePrinter();
14.         // construct a timer that calls the listener
15.         // once every 10 seconds
16.         Timer t = new Timer(10000, listener);
17.         t.start();
19.         JOptionPane.showMessageDialog(null, "Quit program?");
20.         System.exit(0);
21.      }
22.   }
24.   class TimePrinter implements ActionListener
25.   {  
26.      public void actionPerformed(ActionEvent event)
27.      {  
28.         Date now = new Date();
29.         System.out.println("At the tone, the time is " + now);
30.         Toolkit.getDefaultToolkit().beep();
31.      }
32.   }

javax.swing.JOptionPane 1.2

  • static void showMessageDialog(Component parent, Object message)
    displays a dialog box with a message prompt and an Ok button. The dialog is centered over the parent component. If parent is null, the dialog is centered on the screen.

javax.swing.Timer 1.2

  • Timer(int interval, ActionListener listener)
    constructs a timer that notifies listener whenever interval milliseconds have elapsed.

  • void start()
    starts the timer. Once started, the timer calls actionPerformed on its listeners.

  • void stop()
    stops the timer. Once stopped, the timer no longer calls actionPerformed on its listeners

javax.awt.Toolkit 1.0

  • static Toolkit getDefaultToolkit()
    gets the default toolkit. A toolkit contains information about the graphical user interface environment.

  • void beep()
    Emits a beep sound.

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