Working with Interfaces and Inner Classes in Java

This chapter is from the book 

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

The first technique, called interfaces, 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 whenever 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. In particular, you can modify the clone without affecting the original.

Next, 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 enable you to write concise, professional looking code to handle GUI 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.

6.1. Interfaces

In the Java programming language, an interface is not a class but a set of requirements for the 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 the 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 multiple methods. 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 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 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 an implementation of the compareTo method:

public int compareTo(Object otherObject)
{
   Employee other = (Employee) otherObject;
    return Double.compare(salary, other.salary);
}

Here, we use the static Double.compare method that returns a negative if the first argument is less than the second argument, 0 if they are equal, and a positive value otherwise.

As of Java SE 5.0, we can do a little better. We’ll implement the Comparable<Employee> interface type instead.

class Employee implements Comparable<Employee>
{
   public int compareTo(Employee other)
   {
      return Double.compare(salary, other.salary);
   }
   . . .
}

Note that the unsightly cast of the Object parameter has gone away.

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 programming 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 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.

Listing 6.1 presents the full code for sorting an array of instances of the class Employee (Listing 6.2). for sorting an employee array.

Listing 6.1. interfaces/EmployeeSortTest.java

 1  package interfaces;
 2
 3  import java.util.*;
 4
 5  /**
 6   * This program demonstrates the use of the Comparable interface.
 7   * @version 1.30 2004-02-27
 8   * @author Cay Horstmann
 9   */
10  public class EmployeeSortTest
11  {
12     public static void main(String[] args)
13     {
14        Employee[] staff = new Employee[3];
15
16        staff[0] = new Employee("Harry Hacker", 35000);
17        staff[1] = new Employee("Carl Cracker", 75000);
18        staff[2] = new Employee("Tony Tester", 38000);
19
20        Arrays.sort(staff);
21
22        // print out information about all Employee objects
23        for (Employee e : staff)
24           System.out.println("name=" + e.getName() + ",salary=" + e.getSalary());
25     }
26  }

Listing 6.2. interfaces/Employee.java

 1  package interfaces;
 2
 3  public class Employee implements Comparable<Employee>
 4  {
 5     private String name;
 6     private double salary;
 7
 8     public Employee(String n, double s)
 9     {
10        name = n;
11        salary = s;
12     }
13
14     public String getName()
15     {
16        return name;
17     }
18
19     public double getSalary()
20     {
21        return salary;
22     }
23
24     public void raiseSalary(double byPercent)
25     {
26        double raise = salary * byPercent / 100;
27        salary += raise;
28     }
29
30     /**
31      * Compares employees by salary
32      * @param other another Employee object
33      * @return a negative value if this employee has a lower salary than
34      * otherObject, 0 if the salaries are the same, a positive value otherwise
35      */
36     public int compareTo(Employee other)
37     {
38        return Double.compare(salary, other.salary);
39     }
40  }

6.1.1. 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 interface 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 whether an object is of a specific class, you can use instanceof to check whether 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.

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. However, this use of interfaces seems rather degenerate, and we do not recommend it.

While each class can have only 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.

6.1.2. 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);
}

The Employee class would then 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 afford most of the benefits of multiple inheritance while avoiding the complexities and inefficiencies.