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Interfaces, Lambda Expressions, and Inner Classes in Java

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This chapter shows you several advanced techniques that are commonly used in Java. Despite their less obvious nature, you will need to master them to complete your Java tool chest.
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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 move on to lambda expressions, a concise way for expressing a block of code that can be executed at a later point in time. Using lambda expressions, you can express code that uses callbacks or variable behavior in an elegant and concise fashion.

We then discuss 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.

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 following sections, you will learn what Java interfaces are and how to use them. You will also find out how interfaces have been made more powerful in Java SE 8.

6.1.1 The Interface Concept

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. Before Java SE 8, methods were never implemented in interfaces. (As you will see in Section 6.1.4, “Static Methods,” on p. 298 and Section 6.1.5, “Default Methods,” on p. 298, it is now possible to supply simple methods in interfaces. Of course, those methods cannot refer to instance fields—interfaces don’t have any.)

Supplying instance fields and methods that operate on them 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.

We can do a little better by supplying a type parameter for the generic Comparable interface:

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;
 3   import java.util.*;
 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];
16         staff[0] = new Employee("Harry Hacker", 35000);
17         staff[1] = new Employee("Carl Cracker", 75000);
18         staff[2] = new Employee("Tony Tester", 38000);
20         Arrays.sort(staff);
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;
 3   public class Employee implements Comparable<Employee>
 4   {
 5      private String name;
 6      private double salary;
 8      public Employee(String name, double salary)
 9      {
10        this.name = name;
11        this.salary = salary;
12      }
14      public String getName()
15      {
16         return name;
17      }
19      public double getSalary()
20      {
21         return salary;
22      }
24      public void raiseSalary(double byPercent)
25      {
26         double raise = salary * byPercent / 100;
27         salary += raise;
28      }
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   }

6.1.2 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 Section 6.2.3, “Object Cloning,” on p. 306.) If your class implements Cloneable, the clone method in the Object class will make an exact copy of your class’s objects. If you want both cloneability and comparability, simply implement both interfaces. Use commas to separate the interfaces that you want to implement:

class Employee implements Cloneable, Comparable

6.1.3 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 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.

6.1.4 Static Methods

As of Java SE 8, you are allowed to add static methods to interfaces. There was never a technical reason why this should be outlawed. It simply seemed to be against the spirit of interfaces as abstract specifications.

Up to now, it has been common to place static methods in companion classes. In the standard library, you find pairs of interfaces and utility classes such as Collection/Collections or Path/Paths.

Have a look at the Paths class. It only has a couple of factory methods. You can construct a path to a file or directory from a sequence of strings, such as Paths.get("jdk1.8.0", "jre", "bin"). In Java SE 8, one could have added this method to the Path interface:

public interface Path
   public static Path get(String first, String... more) {
      return FileSystems.getDefault().getPath(first, more);

Then the Paths class is no longer necessary.

It is unlikely that the Java library will be refactored in this way, but when you implement your own interfaces, there is no longer a reason to provide a separate companion class for utility methods.

6.1.5 Default Methods

You can supply a default implementation for any interface method. You must tag such a method with the default modifier.

public interface Comparable<T>
   default int compareTo(T other) { return 0; }
       // By default, all elements are the same

Of course, that is not very useful since every realistic implementation of Comparable would override this method. But there are other situations where default methods can be useful. For example, as you will see in Chapter 11, if you want to be notified when a mouse click happens, you are supposed to implement an interface that has five methods:

public interface MouseListener
   void mouseClicked(MouseEvent event);
   void mousePressed(MouseEvent event);
   void mouseReleased(MouseEvent event);
   void mouseEntered(MouseEvent event);
   void mouseExited(MouseEvent event);

Most of the time, you only care about one or two of these event types. As of Java SE 8, you can declare all of the methods as default methods that do nothing.

public interface MouseListener
   default void mouseClicked(MouseEvent event) {}
   default void mousePressed(MouseEvent event) {}
   default void mouseReleased(MouseEvent event) {}
   default void mouseEntered(MouseEvent event) {}
   default void mouseExited(MouseEvent event) {}

Then programmers who implement this interface only need to override the listeners for the events they actually care about.

A default method can call other methods. For example, a Collection interface can define a convenience method

public interface Collection
   int size(); // An abstract method
   default boolean isEmpty()
        return size() == 0;

Then a programmer implementing Collection doesn’t have to worry about implementing an isEmpty method.

An important use for default methods is interface evolution. Consider for example the Collection interface that has been a part of Java for many years. Suppose that a long time ago, you provided a class

public class Bag implements Collection

Later, in Java SE 8, a stream method was added to the interface.

Suppose the stream method was not a default method. Then the Bag class no longer compiles since it doesn’t implement the new method. Adding a nondefault method to an interface is not source compatible.

But suppose you don’t recompile the class and simply use an old JAR file containing it. The class will still load, even with the missing method. Programs can still construct Bag instances, and nothing bad will happen. (Adding a method to an interface is binary compatible.) However, if a program calls the stream method on a Bag instance, an AbstractMethodError occurs.

Making the method a default method solves both problems. The Bag class will again compile. And if the class is loaded without being recompiled and the stream method is invoked on a Bag instance, the Collection.stream method is called.

6.1.6 Resolving Default Method Conflicts

What happens if the exact same method is defined as a default method in one interface and then again as a method of a superclass or another interface? Languages such as Scala and C++ have complex rules for resolving such ambiguities. Fortunately, the rules in Java are much simpler. Here they are:

  1. Superclasses win. If a superclass provides a concrete method, default methods with the same name and parameter types are simply ignored.
  2. Interfaces clash. If a superinterface provides a default method, and another interface supplies a method with the same name and parameter types (default or not), then you must resolve the conflict by overriding that method.

Let’s look at the second rule. Consider another interface with a getName method:

interface Named
   default String getName() { return getClass().getName() + "_" + hashCode(); }

What happens if you form a class that implements both of them?

class Student implements Person, Named

The class inherits two inconsistent getName methods provided by the Person and Named interfaces. Instead of choosing one over the other, the Java compiler reports an error and leaves it up to the programmer to resolve the ambiguity. Simply provide a getName method in the Student class. In that method, you can choose one of the two conflicting methods, like this:

class Student implements Person, Named
   public String getName() { return Person.super.getName(); }

Now assume that the Named interface does not provide a default implementation for getName:

interface Named
   String getName();

Can the Student class inherit the default method from the Person interface? This might be reasonable, but the Java designers decided in favor of uniformity. It doesn’t matter how two interfaces conflict. If at least one interface provides an implementation, the compiler reports an error, and the programmer must resolve the ambiguity.

We just discussed name clashes between two interfaces. Now consider a class that extends a superclass and implements an interface, inheriting the same method from both. For example, suppose that Person is a class and Student is defined as

class Student extends Person implements Named { ... }

In that case, only the superclass method matters, and any default method from the interface is simply ignored. In our example, Student inherits the getName method from Person, and it doesn’t make any difference whether the Named interface provides a default for getName or not. This is the “class wins” rule.

The “class wins” rule ensures compatibility with Java SE 7. If you add default methods to an interface, it has no effect on code that worked before there were default methods.

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