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Interface Implementation (Exam Objective 4.2)
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Identify classes that correctly implement an interface where that interface is either java.lang.Runnable or a fully specified interface in the question. So far in this chapter, we began with Objective 1.2 a look at how to use modifiers in class, method, and variable declarations. Next, for Objective 4.1, we covered the rules for structuring a source code file and declaring the main() method. In this objective, we ll focus on interface declarations and implementations.
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You must know how to implement the java.lang.Runnable interface, without being shown the code in the question. In other words, you might be asked to choose from a list of six classes which one provides a correct implementation of Runnable . Be sure you memorize the signature of the one and only one Runnable interface method:
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public void run() { }
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For any other interface-related question not dealing with Runnable , if the specification of the interface matters, the interface code will appear in the question. A question, for example, might show you a complete interface and a complete class, and ask you to choose whether or not the class correctly implements the interface. But if the question is about Runnable , you won t be shown the interface. You re expected to have Runnable memorized!
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Declaring an Interface
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When you create an interface, you re defining a contract for what a class can do, without saying anything about how the class will do it. An interface is a contract. You could write an interface Bounceable, for example, that says in effect, This is the Bounceable interface. Any class type that implements this interface must agree to write the code for the bounce() and setBounceFactor() methods.
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2: Declarations and Access Control
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By defining an interface for Bounceable, any class that wants to be treated as a Bounceable thing can simply implement the Bounceable interface and provide code for the interface s two methods. Interfaces can be implemented by any class, from any inheritance tree. This lets you take radically different classes and give them a common characteristic. For example, you might want both a Ball and a Tire to have bounce behavior, but Ball and Tire don t share any inheritance relationship; Ball extends Toy while Tire extends only java.lang.Object. But by making both Ball and Tire implement Bounceable, you re saying that Ball and Tire can be treated as, Things that can bounce, which in Java translates to Things on which you can invoke the bounce() and setBounceFactor() methods. Figure 2-8 illustrates the relationship between interfaces and classes. Think of an interface as a 100-percent abstract class. Like an abstract class, an interface defines abstract methods that take the form,
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abstract void bounce(); // Ends with a semicolon rather than curly braces
But while an abstract class can define both abstract and nonabstract methods, an interface can have only abstract methods. Another place interfaces differ from
FIGURE 2-8
The relationship between interfaces and classes
Interface Implementation (Exam Objective 4.2)
abstract classes is that interfaces have very little flexibility in how the methods and variables defined in the interface are declared. The rules are strict, but simple:
All interface methods are implicitly public and abstract. Interface methods must not be static. You do not need to actually type the public or abstract modifiers in
the method declaration, but the method is still always public and abstract.
All variables defined in an interface must be public, static, and final in
other words, interfaces can declare only constants, not instance variables.
Because interface methods are abstract, they cannot be marked final,
native, strictfp, or synchronized.
An interface can extend one or more other interfaces. An interface cannot extend anything but another interface. An interface cannot implement another interface or class. An interface must be declared with the keyword interface. Interface types can be used polymorphically (see 5 for more details).
The following is a legal interface declaration:
public abstract interface Rollable { }
Typing in the abstract modifier is considered redundant; interfaces are implicitly abstract whether you type abstract or not. You just need to know that both of these declarations are legal, and functionally identical:
public abstract interface Rollable { } public interface Rollable { }
The public modifier is required if you want the interface to have public rather than default access. We ve looked at the interface declaration but now we ll look closely at the methods within an interface:
public interface Bounceable { public abstract void bounce(); public abstract void setBounceFactor(int bf); }
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