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You need to remember a few rules for interface constants. They must always be
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public static final
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So that sounds simple, right After all, interface constants are no different from any other publicly accessible constants, so they obviously must be declared public, static, and final. But before you breeze past the rest of this discussion, think about the implications. First, because interface constants are defined in an interface, they don t have to be declared as public, static, or final. They must be public, static, and final, but you don t have to actually declare them that way. Just as interface methods are always public and abstract whether you say so in the code or not, any variable defined in an interface must be and implicitly is a public constant. See if you can spot the problem with the following implementation of Bounceable:
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class Check implements Bounceable { // Implementation code goes here public void adjustGravityFactors(int x) { if (x > LOW_GRAVITY) { LOW_GRAVITY = x; MEDIUM_GRAVITY = x + 1; HIGH-GRAVITY = x + 2; } } }
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You can t change the value of a constant! Once the value has been assigned, the value can never be modified. The assignment happens in the interface itself (where the constant is declared), so the implementing class can access it and use it, but as a read-only value.
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Look for interface definitions that define constants, but without explicitly using the required modifiers. For example, the following are all identical:
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public int x = 1; // Looks non-static and non-final, but isn t! int x = 1; // Looks default, non-final, and non-static, but isn t! static int x = 1; // Doesn t show final or public final int x = 1; // Doesn t show static or public public static int x = 1; // Doesn t show final public final int x = 1; // Doesn t show static static final int x = 1 // Doesn t show public public static final int x = 1; // Exactly what you get implicitly
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Any combination of the required (but implicit) modifiers is legal, as is using no modifiers at all! On the exam, you can expect to see questions you won t be able to answer correctly unless you know, for example, that an interface variable is final and can never be given a value by the implementing (or any other) class.
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Implementing an Interface
When you implement an interface, you re agreeing to adhere to the contract defined in the interface. That means you re agreeing to provide legal implementations for every method defined in the interface, and that anyone who knows what the interface methods look like (not how they re implemented, but how they can be called and what they return) can rest assured that they can invoke those methods on an instance of your implementing class. For example, if you create a class that implements the Runnable interface (so that your code can be executed by a specific thread), you must provide the public void run() method. Otherwise, the poor thread could be told to go execute your Runnable object s code and surprise surprise the thread then discovers the object has no run() method! (At which point, the thread would blow up and the JVM would crash in a spectacular yet horrible explosion.) Thankfully, Java prevents this meltdown from occurring by running a compiler check on any class that claims to implement an interface. If the class says it s implementing an interface, it darn well
2: Declarations and Access Control
better have an implementation for each method in the interface (with a few exceptions we ll look at in a moment). We looked earlier at several examples of implementation classes, including the Ball class that implements Bounceable, but the following class would also compile legally:
public class Ball implements Bounceable { // Keyword 'implements' public void bounce() { } public void setBounceFactor(int bf) { } }
OK, we know what you re thinking: This has got to be the worst implementation class in the history of implementation classes. It compiles, though. And runs. The interface contract guarantees that a class will have the method (in other words, others can call the method subject to access control), but it never guaranteed a good implementation or even any actual implementation code in the body of the method. The compiler will never say to you, Um, excuse me, but did you really mean to put nothing between those curly braces HELLO. This is a method after all, so shouldn t it do something Implementation classes must adhere to the same rules for method implementation as a class extending an abstract class. In order to be a legal implementation class, a nonabstract implementation class must do the following:
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