visual basic barcode program Provide concrete (nonabstract) implementations for all methods from the in Java

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Provide concrete (nonabstract) implementations for all methods from the
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declared interface.
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Follow all the rules for legal overrides (see 5 for details). Declare no checked exceptions on implementation methods other than
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those declared by the interface method, or subclasses of those declared by the interface method.
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Maintain the signature of the interface method, and maintain the same
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return type (but does not have to declare the exceptions declared in the interface method declaration). But wait, there s more! An implementation class can itself be abstract! For example, the following is legal for a class Ball implementing the Bounceable interface:
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abstract class Ball implements Bounceable { }
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Interface Implementation (Exam Objective 4.2)
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Notice anything missing We never provided the implementation methods. And that s OK. If the implementation class is abstract, it can simply pass the buck to its first concrete subclass. For example, if class BeachBall extends Ball, and BeachBall is not abstract, then BeachBall will have to provide all the methods from Bounceable:
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class BeachBall extends Ball { // Even though we don't say it in the class declaration above, //BeachBall implements Bounceable, since BeachBall's abstract //superclass (Ball) implements Bounceable public void bounce() { // interesting BeachBall-specific bounce code } public void setBounceFactor(int bf) { // clever BeachBall-specific code for setting a bounce factor } // if class Ball defined any abstract methods, they'll have to be // implemented here as well. }
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Look for methods that claim to implement an interface but don t provide the correct method implementations. Unless the implementing class is abstract, the implementing class must provide implementations for all methods defined in the interface.
Two more rules you need to know and then we can put this topic to sleep (or put you to sleep; we always get those two confused):
1. A class can extend more than one interface.
It s perfectly legal to say, for example, the following:
public class Ball implements Bounceable, Serializable, Runnable { }
You can extend only one class, but implement many. But remember that subclassing defines who and what you are, whereas implementing defines a role you can play or a hat you can wear, despite how different you might be from some other class implementing the same interface (but from a different inheritance tree). For example, a person extends HumanBeing (although for some, that s debatable). But a person may also implement programmer, snowboarder, employee, parent, or personcrazyenoughtotakethisexam.
2: Declarations and Access Control
2. An interface can itself extend another interface, but never implement anything.
The following code is perfectly legal:
public interface Bounceable extends Moveable { }
What does that mean The first concrete (nonabstract) implementation class of Bounceable must implement all the methods of Bounceable, plus all the methods of Moveable! The subinterface, as we call it, simply adds more requirements to the contract of the superinterface. You ll see this concept applied in many areas of Java, especially J2EE where you ll often have to build your own interface that extends one of the J2EE interfaces. Hold on though, because here s where it gets strange. An interface can extend more than one interface! Think about that for a moment. You know that when we re talking about classes, the following is illegal:
public class Programmer extends Employee, Geek { } // Illegal!
A class is not allowed to extend multiple classes in Java. It that were allowed, it would be multiple inheritance, a potential nightmare in some scenarios (more on that in 5). An interface, however, is free to extend multiple interfaces.
interface Bounceable extends Moveable, Spherical { void bounce(); void setBounceFactor(int bf); } interface Moveable { void moveIt(); } interface Spherical { void doSphericalThing(); }
Ball is required to implement Bounceable, plus all methods from the interfaces that Bounceable extends (including any interfaces those interfaces extend and so on
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until you reach the top of the stack or is it bottom of the stack well, you know what we mean). So Ball would need to look like the following:
class Ball implements Bounceable { // Implement the methods from Bounceable public void bounce() { } public void setBounceFactor(int bf) { } // Implement the methods from Moveable public void moveIt() { } // Implement the methods from Spherical public void doSphericalThing() { } }
If class Ball fails to implement any of the methods from Bounceable, Moveable, or Spherical, the compiler will jump up and down wildly, red in the face, until it does. Unless, that is, class Ball is marked abstract. In that case, Ball could choose to implement any, all, or none of the methods from any of the interfaces, thus leaving the rest of the implementations to a concrete subclass of Ball, as follows:
abstract class Ball implements Bounceable { public void bounce() { } // Define bounce behavior public void setBounceFactor(int bf) { } // Don't implement the rest; leave it for a subclass } class SoccerBall extends Ball { // class SoccerBall must implement the interface methods that Ball didn't public void moveIt() { } public void doSphericalThing() { } // SoccerBall can choose to override the Bounceable methods // implemented by Ball public void bounce() { } }
Figure 2-9 compares the legal and illegal use of extends and implements, for both classes and interfaces.
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