visual basic barcode program 2: Declarations and Access Control in Java

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2: Declarations and Access Control
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Typing in the public and abstract modifiers on the methods is redundant, though, since all interface methods are implicitly public and abstract. Given that rule, you can see that the following code is exactly equivalent to the preceding interface:
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public interface Bounceable { void bounce(); // No modifiers void setBounceFactor(int bf); // No modifiers }
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You must remember that all interface methods are public and abstract regardless of what you see in the interface definition.
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Look for interface methods declared with any combination of public, abstract, or no modifiers. For example, the following five method declarations, if declared within an interface, are legal and identical!
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void bounce(); public void bounce(); abstract void bounce(); public abstract void bounce(); abstract public void bounce();
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The following interface method declarations won t compile:
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final void bounce(); // final and abstract can never be used static void bounce(); // interfaces define instance methods private void bounce(); // interface methods are always public protected void bounce(); // (same as above) synchronized void bounce(); // can t mix abstract and synchronized native void bounce(); // can t mix abstract and native strictfp void bounce(); // can t mix abstract and strictfp
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Declaring Interface Constants
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You re allowed to put constants in an interface. By doing so, you guarantee that any class implementing the interface will have access to the same constant. Imagine that a Bounceable thing works by using int values to represent gravity where the Bounceable thing is, its degree of bounciness (bounce-osity ), and so on. Now imagine that for a Bounceable thing, gravity is set such that a 1 is low, 2 is medium, 3 is high, and for bounciness, 4 is a little bouncy, 8 is very bouncy, and 12 is
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Interface Implementation (Exam Objective 4.2)
extremely bouncy. Those numbers are tough to remember when you re trying to decide how to set the values ( let s see, was it 8 for high gravity and 3 for medium bounce Or was it the other way around ). Now let s say that you (the developer of Bounceable) decide that it would be much easier for programmers to remember names like HIGH_GRAVITY, LOW_BOUNCE, and HIGH_BOUNCE as opposed to knowing the exact int values corresponding to each of those. So, you know you want to define some constants so the programmer can just use the constant name rather than the int value. You need something like the following:
public final static int LOW_BOUNCE = 4; public final static int HIGH_GRAVITY = 3; // and so on
That way, if a method takes the int values,
public void animateIt(int gravity, int bounceFactor) { }
then the code that calls animateIt() can substitute the constants wherever the int values are expected, as follows:
animator.animateIt(LOW_GRAVITY, HIGH_BOUNCE);
So we ve made a case for using constants with easy-to-remember names (as opposed to using nearly arbitrary numbers), but where do you put these constants so that all Bounceable things (things as in things that implement the Bounceable interface ) can substitute the int constant name everywhere one of the int values is needed You could define them in some companion class called, for example, BounceableConstants. But why not just put them in the Bounceable interface That way you can guarantee that all Bounceable things will always have access to the constants, without having to create another class. Look at the changes we ve made to the Bounceable interface:
public int int int int int int interface Bounceable { LOW_GRAVITY = 1; MEDIUM_GRAVITY = 2; HIGH_GRAVITY = 3; LOW_BOUNCE = 4; MEDIUM_BOUNCE = 8; HIGH_BOUNCE = 12;
void bounce();
2: Declarations and Access Control
void setBounceFactor(int bounceFactor); void setGravity(int gravity); }
By placing the constants right in the interface, any class that implements the interface has direct access to the constants, just as if the class had inherited them. For example, the following would be legal for a Bounceable implementation class:
class Ball implements Bounceable { // Lots of exciting code goes here public void bounce() { animator.animateIt(LOW_GRAVITY, HIGH_BOUNCE); } // Still more action-packed code goes here }
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