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

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2: Declarations and Access Control
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wants to access the member variable x In other words, how does that protected member behave once the subclass has inherited it Does it maintain its protected status, such that classes in the Child s package can see it No! Once the subclass-outside-the-package inherits the protected member, that member (as inherited by the subclass) becomes private to any code outside the subclass. So if class Neighbor instantiates a Child object, then even if class Neighbor is in the same package as class Child, class Neighbor won t have access to the Child s inherited (but protected) variable x. The bottom line: when a subclass-outside-the-package inherits a protected member, the member is essentially private inside the subclass, such that only the subclass own code can access it. Figure 2-3 illustrates the effect of protected access on classes and subclasses in the same or different packages. Whew! That wraps up protected, the most misunderstood modifier in Java. Again, it s used only in very special cases, but you can count on it showing up on the exam. Now that we ve covered the protected modifier, we ll switch to default member access, a piece of cake compared to protected. Let s start with the default behavior of a member in a superclass. We ll modify the Parent s member x to make it default.
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package certification; public class Parent { int x = 9; // No access modifier, means default (package) access }
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Notice we didn t place an access modifier in front of the variable x. Remember that if you don t type an access modifier before a class or member declaration, the access control is default, which means package level. We ll now attempt to access the default member from the Child class that we saw earlier. When we compile the Child file, we get the following error:
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%javac Child.java Child.java:4: Undefined variable: x System.out.println("Variable x is " + x); 1 error
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The compiler gives the same error as when a member is declared as private. The subclass Child (in a different package from the superclass Parent) can t see or
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Declarations and Modifiers (Exam Objective 1.2)
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The effects of protected access
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use the default superclass member x ! Now, what about default access for two classes in the same package
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package certification; public class Parent{ int x = 9; // default access }
2: Declarations and Access Control
And in the second class you have the following:
package certification; class Child extends Parent{ static public void main(String [] args) { Parent sc = new Parent(); sc.testIt(); } public void testIt() { System.out.println("Variable x is " + x); // No problem; } }
The preceding source file compiles fine, and the class Child runs and displays the value of x. Just remember that default members are visible only to the subclasses that are in the same package as the superclass. Local Variables and Access Modifiers Can access modifiers be applied to local variables This one should be simple to remember: NO!
There is never a case where an access modifier can be applied to a local variable, so watch out for code like the following:
class Foo { void doStuff() { private int x = 7; this.doMore(x); } }
You can be certain that any local variable declared with an access modifier will not compile. In fact, there is only one modifier that can ever be applied to local variables final.
That about does it for our discussion on member access modifiers. Table 2-1 shows all the combinations of access and visibility; you really should spend some time with it. Next, we re going to dig into the other (nonaccess) modifiers that you can apply to member declarations.
Declarations and Modifiers (Exam Objective 1.2)
TABLE 2-1
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