visual basic barcode program Determining Access to Class Members in Java

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Determining Access to Class Members
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Visibility From the same class From any class in the same package From any non-subclass class outside the package From a subclass in the same package From a subclass outside the same package
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Nonaccess Member Modifiers
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We ve discussed member access, which refers to whether or not code from one class can invoke a method (or access an instance variable) from another class. That still leaves a boatload of other modifiers you can use on member declarations. Two you re already familiar with final and abstract because we applied them to class declarations earlier in this chapter. But we still have to take a quick look at transient, synchronized, native, strictfp, and then a long look at the Big One static. We ll look first at modifiers applied to methods, followed by a look at modifiers applied to instance variables. We ll wrap up this objective with a look at how static works when applied to variables and methods. Final Methods The final keyword prevents a method from being overridden in a subclass, and is often used to enforce the API functionality of a method. For example, the Thread class has a method called isAlive() that checks whether a thread is still active. If you extend the Thread class, though, there is really no way that you can correctly implement this method yourself (it uses native code, for one thing), so the designers have made it final. Just as you can t subclass the String class (because we need to be able to trust in the behavior of a String object), you can t override many of the methods in the core class libraries. This can t-be-overridden restriction provides for safety and security, but you should use it with great caution. Preventing a subclass from overriding a method stifles many of the benefits of OO including extensibility through polymorphism.
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2: Declarations and Access Control
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A typical final method declaration looks like this:
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class SuperClass{ public final void showSample() { System.out.println("One thing."); } }
It s legal to extend SuperClass, since the class itself isn t marked final, but we can t override the final method showSample(), as the following code attempts to do:
class SubClass extends SuperClass{ public void showSample() { // Try to override the final superclass method System.out.println("Another thing."); } }
Attempting to compile the preceding code gives us the following:
%javac FinalTest.java FinalTest.java:5: The method void showSample() declared in class SubClass cannot override the final method of the same signature declared in class SuperClass. Final methods cannot be overridden. public void showSample() { } 1 error
Final Arguments Method arguments are the variable declarations that appear in between the parentheses in a method declaration. A typical method declaration with multiple arguments looks like this:
public Record getRecord(int fileNumber, int recordNumber) {}
Method arguments are essentially the same as local variables. In the preceding example, the variables fileNumber and recordNumber will both follow all the rules applied to local variables. This means they can also have the modifier final:
public Record getRecord(int fileNumber, final int recordNumber) {}
In this example, the variable recordNumber is declared as final, which of course means it can t be modified within the method. In this case, modified means reassigning a new value to the variable. In other words, a final argument must keep the same value that the parameter had when it was passed into the method.
Declarations and Modifiers (Exam Objective 1.2)
Abstract Methods An abstract method is a method that s been declared (as abstract) but not implemented. In other words, the method contains no functional code. And if you recall from the previous section on abstract classes, an abstract method declaration doesn t even have curly braces for where the implementation code goes, but instead closes with a semicolon. You mark a method abstract when you want to force subclasses to provide the implementation. For example, if you write an abstract class Car with a method goUpHill(), you might want to force each subtype of car to define its own goUpHill() behavior, specific to that particular type of car. (If you ve ever lived in the Rockies, you know that the differences in how cars go uphill (or fail to) is not, um, subtle.) A typical abstract method declaration is as follows:
public abstract void showSample();
Notice that the abstract method ends with a semicolon instead of curly braces. It is illegal to have an abstract method in a class that is not declared abstract. Look at the following illegal class:
public class IllegalClass{ public abstract void doIt(); }
The preceding class will produce the following error if you try to compile it:
%javac IllegalClass.java IllegalClass.java:1: class IllegalClass must be declared abstract. It does not define void doIt() from class IllegalClass. public class IllegalClass{ 1 error
You can, however, have an abstract class with no abstract methods. The following example will compile fine:
public abstract class LegalClass{ void goodMethod() { // lots of real implementation code here } }
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