7.9 Ap0090: self-assessment, the super keyword, final keyword, and  (Page 6/7)

The constant named PI

You may recognize the constant named PI from your high school geometry class.

Whenever you need the value for the constant PI , you shouldn't have to instantiate an object just to get access to it. Furthermore, your class shouldnot be required to have any special package relationship with the Math class just to get access to PI .

The good news ...

Because PI is declared to be both public and static in the Math class, it is readily available to any code in any method in any Java program that has access to the standard Java class library.

How is PI accessed?

PI can be accessed by using an expression as simple as that shown below, which consists simply of the name of the class and the name ofthe variable joined by a period (Math.PI) .

No notion of public local variables

As a result of the above, many of you may have become accustomed to associating the keyword public with the keyword final . However, if you missed this question and you have read the explanation to thispoint, you must also remember that there is no notion of public or private for local variables. Therefore, when this program was compiled under JDK 1.3, a compiler error was produced. That compiler error is partiallyreproduced below:

Back to Question 4

Answer 3

A. Compiler Error

Explanation 3

The final keyword

The final keyword can be applied in a variety of ways in Java. This includes:

• final parameters
• final methods
• final classes
• final variables (constants)

Behaves like a constant

When the final keyword is applied to a variable in Java, that causes the variable to behave like a constant. In other words, the value of the variablemust be initialized when it is declared, and it cannot be changed thereafter (see the exception discussed below) .

Apply to local or member variables

The final keyword can be applied to either local variables or member variables. (In case you have forgotten, local variables are declared inside a method or constructor, while member variables are declared inside a class, but outside amethod.)

So, what is the problem?

The problem with this program is straightforward. As shown in the following code fragment, after declaring a final local variable and initializing its value to 5, the program attempts to change the value stored in that variable to 10.This is not allowed.

A compiler error

JDK 1.3 produces the following error message:

An interesting twist - blank finals

An interesting twist of the use of the final keyword with local variables is discussed below.

Background information

Regardless of whether or not the local variable is declared final , the compiler will not allow you to access the value in a local variable if thatvariable doesn't contain a value. This means that you must always either initialize a local variable or assign a value to it before you can access it.