Scope - Java Tutorial 5.0 英文版

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Scope

A variable's scope is the region of a program within which the variable can be referred to by its simple name. Secondarily, scope also determines when the system creates and destroys memory for the variable. Scope is distinct from visibility, which applies only to member variables and determines whether the variable can be used from outside of the class within which it is declared. Visibility is set with an access modifier.

The location of the variable declaration within your program establishes its scope and places it into one of these four categories:

• member variable
• local variable
• method parameter
• exception-handler parameter

A member variable is a member of a class or an object. It is declared within a class but outside of any method or constructor. A member variable's scope is the entire declaration of the class. However, the declaration of a member needs to appear before it is used when the use is in a member initialization expression. For information about declaring member variables, refer to Declaring Member Variables in the next lesson.

You declare local variables within a block of code. In general, the scope of a local variable extends from its declaration to the end of the code block in which it was declared. In MaxVariablesDemo, all of the variables declared within the main method are local variables. The scope of each variable in that program extends from the declaration of the variable to the end of the main method --indicated by the first right curly bracket } in the program code.

Parameters are formal arguments to methods or constructors and are used to pass values into methods and constructors. The scope of a parameter is the entire method or constructor for which it is a parameter.

Exception-handler parameters are similar to parameters but are arguments to an exception handler rather than to a method or a constructor. The scope of an exception-handler parameter is the code block between { and } that follow a catch statement. Handling Errors with Exceptions talks about using exceptions to handle errors and shows you how to write an exception handler that has a parameter.

Consider the following code sample:

if (...) {
    int i = 17;
    ...
}
System.out.println("The value of i = " + i);    // error

The final line won't compile because the local variable i is out of scope. The scope of i is the block of code between the { and }. The i variable does not exist anymore after the closing }. Either the variable declaration needs to be moved outside of the if statement block, or the println method call needs to be moved into the if statement block.

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