| 1. 10. 1. 变量范围 |
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The scope of a variable is the part of the program over which the variable name can be referenced.
(from Ivor Horton's Beginning Java 2, JDK 5 Edition by Ivor Horton) |
You cannot refer to a variable before its declaration. |
You can declare variables in several different places: |
- In a class body as class fields. Variables declared here are referred to as class-level variables.
- As parameters of a method or constructor.
- In a method's body or a constructor's body.
- Within a statement block, such as inside a while or for block.
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Variable scope refers to the accessibility of a variable. |
The rule 1 is that variables defined in a block are only accessible from within the block.
The scope of the variable is the block in which it is defined.
For example, consider the following for statement. |
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public class MainClass {
public static void main(String[] args) {
for (int x = 0; x < 5; x++) {
System.out.println(x);
}
}
}
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Rule number 2 is a nested block can access variables declared in the outer block.
Consider this code. |
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public class MainClass {
public static void main(String[] args) {
for (int x = 0; x < 5; x++) {
for (int y = 0; y < 3; y++) {
System.out.println(x);
System.out.println(y);
}
}
}
}
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Variables declared as method parameters can be accessed from within the method body.
Class-level variables are accessible from anywhere in the class. |
If a method declares a local variable that has the same name as a class-level variable,
the former will 'shadow' the latter.
To access the class-level variable from inside the method body, use the this keyword. |