Java8变量范围

Question

I took mock exam test as i am preparing for OCAJP exam and i came across this below question on Variables and Scope of Variables.

public class HelloWorld{
     static int x = 2;            
     public static void main(String []args){
        if(x>1)
        {
            x++;
            int x = 4;
        }
        System.out.println(x);
        final int x = 10;
     }
}

and the output for the above code is "3". But i am unable to figure out why the output is 3. I can understand that the "int x=4" within the if block will be seen outside the IF block. But should not "final int x = 10;" throw compiler off-track as there is already x as a static variable?

Answer 1

Let's take this in code order.

static int x = 2;

This declares a static class variable named x that is initialized to 2 .

if(x>1)

This refers to the static class variable, because the other declarations of x haven't occurred yet.

    x++;

This increments the static class variable x to 3 .

    int x = 4;

This declares a new local variable x , distinct from the static class variable x , and initializes it to 4 . This new local variable shadows the static class variable. However, it immediately goes out of scope; its scope is limited to the if block. It is not referenced after declaration and before it goes out of scope.

System.out.println(x);

This prints the only x in scope, the static class variable, which is 3 . The local x declared above is out of scope and no longer shadows the static class variable.

final int x = 10;

This declare another new local variable x , also distinct from the static class variable x and distinct from the already out of scope x previously declared in the if block, and initializes it to 10 . This new local variable shadows the static class variable. However, it also immediately goes out of scope; its scope is limited to the main method block. It is also not referenced after declaration and before it goes out of scope.

The main points are:

• A local variable can shadow a class variable of the same name. However, it only does so in its own local scope.
• Variables not yet declared do not shadow the class variable yet.