Class scope variable vs method scope variable

Question

I know that variable scope is enclosed by a start of block { and an end of block }. If the same variable is declared within the block, then the compile error Variable already defined occurs. But take a look at following example.

public class Test{
int x=0;// Class scope variable

public void m(){
  int  x=9;   //redeclaration of x is valid within the scope of same x. 

  if(true){
      int x=7; // but this redeclaration generates a compile time error.
  }

}

Here, x can be redeclared in a method, although it's already declared in the class. But in the if block, x can't be redeclared.

Why is it that redeclaration of a class scope variable doesn't generate an error, but a method scope variable redeclaration generates an error?

Evgeniy Dorofeev · Accepted Answer · 2013-10-20 02:49:52Z

2

This is because int x=0 is not a variable but an instance field. Local variables are allowed to have the same names as fields. To distinguish between a variable and a field with the same name we use this prefix for instance fields or class name for class fields. E.g.

int x = this.x

edited Oct 20, 2013 at 2:49

answered Oct 20, 2013 at 2:43

Evgeniy Dorofeev

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1 Comment

Radiodef Over a year ago

Yes, it just happens that this is allowed. The field and the variable are different spaces in memory with the same identifier in code. IMO it shouldn't be allowed and nobody should do it.

Dawood ibn Kareem · Accepted Answer · 2013-10-20 02:47:53Z

2

Because that's the way Java was designed. The language designers could easily have made it so that it was impossible to use the same name for a local variable and an instance variable. They simply chose not to.

answered Oct 20, 2013 at 2:47

Dawood ibn Kareem

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Jiri Tousek · Accepted Answer · 2016-11-24 13:13:25Z

Here is a good example of java scopes (from OCA java SE-7). Here z(class variable) is initialized inside the method, doStuff2. Class variables can be initialized inside the method but if the same variable is re-declared inside a method, a new local variable is created on the stack instead of the heap.

            public class ScopeTest
            {
            int z;
            public static void main(String[] args){
            ScopeTest myScope = new ScopeTest();
            int z = 6;
            System.out.println(z);
            myScope.doStuff();
            System.out.println(z);
            System.out.println(myScope.z);
            }
            void doStuff() {
            int z = 5;
            doStuff2();
            System.out.println(z);
            }
            void doStuff2()
            {
            z = 4;
            }
            }

The output:

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