question about variable scope and shadowing in jav

问题:

I got this situation i cant understand about shadowing. For example the following code

class Foo {
   int a = 5;
   void goFoo(int a) { 
       // No problem naming parameter as same as instance variable
       for (int a = 0; a < 5; a++) { }
       //Now the compiler complains about the variable a on the for loop
       // i thought that the loop block had its own scope so i could shadow
       // the parameter, why the compiler didnt throw an error when i named
       // the parameter same as the instance variable?
   }
}

Thank you por you patience.

回答1:

You can make a local variable shadow an instance/static variable - but you can't make one local variable (your loop counter) shadow another local variable or parameter (your parameter).

From the Java Language Specification, section 14.4.3:

If a name declared as a local variable is already declared as a field name, then that outer declaration is shadowed (§6.3.1) throughout the scope of the local variable.

Note the "field name" part - it's specifying that it has to be a field that is shadowed.

And from section 8.4.1:

The scope of a parameter of a method (§8.4.1) or constructor (§8.8.1) is the entire body of the method or constructor.

These parameter names may not be redeclared as local variables of the method, or as exception parameters of catch clauses in a try statement of the method or constructor.

(It goes on to talk about local classes and anonymous classes, but they're irrelevant in your case.)

回答2:

void goFoo(int a) { 

       for (int a = 0; a < 5; a++) { }

}

it is similar to

void goFoo() { 
       int a;
       for (int a = 0; a < 5; a++) { }

}

so multiple declaration of a on the same scope, it is not acceptable.

or simply it is similar to

void goFoo() { 
  int a;
  int a;
} 

Also See

• java-variable-scope-shadowing

回答3:

The scope of the variable depends on the hierarchy of the block as well.

ie if u use like this

void goFoo(int a) { 
       // No problem naming parameter as same as instance variable
       for (int b = 0; b < 5; b++) { }
       //Now the compiler complains about the variable a on the for loop
       // i thought that the loop block had its own scope so i could shadow
       // the parameter, why the compiler didnt throw an error when i named
       // the parameter same as the instance variable?

       int b; // you can do this.

   }

That is if a variable is declared in the outer block then you can not declare the same in the block which is inner. the other way you can do it.

回答4:

But you don't declare the second "a" in that new scope, as your code shows. It's in the scope of the goFoo() block itself.

回答5:

The problem isn't that the loop is shadowing the class field, the name is already used by the parameter.

Two options: One is to change the loop:

for (a = 0; a < 5; a++) { }

This uses the parameter as the index variable. Not clear why you would have a parameter, but all the same...

The other option is to rename the loop variable or the parameter to something else.

回答6:

It is not shadowing, it is a conflict here. Both a are in the method scope. One cannot define two variables of same name in the same scope.