What is the difference between for each and traditional for loop in Java in terms of shadowing?

Question

This is the code I have:

class HelloWorld {

    char[] foo = {'a', 'b'};

    // This will compile
    void foo() {
        for (char foo : foo) {
        }
    }

    // This will not compile
    void bar() {
        for (char foo = 0; foo < foo.length; foo++) {
        }
    }
}

How come foo compiles but compiling bar fails with:

Error: char cannot be dereferenced

What is the difference between the two loop declarations that makes loop in foo compile but bar fail?

Answer 1

We can see the difference by looking at JLS§14.14.2's description of how the enhanced for works when handling an array:

The enhanced for statement is equivalent to a basic for statement of the form:

T[] #a = Expression;
L1: L2: ... Lm:
for (int #i = 0; #i < #a.length; #i++) {
    {VariableModifier} TargetType Identifier = #a[#i];
    Statement
}

Note how the variable is declared within the body of the loop, not the header of the loop. That is, your foo function is like this:

void foo() {
    { // Freestanding block for scope, though not really needed as `foo` has
      // nothing else in it
        char[] a = foo;       // T[] #a = Expression;
        for (int i = 0; i < a.length; i++) {
            char foo = a[i];  // {VariableModifier} TargetType Identifier = #a[#i];
        }
    }
}

That's why you get away with the shadowing in the enhanced for, and not in the traditional for, which needs to access the original array (to get its length, to get the entry for i, etc.).

More about the enhanced for loop in How does the Java 'for each' loop work? and its answers.

Answer 2

The Java Language Specification defines the scopes of your two char foo's differently:

The scope of a local variable declared in the ForInit part of a basic for statement (§14.14.1) includes all of the following:

• Its own initializer
• Any further declarators to the right in the ForInit part of the for statement
• The Expression and ForUpdate parts of the for statement
• The contained Statement

The scope of a local variable declared in the FormalParameter part of an enhanced for statement (§14.14.2) is the contained Statement.

(JLS 8, section 6.3)

That perfectly explains your observed behavior: the local foo is in scope in the control clause of the basic for loop (everywhere to the right of its declaration) and therefore it shadows the other foo there, but in the enhanced for loop its scope is only the loop statement -- it is not in scope anywhere in the loop control clause.