Why must I use the "this" keyword for forward references?

Question

When I use the this keyword for accessing a non-static variable in a class, Java doesn't give any error. But when I don't use it, Java gives an error. Why must I use this?

I know when should normally I use this, but this example is very different from normal usages.

Example:

class Foo {
//  int a = b; // gives error. why ?
    int a = this.b; // no error. why ?
    int b;
    int c = b;

    int var1 = this.var2; // very interesting
    int var2 = this.var1; // very interesting
}

Answer 1

Variables are declared first and then assigned. That class is the same as this:

class Foo {
    int a;
    int b;
    int c = b;

    int var1;
    int var2;

    public Foo() {
        a = b;

        var1 = var2;
        var2 = var1;
    }
}

The reason you can't do int a = b; is because b is not yet defined at the time the object is created, but the object itself (i.e. this) exists with all of its member variables.

Here's a description for each:

    int a = b; // Error: b has not been defined yet
    int a = this.b; // No error: 'this' has been defined ('this' is always defined in a class)
    int b; 
    int c = b;  // No error: b has been defined on the line before