oop - Uninitialized variables and members in Java

Question

Consider this:

public class TestClass {

    private String a;
    private String b;

    public TestClass()
    {
    a = "initialized";
    }

    public void doSomething()
    {
    String c;

        a.notify(); // This is fine
    b.notify(); // This is fine - but will end in an exception
    c.notify(); // "Local variable c may not have been initialised"
    }

}

I don't get it. "b" is never initialized but will give the same run-time error as "c", which is a compile-time error. Why the difference between local variables and members?

Edit: making the members private was my initial intention, and the question still stands...

Answer 1

The language defines it this way.

Instance variables of object type default to being initialized to null. Local variables of object type are not initialized by default and it's a compile time error to access an undefined variable.

See section 4.12.5 for SE7 (same section still as of SE14) http://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.12.5