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

Answer 2

Here's the deal. When you call

TestClass tc = new TestClass(); 

the new command performs four important tasks:

1. Allocates memory on the heap for the new object.
2. Initiates the class fields to their default values (numerics to 0, boolean to false, objects to null).
3. Calls the constructor (which may re-initiate the fields, or may not).
4. Returns a reference to the new object.

So your fields 'a' and 'b' are both initiated to null, and 'a' is re-initiated in the constructor. This process is not relevant for method calling, so local variable 'c' is never initialized.

HTH

PS: for the gravely insomniac, read this.