Instance data member vs Local variables (primitive/object reference)

just a simple question: why does a local variable should be initialized before a program's running stage? considering that this variable will be used somewhere local, while a data member can be used even if it is not initialized in a constructor or an instance method.
does the compiler always assign an offset values in data members when they are not initialized explicitly?

Here is an explanation for both of your questions: oop - Uninitialized variables and members in Java - Stack Overflow
The compiler forces you to initialize local variables partly because it can not figure out the full logic of your program flow.

Edit: My compiler optimizes away the first increment but not the second. Whether this should be an error or a warning is
debatable but that is the way it is in java.

Code java:

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public void foo() {
     int x;
     final int y = 0;
     if(false)
	x++;
     if(y == 0)
	x++;
}

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