Question

What is the different between:

Socket connection = null;

and

Socket connection;

Thanks.

Answer 1

If it’s a field (static or instance variable) then the two are broadly equivalent as null is the default value. (There’s a subtle difference if a super-constructor calls an overridden method which sets the value, but that’s a corner case.)

If it’s a local variable, they’re very different – in the first form, the local variable is definitely assigned, and the value can be read in the very next statement. In the second form, the variable isn’t definitely assigned, and you won’t be able to read it from code until such a point where the compiler can prove that a value will have been written:

void foo() {
  String x = null;
  String y;

  System.out.println(x)  // Fine, prints null
  System.out.println(y); // Compile-time error
}