Difference between local variable initialize null and not initialize?

Question

In Java, what is the difference and best way to do?

Integer x = null; // x later assign some value.
Integer y; // y later initialize and use it.

Answer 1

Local variables must be assigned to something before they are used.

Integer x = null;
myFunction(x);
// myFunction is called with the argument null

Integer y;
myFunction(y);
// This will not compile because the variable has not been initialised

Class variables are always initialised to a default value (null for object types, something like zero for primitives) if you don't explicitly initialise them. Local variables are not implicitly initialised.