Why does javac complain about not initialized variable?

Question

For this Java code:

String var;
clazz.doSomething(var);

Why does the compiler report this error:

Variable 'var' might not have been initialized

I thought all variables or references were initialized to null. Why do you need to do:

String var = null;

??

Answer 1

Instance and class variables are initialized to null (or 0), but local variables are not.

See §4.12.5 of the JLS for a very detailed explanation which says basically the same thing:

Every variable in a program must have a value before its value is used:

• Each class variable, instance variable, or array component is initialized with a default value when it is created:
  • [snipped out list of all default values]
• Each method parameter is initialized to the corresponding argument value provided by the invoker of the method.
• Each constructor parameter is initialized to the corresponding argument value provided by a class instance creation expression or explicit constructor invocation.
• An exception-handler parameter is initialized to the thrown object representing the exception.
• A local variable must be explicitly given a value before it is used, by either initialization or assignment, in a way that can be verified by the compiler using the rules for definite assignment.