Why must local variables, including primitives, always be initialized in Java?

It is necessary to initialize local variables (only when we use them) because they don't get any default value like instance variables.

And as basic rule, we should always initialize any variable before using it. Otherwise it may result in an error like nullPointer, etc

Now, why don't local variables get default value? The reason is that local variables reside on stack and is visible only in local method context, unlike instance variables which resides on heap and has scope throughout the program.

So when the stack will end local method's value will also get destroyed therefore: 1] They are supposed to be initialized explicitly (when we use them) 2] They should not be initialized implicitly (by null,0 or false) like instance variables

Going by the definition of local variable which state, a local variable is declared within a function or is passed as an argument in a function. If you do not initialized a local variable you will encounter exception handling problem because, the compiler will not be able to understand what value the variable holds