There is two variable 'a' and 'b' which is not initialised. 'a' variable is of class Example1 and 'b' variable of class Main

Question

There are two variables, a and b, which are not initialized. a variable is of class Example1 and b variable of class Main. As we know if we not initialized any variable in java then it take default value. But when I want to print variable of Main class it show error of initialization. But when I call the Example1 class variable then it executed successfully and gives default value.

What is the exact reason behind it?

class Example1 {
    int a;
}
public class Main
{
    public static void main(String[] args) {
        int b;
        Example1 e = new Example1();
        System.out.println(e.a);        // output: 0
        System.out.println(b);          // error: variable a might not have been initialized
        
    }
}

Answer 1

When you write:

int b; // this is declaration

int b; is a local variable. Method local variables need to be initialized before using them. And this is why you've got an error.

When you write

 int b = 0; // this is initialization

When you write:

  Example1 e = new Example1();  // This is an initialization

Primitive default value is zero when declared as class members.