local variable and instance variable. Java

Question

So I was testing some code and here they are:

public class Phone {

    String phoneNumber = "1234";

    void setNumber(){
        String phoneNumber;
        phoneNumber = "4321";
    }
}

public class TestPhone {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Phone p1 = new Phone();
        p1.setNumber();
        System.out.println(p1.phoneNumber);
    }

}

Output:

1234

Why not 4321? I called setNumber so phoneNumber should be assigned to 4321, what am I missing?

Answer 1

The local variable phoneNumber shadows the instance variable with the same name.

void setNumber(){
    String phoneNumber;
    phoneNumber = "4321";
}

So this code sets the local variable, doesn't change the instance variable.

If you wanted to change the instance variable, you would have to disambiguate, using the this. prefix, like this:

void setNumber(){
    String phoneNumber;
    this.phoneNumber = "4321";
}

... or better yet, just remove the pointless local variable entirely... (like you said @RobP!)