Question

This is a very basic question, but I’d quite like an explanation of why my question can or cannot be achieved.

If I have a class (A) which contains say a string, with a set method for that string. And I instantiate another class (B) from the first class (A), why can’t I then access the first class (A) from the new class (B) to call the set method for the string in the first class (A).

The only reason I ask is that I’m working on a project with a similar problem, from a main class I create a new class which returns some buttons. And when a button is clicked the ActionListener in the main class is supposed to change the String in the initial class, but I cannot seem to access the set Method of the original class without re-instantiating the class.

Sorry if that sounds rambled, but I really want to understand why this is an issue, and what the correct way of doing it is. I know I’ll probably be shot down on this, but any help is appreciated.

Answer 1

Because class B needs to hold a reference of the instance of A from which it has been created. There is no formal reason for which this should be made by default. For example:

public class B {

    private final A creator; 

    public B(A creator) {
        // you might want to check for non null A
        this.creator = creator;
    }

    public void foo(String value) {
        creator.setText(value);
    }
}