设置另一个班级的组合框

Question

Is it possible to set the selected item of a combobox from a different class? Say I have Class A and Class B, in class AI setup the combobox and add it to the Jframe. Could I do something like ClassA.comboBox.setSelectedItem("Option1"); from class B? That doesn't work but I hope it illustrates what I am trying to do. Do you know how/if I can achieve this result with Java? Many thanks.

Answer 1

This a common enough problem which can solved in several ways

You could...

Pass a reference of A to B , so B can call what ever methods A has available to change it's state.

You should avoid allowing direct access to your classes fields, because it would allow B to do things you might not want it to do.

Equally, you should prefer interface s over passing actually implementations, further restricting what B can do to A .

This, however, does tighten the coupling of your code. What should A have to have a reference to B to do anything (or visa-versa)

You could...

Use an Observer Pattern , allowing B to generate events that interested parties (like A ) so that they can update themselves accordingly, based on there own choices, the event and/or the state of B

This de-couples your code ( B no longer needs an instance of A to do anything) and makes your code more re-usable, other classes can register interest with instances B and B doesn't care

You shouldn't...

Use static . static is a BAD, BAD idea when it comes to cross communication between objects. The moment you have more then one instance of any class, the whole thing just blows up into a horrible mess. You also lose a lot of control over who is doing what to those instances (nothing stopping any one from creating a new instance of a class and assigning it to the static field), all of which just makes your life a misery (or in the case of the developers who have to clean it up, your name becomes a dirty word)

Answer 2

If you have an instance of A in B the better option is to have getters/setters for the elements you need in A :

Class A {
    private JComboBox comboBox = new JComboBox();

    public A(){}
    public JCombobox getComboBox(){ return comboBox;}
}

And in B :

Class B {
    public B(){
    A aInstance = new A();
    aInstance.getComboBox().setSelectedItem("Option1");
}

Answer 3

You can first create the class B and then pass the reference A to the class B , then in the class B you can do what you want with class A . A simple example:

class A extends JFrame {

    private ComboBox comboBox;

    public A() {
      comboBox = new ComboBox();
      B b = new B(this);
    }

    public void addItem(Obejct item) {
        // add item to comboBox
    }

}

class B {
    private A a;

    public B(A a) {
         this.a = a;
         String item = "new Item";
         a.addItem(item);
    }

}

Hope this help!