Question

I encountered similar problem “if” statement vs OO Design – 1 but it is slightly different. Here is the problem that open the popup (different objects/popups) onValueChange of listbox

Popup1 p1; // different objects
Popup2 p2; // different objects
Popup3 p3;
...

listbox.add("p1");
listbox.add("p2");
listbox.add("p3");
...

listbox.addChangeHandler() {
    if(getSelectedItem().equals("p1")){
       p1 = new Popup1();
       p1.show();
    } else if() {...}
      ....
}

I don’t want to write “if” that if p1 then p1 = new Popup1(); p1.center();

How I can handle this situation? Any design-pattern?

Here is my solution but it is so costly

map() {

    map.put("p1", new Popup1());
    map.put("p2", new Popup2());
    map.put("p3", new Popup3()); 
}

onValueChange() {
    map.get(selectedItem).show();
}

One drawback is initialization all the popups. but it is require only when valueChange

Answer 1

I agree with leveraging the common base class when you can, but you can’t always add a method to the base class for every usage that might call for selecting between different subclasses.

Your “map” solution is a decent approach for cases where the selection logic is specific to a piece of code, like matching user action to an object (e.g. popup), and you can’t find a way to leverage the common base class.

One drawback is initialization all the popups. but it is require only when valueChange

You should defer the instantiation until you need it:

interface Showable {
    void show();
}
map() {

    map.put("p1", new Showable() { void show() { new Popup1().show(); } } );
    map.put("p2", new Showable() { void show() { new Popup2().show(); } } );
    map.put("p3", new Showable() { void show() { new Popup3().show(); } } ); 
}

onValueChange() {
    map.get(selectedItem).show();
}

The anonymous classes are stateless, so if you wanted to be extra efficient, you could create the anonymous instances once and reuse them.