单例动作与多实例动作

Question

I have a dude about how to implement Actions in Swing.

My idea is create a Class for each action of my application extending AbstractAction so I can use in many components that must have the same behavior. So I finaly have something as:

public class ActionExample extends AbstractAction {

    @Override
    public void actionPerformed(ActionEvent arg0) {

        System.out.println("Do something");

    }

}

Well, now when I want to use it I have three options in my mind:

public void makeUI1() {

    JButton btn = new JButton(new ActionExample("Do it"));

    JMenuItem mi = new JMenuItem(new ActionExample("Do it"));

}

public void makeUI2() {
    Action a = new ActionExample("Do it");
    JButton btn = new JButton(a);
    JMenuItem mi = new JMenuItem(a);
}

Or use it as a singleton (also changing ActionExample):

public void makeUI2() {

    JButton btn = new JButton(ActionExample.getInstance());
    JMenuItem mi = new JMenuItem(ActionExample.getInstance());
}

public class ActionExample extends AbstractAction {

    private static final ActionExample INSTANCE = new ActionExample("Do it");

    public static Action getInstance() {
        return INSTANCE;
    }

    @Override
    public void actionPerformed(ActionEvent arg0) {

        System.out.println("Do something");

    }

}

My first opinion was make it through singleton instance but I see in oracle tutorial that it make a new instance before setting it into components and in the I also see that many code create new instance for each component so I don't know what it's better and why.

Is preferred one method to be used over the other?

Answer 1

The multi instance action allows you to save data in the moment of the action for further use.

Imagine you want to add undo/redo functionality. You need to save what actions have been done for every action.

Singleton does not provide any advantage in this case.

Answer 2

I think the best thing to do would be to use the MVC pattern. Your AbstractAction class is a controller. It's responsible for extracting the information necessary for the model (ie: business logic) to use. The model/business logic is the part you reuse, but the controller may differ greatly even if it uses the same business logic.

For example, you may have a JComponent that you need to add a KeyListener to. Suddenly, your pre-made AbstractAction has become worthless because it can't be used in this situation. But, as long as you reuse all the business logic code in your KeyListener that you used in your AbstractAction, you're doing things right.