将instanceof更改为多态

Question

How to remove the operator "instanceof" and replace it with polymorphism? Both NamedPlace and DescribedPlace are subclasses to the main class Place.

class RightClickListener extends MouseAdapter {

    @Override
    public void mouseClicked(MouseEvent mev) {           
        p = (Place) mev.getSource();

        if (mev.getModifiers() == InputEvent.BUTTON3_MASK) {
            if (p instanceof NamedPlace) {
                JOptionPane.showMessageDialog(null, p.getName() + " " + p.getPosition(), "Platsinfo: ", JOptionPane.INFORMATION_MESSAGE);
            }
            if (p instanceof DescribedPlace) {
                JOptionPane describedPane = new JOptionPane();
                describedPane.setMessage("Name: " + p.getName() + " " + p.getPosition() + "\n" + "Description: " + ((DescribedPlace) p).getDescription());
                describedPane.setMessageType(JOptionPane.INFORMATION_MESSAGE);
                JDialog dialog = describedPane.createDialog(p, "Platsinfo:");
                dialog.setVisible(true);
            }

        }

    }

}

Answer 1

You can give Place the abstract method and add the implementation to the subclasses.

abstract class Place {
    // ..
    public abstract void action();
}

class NamedPlace extends Place {
    public void action() {
            JOptionPane.showMessageDialog(null, p.getName() + " " + p.getPosition(), "Platsinfo: ", JOptionPane.INFORMATION_MESSAGE);
    }
}

class DescribedPlace extends Place {
    public void action() {
            JOptionPane describedPane = new JOptionPane();
            describedPane.setMessage("Name: " + p.getName() + " " + p.getPosition() + "\n" + "Description: " + ((DescribedPlace) p).getDescription());
            describedPane.setMessageType(JOptionPane.INFORMATION_MESSAGE);
            JDialog dialog = describedPane.createDialog(p, "Platsinfo:");
            dialog.setVisible(true);
    }
}

and then use this in the listener.

class RightClickListener extends MouseAdapter {

    @Override
    public void mouseClicked(MouseEvent mev) {           
        p = (Place) mev.getSource();

        if (mev.getModifiers() == InputEvent.BUTTON3_MASK) {
            p.action();
        }
    }
}

But, depending on what Place is exactly, that might not be a good idea (eg if it's not a GUI class).

Answer 2

The only way would be to first add an abstract method like doTheThing() to your base class.

So that you then simple call p.doTheThing() in that listener.

And of course, NamedPlace implements that method to do:

JOptionPane.showMessageDialog(null, p.getName() + " " + p.getPosition(), "Platsinfo: ", JOptionPane.INFORMATION_MESSAGE);

and the other class implements it to do

JOptionPane describedPane = new JOptionPane();
...

That technically does what you ask for, but it is also not a very elegant solution. As those two methods are doing really different things.

A more reasonable solution could be that you have an abstract method that returns a (formatted) string for example, that then gets added to a (uniform) message window, or something alike.

The point of polymorphism is to allow doing "similar" things using a common interface. It doesn't help with doing completely different things under the same hood.

Answer 3

If your Place classes shouldn't be aware of the specific response implementation to a click, then you can use visitor pattern , and separate your Place hierarchy from it's response implementations.

Then you can provide different implementationsfor different situations, and your Place classes will not grow and tightly couple with more and more situation specific code.

To achieve that, update your parent class (I prefer to use interface) Place with additional method:

public interface Place {
    void accept(Visitor visitor);
}

And implement it:

public final class NamedPlace {
    @Override
    public void accept(Visitor visitor) {
        visitor.visitNamedPlace(this);
    }
}

public final class DescribedPlace {
    @Override
    public void accept(Visitor visitor) {
        visitor.visitDescribedPlace(this);
    }
}

Now the visitor interface will look like this:

public interface Visitor {
    void visitNamedPlace(NamedPlace place);
    void visitDescribedPlace(DescribedPlace place);

}

The visitor pattern is now ready. And let's use it in your example:

class RightClickListener extends MouseAdapter {

    @Override
    public void mouseClicked(MouseEvent mev) {           
        ((Place) mev.getSource()).accept(new Visitor() {
            @Override
            public void visitNamedPlace(NamedPlace p) {
                JOptionPane.showMessageDialog(null, p.getName() + " " + p.getPosition(), "Platsinfo: ", JOptionPane.INFORMATION_MESSAGE);
            }
            @Override
            public void visitDescribedPlace(DescribedPlace p) {
                JOptionPane describedPane = new JOptionPane();
                describedPane.setMessage("Name: " + p.getName() + " " + p.getPosition() + "\n" + "Description: " + ((DescribedPlace) p).getDescription());
                describedPane.setMessageType(JOptionPane.INFORMATION_MESSAGE);
                JDialog dialog = describedPane.createDialog(p, "Platsinfo:");
                dialog.setVisible(true);
            }
        });
    }
}

}

Your choice is actually:

• Either to have fixed implementation of response in your classes, and you can extend your hierarchy by adding more of them (earlier responses)
• Or to have fixed hierarchy of classes, and provide dynamically implementations (visitor pattern).

It depends on your use cases.