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The Joy of Generics

Posted by timboudreau on July 22, 2006 at 1:02 AM PDT

Okay, Generics are old news. But since NetBeans was still targetting JDK 1.4 until a few months ago, and most of my coding is NetBeans plug-ins, I'm just starting to really appreciate them. In particular, I'm finding they have a very nice effect on the things I choose to write and how I write them.

Of course the basics are quite convenient - just being able to eliminate casts and get type checking on uses of collections is handy. But what I find I am doing that's very useful is making different choices about what to write and how to write it.

As a case in point, this week I wrote a little standalone Bézier Spline editor - a project I sort of distracted myself into - I needed to design a very precise GeneralPath as part of some tweaks to NetBeans popup windows' appearance on Mac OS. So rather than sit around with graph paper, I wrote a little app that would let me draw what I needed around a screen shot, and spit out the source code for a corresponding GeneralPath object.

Anyway, since I still have some plans for the Imagine project, I was thinking about how best to design an image editor, and wanted to do some experimenting that might be useful later. What I ended up with is a concept where there are EditingModes, and an EditingMode makes available a set of tools that should be shown in a toolbar and a menu. The user can select editing modes, and the user can select tools (and also EditingModes have lists of actions of their own).

So I've got three things that need basically the same thing - each has a list of objects that share a type. For each list of objects, I need a corresponding list of toggle buttons and menu items. But the list contents are all different types.

This is where generics come to the rescue - in the past, I probably wouldn't have thought to take this approach, simply because all of the casting required would be ugly, and it wouldn't be terribly type-safe. What I did is create a single class with a generic type; its constructor takes an argument of a 3 method interface which must have the same generic type. So all of the logic about manufacturing menu items and such is shared; I just implement the Logic interface for each one and pass it to the constructor and I'm done. The Logic interface handles fetching the display name of an object from the list, fetching the icon for the object, and performing whatever action should be performed when the menu item or button is invoked.

The point here isn't that the code is wildly beautiful - but that it's really enjoyable to find a technique that changes the way you think about what you can do. In the past, what I would have written to do this would be either uglier, or there simply would have been three cut-and-paste implementations, depending on how expedient I was being. So I'm finding that generics are adding a bit of extra fun into programming - I can have more logic be, uh, generic, and write less boilerplate as a result. To illustrate, here's what I mean:

package pathdesign.app;
import java.awt.event.ActionEvent;
import java.util.ArrayList;
import java.util.List;
import javax.swing.AbstractAction;
import javax.swing.Action;
import javax.swing.ButtonGroup;
import javax.swing.Icon;
import javax.swing.JMenuItem;
import javax.swing.JToggleButton;

public class ActionMapper <T extends Object> {
    private List <T> objects;
    private final Logic logic;
    private T selection;
   
    interface Logic <T> {
        void invoke (T t);
        String getDisplayName (T t);
        Icon getIcon (T t);
    }
   
    public ActionMapper(List <T> objects, Logic<T> invoker, T initialSelection) {
        this.objects = objects == null ? new ArrayList <T> () :
            new ArrayList<T>(objects);
        this.selection = initialSelection == null ? objects.size() > 0 ?
            objects.get(0) : null : initialSelection;
       
        if (initialSelection == null) {
            throw new NullPointerException ("Can't determine initial selection");
        }
        selection = initialSelection;       
        this.logic = invoker;
        if (invoker == null) {
            throw new NullPointerException ("Invoker null"); //NOI18N
        }
    }
   
    public interface MapperAction <T> extends Action {
        public T getItem();
    }
   
    public boolean setObjects (List <T> objects, T selection) {
        if (!this.objects.equals(objects)) {
            this.objects = new ArrayList<T>(objects);
            setSelection (selection);
            buttons = null;
            actions = null;
            menuItems = null;
            return true;
        } else {
            return setSelection (selection);
        }
    }
   
    public List <T> getObjects() {
        return new ArrayList <T> (objects);
    }
   
    private boolean inSetSelection = false;
    public boolean setSelection (T selection) {
        if (inSetSelection) return false;
        inSetSelection = true;
        try {
            if (!objects.contains(selection)) {
                throw new IllegalArgumentException ("I don't know about "  //NOI18N
                        + selection);
            }
            boolean result = !selection.equals (this.selection);
            if (result) {
                this.selection = selection;
                updateButton(selection);
            }
            return result;
        } finally {
            inSetSelection = false;
        }
    }
   
    public T getSelection() {
        return selection;
    }
   
    private List <MapperAction<T>> actions;
    public List <MapperAction<T>> getActions() {
        if (actions == null) {
            actions = createActions();
        }
        return actions;
    }
   
    private List <JToggleButton> buttons;
    public List <JToggleButton> getButtons() {
        if (buttons == null) {
            buttons = createButtons();
        }
        return buttons;
    }
   
    public List <JMenuItem> menuItems;
    public List <JMenuItem> getMenuItems() {
        if (menuItems == null) {
            menuItems = createMenuItems();
        }
        return menuItems;
    }
   
    private List <MapperAction <T>> createActions() {
        List <MapperAction<T>> result = new ArrayList <MapperAction<T>> (objects.size());
        for (T item : objects) {
            MapperAction <T> action = new MapperActionImpl <T>(item);
            action.putValue(Action.NAME, logic.getDisplayName (item));
            result.add (action);
        }
        return result;
    }
   
    private List <JToggleButton> createButtons() {
        List <JToggleButton> result = new ArrayList <JToggleButton> (objects.size());
        ButtonGroup grp = new ButtonGroup();
        for (MapperAction<T> action : getActions()) {
            JToggleButton button = new JToggleButton();
            button.setAction (action);
            button.setIcon (logic.getIcon (action.getItem()));
            result.add (button);
            if (getSelection() == action.getItem()) {
                button.setSelected(true);
            }
            grp.add (button);
        }
        return result;
    }
   
    private List <JMenuItem> createMenuItems() {
        List <JMenuItem> result = new ArrayList <JMenuItem> (objects.size());
        ButtonGroup grp = new ButtonGroup();
        for (MapperAction<T> action : getActions()) {
            JMenuItem button = new JMenuItem();
            button.setAction (action);
            button.setIcon (logic.getIcon (action.getItem()));
            result.add (button);
            if (getSelection() == action.getItem()) {
                button.setSelected(true);
            }
            grp.add (button);
        }
        return result;
    }
   
    private void updateButton (T t) {
        JToggleButton b = buttonFor (t);
        if (b != null) {
            b.setSelected (true);
        }
    }
   
    private JToggleButton buttonFor (T t) {
        if (buttons != null) {
            for (JToggleButton button : buttons) {
                MapperAction<T> action = (MapperAction<T>) button.getAction();
                if (action.getItem().equals (t))  {
                    return button;
                }
            }
        }
        return null;
    }
   
    private final class MapperActionImpl <T> extends AbstractAction implements MapperAction <T> {
        private final T item;
        public MapperActionImpl (T item) {
            this.item = item;
        }
       
        public void actionPerformed(ActionEvent e) {
            logic.invoke(getItem());
        }

        public T getItem() {
            return item;
        }
    }
}

That being said, there are things that I find ugly about generics -
Map m = new HashMap() is less work to read then Map <String, Foo> m = new HashMap <String, Foo> (), and it can get really ugly, e.g. Map <String, Reference<Bar<Baz>>> m = new HashMap <String, Reference<Bar<Baz>>> () - but for cases like the above code, they're darned useful - and moreover, fun.

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