Monthly Archives: June 2011

Don’t Rely on Autoboxing in Java

Question: What does the following code display?

import java.awt.Color;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JLayeredPane;
import javax.swing.JPanel;

public class Demo {
  public static void main(String[] args) {
    JFrame frame = new JFrame();
    JLayeredPane container = new JLayeredPane();

    JPanel bottom = new JPanel();
    bottom.setBounds(0, 0, 400, 200);
    container.add(bottom); // default level is 0

    JLabel intLabel = new JLabel("I'm an int");
    intLabel.setBounds(0, 10, 400, 30);
    int layer = 10;
    container.add(intLabel, layer);

    JLabel integerLabel = new JLabel("I'm an Integer");
    integerLabel.setBounds(0, 50, 400, 30);
    container.add(integerLabel, new Integer(10));

    JLabel literalLabel = new JLabel("I'm a literal");
    literalLabel.setBounds(0, 100, 400, 30);
    container.add(literalLabel, 10);

    JLabel longLabel = new JLabel("I'm a long");
    longLabel.setBounds(0, 150, 400, 30);
    container.add(longLabel, new Long(10));

    frame.setSize(400, 200);


I'm an Integer


If you open docs for JLayeredPane, you can see the following code snippets:

layeredPane.add(child, new Integer(10));
layeredPaneParent.setLayer(child, 10);

You can see Integer there and literal 10 here. Subconsciously, you think: who cares, 10 is the same thing as new Integer(10) because Java has autoboxing, right? WRONG!

The only right way to interact with layeredPane.add is an Integer that you box yourself. Let’s take a peek at its source code…

protected void addImpl(Component comp, Object constraints, int index) {
    int layer = DEFAULT_LAYER.intValue();
    int pos;

    if(constraints instanceof Integer) { // A-ha!
        layer = ((Integer)constraints).intValue();
        setLayer(comp, layer);
    } else
        layer = getLayer(comp);

    pos = insertIndexForLayer(layer, index);
    super.addImpl(comp, constraints, pos);

Ironically, even though it uses int internally, it wouldn’t recognize it as a parameter.


Autoboxing works, except for when it doesn’t. Be defensive with APIs (even core Java) and when something is wrong do try all those ideas that seem obviously crazy at first. When writing your own API that uses type identification, remember that int and Integer are completely different beasts and both need to be included.

This example is from Swing, but it can happen anywhere. I’ve tripped on this issue before when I was implementing a CRUD framework or renderers, but did not expect it in core library.


As frimble pointed out at reddit, there is one more reason why it would never work. In addition to add(Component comp, Object constraints), Container also has add(Component comp, int index).

Having two overloaded versions of a method, one of them accepting Integer and another for int, when both do completely different things, is even worse than what I criticized in the first place.

Rant: Yet another example of extremely poor design in core Java API. Just like Object.equals() having specification that’s impossible to meet in a reasonable way or the hierarchy of java.util.Date. They use Java to teach OOP. Apparently it’s just as good for teaching how NOT to program.

Do Leave Failing Tests

Thou shalt do your coding in small increments. No work item may be longer than one day. And if thy work is unfinished at the end of the day, cut it off, and cast from thee. We heard it, believe it and are ready to die for it.


Here’s an example. Recently I spent over two weeks on a single development task. It had several moving parts that I had to create from scratch, carefully designing each of them and their interactions. Together they form a system that must be as robust as possible, otherwise our users won’t be able to use our application at all (hello, Web Start!). Finally, I tried to make it as transparent and as maintainable as possible.

I took the iterative approach. Create a spike or two. Lay out some high-level design and test. Implement some detail. Another spike. Another detail. An a-ha moment, step back and rewrite. You know how it goes.

Now, it was an all-or-nothing piece of functionality. It was critical that everything perfectly fits together and there are no holes. I just had to grasp the whole thing, even though it took days. Secondly, there were no parts that made much sense individually. Design and interfaces varied wildly, as is often the case in the young, unstable stage of development.

Sometimes your task is just like that. It simply is too big for one day or even one week, and too critical and cohesive to be partitioned.

Here’s an advice.

When you have to leave, what is the best moment to stop? It’s when you’re done with the part you were working on now, right? And then, when you get back to it on Monday, you spend a few lazy sleepy hours trying to figure out where you left 3 days ago? Wrong!

When you’re leaving, leave a failing test. Code that fails to compile. A bug that jumps at you and tries to bite your head off as soon as you launch. When you return, you’ll see the fire and jump right in to action. You will know exactly where you left, and won’t take long to figure out what to do next.

Thanks to Tynan for helping me realize it. Though his post is a general lifestyle advice, I think it can be applied to software engineering as well.