Duffblog 2.0

(With thanks to my colleague Armand "jojo" Dijamco)What does this code print?Map map = new HashMap();

Collection things = new HashSet();
things.add( map );
System.out.println( things.contains( map ) );

map.put( "foo", "bar" );
System.out.println( things.contains( map ) ); Be wary of storing mutable objects in a HashSet, or using them as keys in a HashMap. An implementation side effect of using a collection that relies on hash codes is that making changes to the object in a way that alters its hash code will cause it to no longer be considered the same object. There's no way (short of removing and re-adding the item from the collection) to inform a collection that a mutable object needs to be re-hashed.Item 13 in Josh Bloch's Effective Java tells us to "Favor Immutability". He also tells us in item 8 to "Always override hashCode when you override equals". A corollary is that you should not use mutable objects as keys in HashMaps or va…

Testing Swing components seems to fill many developers with fear. "How can I unit test this thing?" they often ask, "it's full of icky event handling code and troublesome painting...".Frameworks such as Abbot are often the solution developers find for this problem. Essentially, many developers abandon the idea of writing regular unit tests for Swing components, and resort to "click simulators" which are frequently functional tests rather than unit tests.There is a lot you can do just to test regular Swing components using bog-standard JUnit (or Test-NG) tests. Consider the following fairly basic Swing component.package org.dubh.blog;

import java.awt.Graphics;

import java.awt.event.MouseAdapter;

import java.awt.event.MouseEvent;

import java.util.List;

import java.util.concurrent.CopyOnWriteArrayList;

import javax.swing.JComponent;

/**
* A user interface widget.
*/
public final class Widget extends JComponent {

private List<WidgetListener> _listeners = 
new C…

Well... If I was being more accurate, this would really be 4.0.In the beginning, there was Brian Duff's Weblog over on Radio Userland (good grief, that wasn't even Web 1.0. You had to install a thick client application just to post. Yowzers), then I set up Orablogs, and moved over to there. Until fairly recently, I blogged (or rather, mostly didn't out of laziness) at blogs.oracle.com.So why move now?Well. I got a wee bit frustrated with the blog management software I was using - it had a troublesome tendency to mangle Java code in particular, which is something of a pain when you're trying to write a Java coding related blog. So far, I'm pretty happy with blogger.com's interface.Another reason for the move is that I want to blog about more random stuff outside the world of Oracle. For instance, if I decide to play with Eclipse or JBoss (perish the thought), blogs.oracle.com doesn't really feel like the appropriate place. Even though my blog there tries not…

I recently ran into a weird NullPointerException in part of our code: Exception in thread "main" java.lang.NullPointerException at org.dubh.blog.Graph$Vertex.access$000(Graph.java:17) at org.dubh.blog.Graph.getVerticesConnectedTo(Graph.java:14) at org.dubh.blog.Graph.main(Graph.java:26) Hmm... access$000? That's odd. The code looked something like this:01package org.dubh.blog;
02
03import java.util.ArrayList;
04import java.util.HashMap;
05import java.util.List;
06
07final class Graph
08{
09private HashMap _vertices = new HashMap();
10
11public List getVerticesConnectedTo( Object vertexKey )
12{
13Vertex vertex = (Vertex) _vertices.get( vertexKey );
14return vertex.fromEdges;
15}
16
17private final class Vertex
18{
19private final List fromEdges = new ArrayList();
20}
21
22public static void main( String[] args )
23{
24Graph graph = new Graph();
25graph.getVerticesConnectedTo( getSomeVertex() );
26}
27} Can you tell quickly from looking at these two pieces of information what the problem is? It …