More than one URL object

Question

OK - I've managed to get this far without posting to the forum with this project, which shows that I'm actually beginning to grasp java.
I'm trying to get graphics to show up on JButton in jar files - I know that you're thinking that I should search the forums and I've already done that and found part of the solution - which is so create a URL object which uses the getResource method.
This works when I have one button - and works fine - when I have more than one for some reason the whole application crashes. If I have two buttons implemented through this way the first works fine, but the second causes the application to crash and I get:
Uncaught error fetching image:
java.lang.NullPointerException
        at sun.awt.image.URLImageSource.getConnection(Unknown Source)
        at sun.awt.image.URLImageSource.getDecoder(Unknown Source)
        at sun.awt.image.InputStreamImageSource.doFetch(Unknown Source)
        at sun.awt.image.ImageFetcher.fetchloop(Unknown Source)
        at sun.awt.image.ImageFetcher.run(Unknown Source)My code is
if (letter=="a"){
     URL urlA = JIEventFrame.class.getResource("alphabet_media/Angelfish.png");     
     ImageIcon picA = new ImageIcon(Toolkit.getDefaultToolkit().getImage(urlA));
          jiEventOK = new JButton(picA);  jiEventText = new JLabel ("a, Angelfish", JLabel.CENTER);}
if (letter=="b"){
          URL urlB = JIEventFrame.class.getResource("alphabet_media/Baloons.png");     
          ImageIcon picB = new ImageIcon(Toolkit.getDefaultToolkit().getImage(urlB));
          jiEventOK = new JButton (picB); jiEventText = new JLabel ("b, Balloons", JLabel.CENTER);}letter is a String which is passed from a preceding class which creates an instance of this one.
Any ideas why one would work and not two, even though they've got different identifiers ?
Thanks in advance.
adam

Answer 1

Map pngs = new HashMap();
pngs.put("a", "Angelfish");
pngs.put("b", "Balloons");
// and so on
Your code which you were having a
problem then doesn't need to have 26 if-statements, it
should look more like this:
String pngName = (String)pngs.get(letter);
URL url = JIEventFrame.class.getResource("alphabet_media/" + pngName + ".png");
ImageIcon pic = new ImageIcon(Toolkit.getDefaultToolkit().getImage(url));
jiEventOK = new JButton(pic);
jiEventText = new JLabel (letter + ", " + pngName,
JLabel.CENTER);Okay - this is a neat looking piece of code and I'm really learning. Many thanks for this. However, I'm not sure what pngName will be in this instance - will it be "a" + "Angelfish" or will it be "a", or "Angelfish"? How are the two parts separated, if at all? I've read the API for HashMap, but it isn't all that clear. I'll scour the tutorials next. How does the String know that letter is the first element "a" and that pngName is "Angelfish"?
I want to have an ImageIcon called Angelfish.png and then another JLabel where I will show the text "a, Angelfish".
Also there is a smallish problem in that not all the graphics are png; I also have jpg, but I could fix that by reformatting them. However, it's not an ideal fix.