可运行的JAR文件不会

Question

I created this card game in Java. What it does it it presents one card face up and 4 more cards face down. You wager from 1 to 100 coins and try to pick a higher card from the face down cards. If you pick a higher card, your wager is doubled and you can choose to go double or nothing on another round.

The program uses 3 .java files in one package:

• HigherNumber: Main class, contains the bulk of the code.
• Deck: Contains definition for a class representing a deck of cards.
• Card: Contains definition for a class representing an individual card.

So naturally, this program uses a lot of pictures, to represent the cards. In my original implementation, I just passed ImageIcon a string to represent the location of the cards. So like, for the icon for a face down card,

faceDown = new ImageIcon("multimedia/redBack.gif");

When I did this, the program ran perfectly when run through Eclipse. So I used Eclipse to Export to a runnable JAR file. This JAR file then ran without a problem, except if I moved the JAR file anywhere else, none of the images showed up.

So I researched and found out about using URLs to combat this. I reworked the program to use URLs, so now I have stuff like this:

//Set URL for default faceDown icon.
faceDownURL = this.getClass().getResource(pictureRoot +"redBack.gif");
//Set location for default back face of cards.
faceDown = new ImageIcon(faceDownURL);

Now it runs fine in Eclipse, but I cannot get the exported runnable JAR to work. When run from Windows, it just kinda blinks and does nothing. When I run through the command line, I get this:

C:\Documents and Settings\mstabosz>java -jar C:\Temp\HigherNumber.jar
Exception in thread "main" java.lang.NullPointerException
        at javax.swing.ImageIcon.<init>(Unknown Source)
        at higherNumber.Card.setImage(Card.java:150)
        at higherNumber.Card.<init>(Card.java:36)
        at higherNumber.Deck.<init>(Deck.java:22)
        at higherNumber.HigherNumber.<init>(HigherNumber.java:16)
        at higherNumber.HigherNumber.main(HigherNumber.java:857)

Trying to follow this code, it looks like the source of the problem is in the Card class at line 150. At line 150, the class is in the setImage() function, which is building a string called iconName to be used to set the image for each card as it is created. It then returns an ImageIcon to the Card class's constructor.

//Set up the icon for the card.
this.cardIcon = setImage();

Line 150 is the return statement. Here are the statements that create the URL cardIconURL which is used in the ImageIcon.

//Create a URL based on the constructed string.
URL cardIconURL = this.getClass().getResource(iconName);

return new ImageIcon(cardIconURL);

I just don't get what's going wrong here. The program worked fine as a runnable JAR when I was using Strings instead of URLs. It works fine when run through Eclipse. It doesn't work as a runnable JAR now.

I did read up on something called manifests, which I had trouble understanding. I did have Eclipse generate a manifest for this program:

Manifest-Version: 1.0
Main-Class: higherNumber.HigherNumber

What am I missing?

Answer 1

i use something like:

URL myurl = this.getClass().getResource("file.png");
myIconImage = Toolkit.getDefaultToolkit().getImage(myurl);

you're doing:

return new ImageIcon(cardIconURL);

Maybe try my second line. Also i store the images in the jar.

Answer 2

Okay it looks like I got the image files in the runnable JAR by dragging and dropping the "multimedia" folder which contains the pictures into the "highernumber" package in Eclipse. I'm still getting the NullPointerException though.