运行.jar文件时出错

Question

I created a .jar file of my java game. The program ran well with a java compiler, but when I try to run the .jar file, it showed no result. I ran it again via CMD using:

java -jar PokemonGame.jar`

...it gave me an error:

Exception in thread "main" java.lang.NullPointerException
at javax.swing.ImageIcon.(init)(ImageIcon.java:167)
at MainFile.(init)(MainFile.java:25)
at MainFile.main(MainFile.java:86)

I traced the line numbers which I commented:

public class MainFile extends JPanel implements MouseListener{
    MainFile m;
    static JFrame mainWindow = new JFrame("POKEMON MEMORY GAME");
    static TimerFile timerPanel;
    static GridFile gridPanel;
    static LogsFile logsPanel;
    static ButtonMenuFile buttonMenuPanel;
    JPanel blockPanel;
    URL url;
    BufferedImage winlose;
    JPanel winlosePanel;
    //MainFile line 25
    ImageIcon gameBackground = new ImageIcon(getClass().getResource("Assets\\Pokedex.png"));
    Image gameImage = gameBackground.getImage();
    GameSoundFile gameSound = new GameSoundFile();
    GameSoundFile screenSound = new GameSoundFile();

    //...some codes here

    public static void main(String[] args) {    
        //MainFile line 86
        MainFile mainPanel = new MainFile();
        mainPanel.setMainPanel(mainPanel);          

        mainWindow.add(mainPanel);
        mainWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  
        mainWindow.pack();
        mainWindow.setLocationRelativeTo(null);
        mainWindow.setResizable(false);
        mainWindow.setVisible(true);

    }

    public MainFile(){
        //...MainFile codes here
    }
}

Can anyone tell me the flaw here because it seems like a weird behavior that this program was able to run with a java compiler but not on the .jar execuatble file.

Answer 1

Pack Pokedex.png into jar at the top level:

jar uf PokemonGame.jar Pokedex.png

and reference it like that:

getClass().getResource("/Pokedex.png")

Answer 2

Within JAR files, the windows directory separator character doesn't exist. Use / to separate your directories, and assuming the file is there, it will be found.