声明新的ImageIcon时为nullpointerexception [英] nullpointerexception when declaring new ImageIcon

问题描述

我试图在同一窗口上加载多个图像，因此为了防止大量复制和粘贴，我制作了一个名为badgeIMG的图像类，如下所示:

I am trying to load multiple images on the same window, so to prevent a lot of copy and paste, I made an image class called badgeIMG that looks like this:

    package BattleSim;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;

import javax.swing.ImageIcon;
import javax.swing.JPanel;

public class badgeIMG extends JPanel{

    Image badgeIcon;
    String badgePath;
    int x = 0;
    int y = 0;

    public badgeIMG() {
        ImageIcon ii = new ImageIcon(this.getClass().getClassLoader().getResource(badgePath));
        badgeIcon = ii.getImage();
    }

    public void paint(Graphics g) {

        Graphics2D g2d = (Graphics2D) g;
        g2d.drawImage(badgeIcon, x, y, null);
    }
}

然后在另一个名为badgeSelectionWindow的类中，我有这段代码:

Then in another class, called badgeSelectionWindow, I have this piece of code:

        badgeIMG allOrNothingBadge = new badgeIMG();
    badgeIMG closeCall = new badgeIMG();

    allOrNothingBadge.badgePath = "/Badges/allornothing.gif";
    allOrNothingBadge.x = 128;
    allOrNothingBadge.y = 144;

    closeCall.badgePath = "/Badges/closecall.gif";
    closeCall.x = 256;
    closeCall.y = 144;

    add(allOrNothingBadge);
    add(closeCall);

问题是，当从上述代码中声明badgePath时，我得到了NullPointerException，但是当我用实际文件路径之一替换了badgePath时，它没有给我错误，但是我希望能够插入一个带有文件路径的字符串，并使其显示多个图像.有什么想法吗?

The problem is, I get a NullPointerException when declaring a badgePath from the above code, but when I put replace badgePath with one of the real file paths, it does not give me error, but I want to be able to plug in a String with the file path and have it display multiple images. Any ideas?

这是错误:

Exception in thread "main" java.lang.NullPointerException
at sun.misc.MetaIndex.mayContain(Unknown Source)
at sun.misc.URLClassPath$JarLoader.getResource(Unknown Source)
at sun.misc.URLClassPath.getResource(Unknown Source)
at sun.misc.URLClassPath.getResource(Unknown Source)
at java.lang.ClassLoader.getBootstrapResource(Unknown Source)
at java.lang.ClassLoader.getResource(Unknown Source)
at java.lang.ClassLoader.getResource(Unknown Source)
at BattleSim.badgeIMG.<init>(badgeIMG.java:17)
at BattleSim.badgeSelectionWindow.<init>(badgeSelectionWindow.java:11)
at BattleSim.badgeSelectionWindow.main(badgeSelectionWindow.java:36)

推荐答案

您的badgePath是null.

构造函数将badgePath用作ImageIcon构造函数的参数，但并未首先对其进行初始化.使用这样的构造函数:

The constructor uses the badgePath as argument for the ImageIcon constructor, but it didn't initialize it first. Use a constructor like this:

public badgeIMG(String path)
{
    ImageIcon ii = new ImageIcon(this.getClass().getClassLoader().getResource(path));
    badgeIcon = ii.getImage();
    badgePath = path;
}

注意:非常重要:Java命名约定是类以大写字符开头.因此，将类名和文件名更改为:BadgeImg或BadgeIMG.

Note: Very important: Java naming conventions are that classes start with an uppercase char. So change the class name and file name to: BadgeImg or BadgeIMG.

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