Why does my icon handling code throw a NullPointerException?

问题

I have added an image for my button,but when I run that frame this exception will be thrown .why?please help me.

init:

deps-jar:
compile-single:
run-single:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at javax.swing.ImageIcon.<init>(ImageIcon.java:138)
        at ClientGUI.IdAndPasswordFrame.initComponents(IdAndPasswordFrame.java:91)
        at ClientGUI.IdAndPasswordFrame.<init>(IdAndPasswordFrame.java:22)
        at ClientGUI.IdAndPasswordFrame$4.run(IdAndPasswordFrame.java:200)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)
BUILD SUCCESSFUL (total time: 1 second)

line 138:

public ImageIcon (URL location) {
    this(location, location.toExternalForm());
}

line91:

 jButton1.setIcon(new javax.swing.ImageIcon(getClass().getResource("/Images/yahoo_1.gif"))); // NOI18N

I use this poor checking (Peter Lang recommended)which is:System.out.println(getClass().getResource("/Images/yahoo_1.gif")); and it returns null,why? please help me.

回答1:

This means, that getClass().getResource("/Images/yahoo_1.gif") returns null.

JavaDoc states that this happens if

the resource could not be found or the invoker doesn't have adequate privileges to get the resource.

1. Check if getResource really returns null:
   System.out.println(getClass().getResource("/Images/yahoo_1.gif"));

2. Make sure that your path is correct and that it is in your classpath.

EDIT:

I just tried it with NetBeans. I created the following structure

Source Packages
  Images
    yahoo_1.gif

and your code worked fine. Is this your structure?

Try to right-click on your application and select Clean and Build.

回答2:

In order to fix this, the images need to be copied in the bin directory - not in src directory.

Otherwise you will get null all the time on getClass().getResource("image.png"). The path is not null and you can set it as the above - only if you copy the images that you need inside the binary directory, where .class files for your project are located.

This fixed the problem. Let me know if I helped in this.

Ioana

回答3:

It looks like getClass().getResource("/Images/yahoo_1.gif") returns null i.e. the .gif cannot be found on your classpath. (Images versus images maybe?)

回答4:

The URL being passed in is null from this line:

getClass().getResource("/Images/yahoo_1.gif")

From the JDK documentation:

[getResource(..) returns] A URL object for reading the resource, or null if the resource could not be found or the invoker doesn't have adequate privileges to get the resource

Maybe you meant ("Images/yahoo_1.gif") - i.e. relative path not absolute?

回答5:

I had the same problem. What worked for me was:

1. Look into the jar file or in the bin folder(the one with .class files) and see the path of image.
2. List item

回答6:

private class HandlerClass implements ActionListener{
        public void actionperformed(ActionEvent event){
            JOptionPane.showMessageDialog(null, String.format("%s", event.getActionCommand()));
        }

}

来源：https://stackoverflow.com/questions/2019914/why-does-my-icon-handling-code-throw-a-nullpointerexception