Exception in thread "main" java.lang.NullPointerException是什么意思???
E:\javaprogram>java GUIDemo3
Exception in thread "main" java.lang.NullPointerException
at GUIDemo3.<init>(GUIDemo3.java:28)
at GUIDemo3.main(GUIDemo3.java:86)
出现这种错误是什么原因,请各位大虾帮忙看一下!!!