运行Java程序时出错

Question

Every program I run in java gives similiar error.

This is a helloworld program:

Exception in thread "main" java.lang.NoClassDefFoundError: helloworld (wrong nam
e: helloworldapp)
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClassCond(Unknown Source)
        at java.lang.ClassLoader.defineClass(Unknown Source)
        at java.security.SecureClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$000(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
Could not find the main class: helloworld.  Program will exit.

Answer 1

The class file helloworld.class contains a class called helloworldapp.class . The only real reason for this that I can think of is that you manually renamed the .class file.

That won't work! A class called helloworldapp must be found in a .class file called helloworldapp.class .

If you want to change the name of the class, rename the .java source file, edit the class definition and recompile it.

Answer 2

How do you start the Java program? Are you passing the arguments correctly? Pay attention to the class path. If you have helloworld.java file and have compiled it to helloworld.class, you should run it like this (from a command prompt in the same directory the class file is located):

java -cp . helloworld

Answer 3

Try this one:

public class HelloWorld 
{
  public static void main(String[] args) 
  {
    System.out.println("Hello World!");
  }
}

Read this tutorial about this problem.