Lesson: Common Problems (and Their Solutions) (The Java™ Tutorials > Getting Started)

Lesson: Common Problems (and Their Solutions) (The Java™ Tutorials > Getting Started): "Exception in thread 'main' java.lang.NoClassDefFoundError: HelloWorldApp
If you receive this error, java cannot find your bytecode file, HelloWorldApp.class.
One of the places java tries to find your bytecode file is your current directory. So, for example, if your bytecode file is in /home/jdoe/java, you should change your current directory to that. To change your directory, type the following command at the prompt and press Return:
cd /home/jdoe/java

If you enter pwd at the prompt, you should see /home/jdoe/java. If you enter ls at the prompt, you should see your .java and .class files. Now enter java HelloWorldApp again.
If you still have problems, you might have to change your CLASSPATH environment variable. To see if this is necessary, try clobbering the classpath with the following command.
unset CLASSPATH

Now enter java HelloWorldApp again. If the program works now, you'll have to change your CLASSPATH variable in the same manner as the PATH variable above."