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Posted on: July 26, 2006 at 12:00 AM
CLASSPATH tells Java where to search for programs.

Java Notes


Do you get the following error message?

Exception in thread "main" java.lang.NoSuchMethodError: main

If so, the following are common causes.

  1. File/class name mismatch. Check the file and class names -- they much match exactly, even with upper and lower case. Windows doesn't care about file names that differ only in case, but Java does. If you have trouble renaming a file to fix the case, first rename it to something different, then rename to the correct case.
  2. Typo in main header. Something is wrong with your declaration of main. It must be
        public static void main(String[] args)
    Slight variations in the parameter declaration are allowed, but the above is by far the most common.
  3. CLASSPATH is explained below in more detail.

CLASSPATH tells Java where to search for programs

Where to look? The Java runtime system needs to know where to find programs that you want to run and libraries that are needed. It knows where the predefined Java packages are, but if you are using additional packages, you must tell specify where they are located.

CLASSPATH. For Windows the easiest way is to set the environment variable CLASSPATH, which Java uses to see where it should find Java programs and libraries.

Automatic? Some IDEs tell the Java runtime system where to search, and double-clickable Jar (Java Archive) files don't have this problem

When do you need it? If you compile "by hand" (typing javac and java in a command window), or sometimes with TextPad or other editors.

Setting CLASSPATH on Windows XP

The CLASSPATH variable can be set on Windows XP with the following steps.
  • Click the Start button in the lower left of the screen.
  • Select Control Panel from the pop-up menu.
  • Choose System from the submenu.
  • Click the Advanced tab.
  • Click the Environment Variables button near the bottom and you will see two lists of variables.
  • Look in the System variables list for a variable named CLASSPATH. If you find it, click Edit. If you don't find it, click New and enter CLASSPATH in the Variable name field.
  • The Variable value field is a list of file paths of directories or jar files. The first thing in this list should be the current directory, which is represented in windows just as it is in Unix, as a single period. If there's more than one thing in this list, separate items with a semicolon. For example, my CLASSPATH variable starts with these three items (there are a couple more, but this should be enough to give you the idea). The only part you need is the first "dot".
    I put extra libraries that I want to be searched in a directory named classpath, but you can choose any name.


A more complete explanation of CLASSPATH can be found in Java Glossary : classpath.
Copyleft 2005 Fred Swartz MIT License