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What is Classpath in Java? - Definition & Example

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  • 0:04 Which Path?
  • 0:22 Classpath
  • 0:53 When to Change
  • 2:08 Reset
  • 2:25 Lesson Summary
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Lesson Transcript
Instructor: Martin Gibbs

Martin has 16 years experience in Human Resources Information Systems and has a PhD in Information Technology Management. He is an adjunct professor of computer science and computer programming.

Even computer languages need to know where to find things, including Java. The CLASSPATH variable lets Java know where to find class libraries. This lesson will define CLASSPATH and provide code examples for its use.

Which Path?

In this lesson, we'll explore a path many of us remember fondly - or didn't know existed. That path is the DOS prompt. Yes, the pre-Windows, command-line interface. In Windows, you can type Start -> Run and the keyword cmd to access it. It looks like this:

Command Prompt

CLASSPATH

CLASSPATH is actually an environment variable in Java, and tells Java applications and the Java Virtual Machine (JVM) where to find the libraries of classes. These include any that you have developed on your own. An environment variable is a global system variable, accessible by the computer's operating system (e.g., Windows). Other variables include COMPUTERNAME and USERNAME. In Java, CLASSPATH holds the list of Java class file directories and the JAR file, which is Java's delivered class library file.

When to Change

If you're trying to run a stand-alone Java program, you may find it necessary to change the CLASSPATH variable. When the program runs, Java's run-time system, called the interpreter, is working through your code. If it comes across a class name, it will look at each directory listed in the CLASSPATH variable. If it does not find the class name, it will error out.

We can set the value of CLASSPATH in DOS. The following example changes the variable to a local folder that we've created called CustomClasses; it's located in a folder on the C: drive called Java:

If you have your classes saved in a zip file, you can use the same command as above, except add the name of the zip file like this:

Java classpath zip file

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