Why Am I Getting \'java.lang.NoClassDefFoundError\' When Running Java from the Command Line?

Troubleshooting Java Execution from Command Line

Running a compiled Java class (.class) from the command line can sometimes encounter errors. One such common issue is the "java.lang.NoClassDefFoundError: [class name]" error. This occurs when the Java runtime cannot locate the necessary class definition in the classpath.

Problem Synopsis

A Java class, Echo.class, exists in the current directory, and the user attempted to execute it using:

java Echo "hello"

However, the above command resulted in an error message indicating that the Echo class was not found.

Resolution

To resolve the issue, one must specify the current directory in the classpath, which can be achieved using the -cp option in the java command:

java -cp . Echo "hello"

Alternatively, one can set the CLASSPATH environment variable to include the current directory:

SET CLASSPATH=%CLASSPATH;.

java Echo "hello"

Additional Considerations

After resolving the NoClassDefFoundError, the following error might arise:

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

This occurs if the Echo class does not contain a public static void main(String[] args) method. To resolve it, ensure that the class has a main method that conforms to the main class pattern.

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