When a class file is not found on the classpath, it means that the Java Virtual Machine (JVM) cannot locate the specified class during runtime. The classpath is a parameter that tells the JVM where to look for class files. If a class is not on the classpath, it cannot be loaded and used by the JVM.
The classpath is a list of directories and JAR files where the JVM searches for class files. It can be set using the CLASSPATH environment variable or specified explicitly when running the Java program using the -classpath or -cp option. The classpath can include both absolute paths and relative paths to directories or JAR files.
Here’s an example to understand it better. Let’s say we have a Java program called “MyProgram.java” that depends on a class called “MyClass”. If “MyClass” is not on the classpath, we will encounter a “java.lang.NoClassDefFoundError” at runtime when trying to run “MyProgram”.
To fix this issue, we need to ensure that the classpath includes the directory or JAR file containing “MyClass”. For example, if “MyClass” is in a JAR file called “mylibrary.jar” located in the “lib” directory, we can set the classpath as follows:
java -cp /path/to/lib/mylibrary.jar MyProgram
In this example, we specify the absolute path to the “mylibrary.jar” JAR file using the -cp option. The JVM will then be able to find the “MyClass” class inside the JAR file and successfully run the “MyProgram” program.