After you compile your code, you end up with
.class files for each class in your program. These binary files are the bytecode that Java interprets to execute your program. The
NoClassDefFoundError indicates that the classloader (in this case
java.net.URLClassLoader), which is responsible for dynamically loading classes, cannot find the
.class file for the class that you’re trying to use.
Your code wouldn’t compile if the required classes weren’t present (unless classes are loaded with reflection), so usually this exception means that your classpath doesn’t include the required classes. Remember that the classloader (specifically
java.net.URLClassLoader) will look for classes in package a.b.c in folder a/b/c/ in each entry in your classpath.
NoClassDefFoundError can also indicate that you’re missing a transitive dependency of a .jar file that you’ve compiled against and you’re trying to use.
For example, if you had a class
com.example.Foo, after compiling you would have a class file
Foo.class. Say for example your working directory is
.../project/. That class file must be placed in
.../project/com/example, and you would set your classpath to
Side note: I would recommend taking advantage of the amazing tooling that exists for Java and JVM languages. Modern IDEs like Eclipse and IntelliJ IDEA and build management tools like Maven or Gradle will help you not have to worry about classpaths (as much) and focus on the code! That said, this link explains how to set the classpath when you execute on the command line.