Strange exception when implementing interface

Question

I have Exception in thread "main" java.lang.NoClassDefFoundError: A (wrong name: a) and I dont't have any idea what this can caused by

public class Test
{
    public static void main(String[] args)
    {
        new B();
    }
}

interface a { }

class A implements a { }

class B extends A { }

Edit: in online compiler https://www.onlinegdb.com/online_java_compiler it compiles

Answer 1

When Java compiles your source code, it creates multiple .class files. For example, it creates Test.class for public class Test, a.class for interface a, and A.class for class A. The problem here is that file names in some operating systems are case-insensitive. This means that the operating system sees a.class and A.class as the same file so one will overwrite the other.

The online compiler most likely treats these file names as different due to case-sensitivity.

The solution here is to use different names so that you avoid these name collisions at the operating system level.

The established Java convention is to start all class and interface names with an upper case letter. If you follow this convention, then you will avoid this problem.

Answer 2

If you run javac path/to/your/file, you should see the list of .classfiles created by the java compiler in that dir. The problem with your approach is you have duplicate names for the interface and the class i.e A (case insensitive) and as a result only one .class gets created. Try again by changing the name of either interface or class and your problem should go away.