Why is there a ClassLoader exception when there are 2 objects with similar type names

Question

I have a class with 2 methods.

• In method1(), I create a local record called Abc. This local record is only available to method1() because it was defined in method1() (here are the rules on it according to Java).

• In method2(), I create a local interface called ABC. Notice that there is a capitalization difference here. The local record was named Abc, but this local interface is named ABC.

Here is the class.

//package ..... //commenting out package information, as I sincerely doubt that is the cause

/** There seems to be a class loader error when running the below method in main(). */
public class ClassLoaderIssue
{

   /** Method 1. */
   private void method1()
   {
   
      record Abc(int a)
      {
      
         public static String iDontCare()
         {
         
            return "ignore this method";
         
         }
      
      }
      
      System.out.println(Abc.iDontCare()); //error
   
   }
   
   /** Method 2. */
   private void method2()
   {
   
      interface ABC
      {
      
      }
      
   }
   
 
   /**
    * 
    * Main method.
    * 
    * @param   args  commandline arguments that we don't care about for this example.
    * 
    */
   public static void main(String[] args)
   {
   
      new ClassLoaderIssue().method1();
   
   }
   
}

And here is the Exception that I get.

Exception in thread "main" java.lang.NoClassDefFoundError: ClassLoaderIssue$1ABC (wrong name: ClassLoaderIssue$1Abc)
    at java.base/java.lang.ClassLoader.defineClass1(Native Method)
    at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1012)
    at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150)
    at java.base/jdk.internal.loader.BuiltinClassLoader.defineClass(BuiltinClassLoader.java:862)
    at java.base/jdk.internal.loader.BuiltinClassLoader.findClassOnClassPathOrNull(BuiltinClassLoader.java:760)
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:681)
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:639)
    at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520)
    at ClassLoaderIssue.method1(ClassLoaderIssue.java:23)
    at ClassLoaderIssue.main(ClassLoaderIssue.java:49)

And here is some relevant information.

Java compile command = C:\Program Files\Java\jdk-17.0.2\bin\javac.exe
Java run command = C:\Program Files\Java\jdk-17.0.2\bin\java.exe

Obviously, this is just a runnable example of the problem, but I have a situation where it is extremely convenient and helpful to have local class/enum/record to handle weird state at the very edges of my domain. Therefore, I use this pattern a lot, but this is the first time I have come across this issue.

Now obviously, there are many ways to go around this. The simplest being that I could just rename the enum for one. I already tried that and that worked. Same for the interface.

But why does this happen?

EDIT - I also tested this with Java 18 and got the same result. I used a brand new file, a brand new folder, and I copied and pasted the text (not the file), into an empty file with the same name. Same error. I can't really get a cleaner test than that.

Answer 1

On Windows, file names are case-insensitive. This means that files called Something$ABC.class and Something$Abc.class cannot be distinguished.

Meanwhile, of course, the language itself is case-sensitive, and expects the file name and the name of the class within the file to match.

Your only recourse seems to be to rename either the class or the interface.