What is the use of the forward slash in Java?

I'm updating an old Java application to work on modern operating systems, and a very common error throughout this program is the use of a forward slash in class names. It's almost like it's trying to be used as a way to differentiate between subclasses or something. I can't seem to find much information on this online, as this is either written in Java 3 or 4 and I've never programmed in Java before.

A few examples of its usage in this program:

final class classname/a0 implements ActionListener { // <----------- these lines
  private final t a;
  
  public void actionPerformed(ActionEvent paramActionEvent) {
    this.a.a(this.a.k.a, true, 0, 0);
  }

final class classname/a1 implements ActionListener {
  private final t a;
  
  public void actionPerformed(ActionEvent paramActionEvent) {
    this.a.a(this.a.k.a, true, 0, 0);
  }

final class classname/a2 implements ActionListener {
  private final t a;
  
  public void actionPerformed(ActionEvent paramActionEvent) {
    this.a.a(this.a.k.a, true, 0, 0);
  }
}

They all throw the error - Syntax error on token "/", extends expected and - The type classname is already defined

Any info on what this means so that I can start work on a solution to this?

This is invalid syntax:

final class classname/a2 implements ActionListener 

You cannot use / in an identifier in Java.

As far as I can remember, the syntactical rules for Java indentifiers are as follows:

• numbers and letters only
• cannot start an identifier with a number

In addition, as far as I can see, there are not enough closing braces to syntactically "close" each of the classes code definitions.

It should be like so (assuming no knowledge of the type t):

final class classnamea0 implements ActionListener {
  private final t a;
  
  public void actionPerformed(ActionEvent paramActionEvent) {
    this.a.a(this.a.k.a, true, 0, 0);
  }
}

final class classnamea1 implements ActionListener {
  private final t a;
  
  public void actionPerformed(ActionEvent paramActionEvent) {
    this.a.a(this.a.k.a, true, 0, 0);
  }
}

final class classnamea2 implements ActionListener {
  private final t a;
  
  public void actionPerformed(ActionEvent paramActionEvent) {
    this.a.a(this.a.k.a, true, 0, 0);
  }
}

If I had to hazard a guess, it looks like you've tried to reverse engineer some byte code to Java readable source code using a tool and it's showing you inner, compile time derived, anonymous implemenations of interfaces from the source code.

In other words the following Kotlin code:

interface Bar {
    fun bar()
}

class Foo {
    fun foo() {
        val bar = object : Bar() {
            override fun bar() { } 
        }   
    }
}

At compile time, this is turned into byte code where there is a named inner class of Bar (usually something like Foo$Bar0) which maps to the anonymous implementation of Bar in the Kotlin source of Foo class. In other words, an anonymous implementation in source code is turned into a named implementation in the compiled java byte code (which itself is JIT compiled at run time).

This guess I am making is based of the fact the resultant "Java" code in the OP is A) not valid, B) obfuscated and C) showing evidence of decompilation from byte code of anonymous interface/abstract class implementations in the original source code.

Looks like the original engineer didn't intend for the code to be so easily reverse engineered (you may need mapping files from byte code -> source code to properly decode the byte code into readable source - these are usually produced as a side effect of a build process and may or may not still exist as data somewhere).

I.e. we may need more context to properly help out here.