Why does javac create an additional class? [duplic

Question

This question already has an answer here:

• Why is an anonymous inner class containing nothing generated from this code? 5 answers

I have compiled the following code (Methods and variables are elided for brevity):

// Outer.java
public class Outer
{
    private class Inner
    {
    }
    void someMethod()
    {
        Inner inObj = this.new Inner();
    }
    public static void main(String s[])
    {
        Outer outerObj = new Outer();
    }
}

When I listed the classes created, it displayed the following:

Outer$1.class
Outer$Inner.class
Outer.class

Outer and Outer$Inner appear logical. What is the purpose of Outer$1 class? What is the order of creation of these in time scale?

Answer 1

Curious. I'm not sure what this is for. But if you decompile the classes, you can see how it is used:

public class Outer {
  public Outer();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  void someMethod();
    Code:
       0: new           #2                  // class Outer$Inner
       3: dup
       4: aload_0
       5: aconst_null
       6: invokespecial #3                  // Method Outer$Inner."<init>":(LOuter;LOuter$1;)V
       9: astore_1
      10: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #4                  // class Outer
       3: dup
       4: invokespecial #5                  // Method "<init>":()V
       7: astore_1
       8: return
}

class Outer$Inner {
  final Outer this$0;

  Outer$Inner(Outer, Outer$1);
    Code:
       0: aload_0
       1: aload_1
       2: invokespecial #1                  // Method "<init>":(LOuter;)V
       5: return
}

class Outer$1 {
}

So, Outer$1 seems to contain nothing - not even a default constructor. But a reference to a Outer$1 has to be passed to Outer$Inner to construct it. Mysteriously, the value passed in someMethod is null (line 5 in someMethod).