Question

package com.test;

public class OuterClass {
    public class InnerClass {
        public class InnerInnerClass {

        }
    }

    public class InnerClass2 {

    }

    //this class should not exist in OuterClass after dummifying
    private class PrivateInnerClass {
        private String getString() {
            return "hello PrivateInnerClass";
        }
    }

    public String getStringFromPrivateInner() {
        return new PrivateInnerClass().getString();
    }
}

When run through javac on the command line with Sun JVM 1.6.0_20, this code produces 6 .class files:

OuterClass.class
OuterClass$1.class
OuterClass$InnerClass.class
OuterClass$InnerClass2.class
OuterClass$InnerClass$InnerInnerClass.class
OuterClass$PrivateInnerClass.class

When run through JDT in eclipse, it produces only 5 classes.

OuterClass.class
OuterClass$1.class
OuterClass$InnerClass.class
OuterClass$InnerClass2.class
OuterClass$InnerClass$InnerInnerClass.class
OuterClass$PrivateInnerClass.class

When decompiled, OuterClass$1.class contains nothing. Where is this extra class coming from and why is it created?

Answer 1

I don’t have the answer, but I’m able to confirm that, and reduce the snippet to the following:

public class OuterClass {
    private class PrivateInnerClass {
    }
    public void instantiate() {
        new PrivateInnerClass();
    }
}

This creates OuterClass$1.class

Compiled from "OuterClass.java"
class OuterClass$1 extends java.lang.Object{
}

And here’s javap -c for OuterClass.class:

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

public void instantiate();
  Code:
   0:   new     #2; //class OuterClass$PrivateInnerClass
   3:   dup
   4:   aload_0
   5:   aconst_null
   6:   invokespecial #3; //Method OuterClass$PrivateInnerClass."<init>":
                          //(LOuterClass;LOuterClass$1;)V
   9:   pop
   10:  return

}

And for OuterClass$PrivateInnerClass:

Compiled from "OuterClass.java"
class OuterClass$PrivateInnerClass extends java.lang.Object{
final OuterClass this$0;

OuterClass$PrivateInnerClass(OuterClass, OuterClass$1);
  Code:
   0:   aload_0
   1:   aload_1
   2:   invokespecial   #1; //Method "<init>":(LOuterClass;)V
   5:   return

}

As you can see, the synthesized constructor takes an OuterClass$1 argument.

So javac creates the default constructor to take an extra argument, of type $1, and the value of that default argument is 5: aconst_null.

---

I’ve found that $1 doesn’t get created if either of the following is true:

• You make public class PrivateInnerClass
• You declare a nullary constructor for PrivateInnerClass
• Or you don’t call the new on it
• Probably other things (e.g. static nested, etc).

---

Possibly related

• Bug ID:4295934: Compiling a private inner class creates an anonymous class file in the wrong dir

Create the following source in a directory called test:

package test;

public class testClass
{
    private class Inner
    {
    }
    public testClass()
    {
        Inner in = new Inner();
    }
}

Compile the file from the parent directory javac test/testClass.java

Notice that the file testClass$1.class is created in the current directory.
Not sure why this file is even created since there is also a test/testClass$Inner.class created as well.

EVALUATION

The testClass$1.class file is for a dummy class needed by an "access
constructor" for the private constructor of the private inner class
testClass$Inner. Dissassembly shows that the fully-qualified name of
this class is correctly noted, so it is unclear why the class file ends
up in the wrong directory.