062bFollow-Up and Happy New Year!

Author: Dr. Heinz M. KabutzDate: 2002-12-31Java Version: 1.4Category: Language

Abstract: Java 1.4 changed inner classes slighty to set the link to the outer object prior to calling super(). This avoids some hard to explain NullPointerExceptions.

First of all, I wish you all the best for the year 2003!

Two of our readers, Dimitris Andreou from Greece and another by the name of Snow, pointed out that in JDK 1.4 the NullPointerException has been fixed. Have a look at Sun's website. Try compile the following class with the -target 1.4 option:

public class NestedBug3 {
  private Integer wings = new Integer(2);
  public NestedBug3() {
    new ComplexBug();
  }
  private class ComplexBug extends Insect {
    ComplexBug() {
      System.out.println("Inside ComplexBug Constructor");
    }
    public void printDetails() {
      System.out.println(wings);
    }
  }
  public static void main(String[] arguments) {
    new NestedBug3();
  }
}

When you compile this with -target 1.4 you will only be able to run it with a JVM 1.4 and later. Here is the output:

Inside Insect() Constructor
2
Inside ComplexBug Constructor

What happens in the inner class? Let's have a quick look:

jad -noinner NestedBug3$ComplexBug.class

class NestedBug3$ComplexBug extends Insect {
  NestedBug3$ComplexBug(NestedBug3 nestedbug3) {
    this$0 = nestedbug3;
    super();
    System.out.println("Inside ComplexBug Constructor");      
  }
  public void printDetails() {
    System.out.println(NestedBug3.access$000(this$0));
  }
  private final NestedBug3 this$0; /* synthetic field */
}

You can seen that the assignment of this$0 and the call to super() have been swapped around.

In case you were wondering where to get hold of JAD, I wrote to the author and he told me to tell you to look at JAD. I want to hereby publicly thank Pavel Kuznetsov for making this great tool freely available. I have learnt more from this tool than from any other Java tool.

In case you were wondering why I am spending my New Year's Party writing a newsletter, I just want to get this done quickly so that I can put the follow-up into the 2002 folder ;-)

Heinz

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