Description Luc Bourlier 2002-10-04 12:34:54 EDT

I get a problem with the compiled code, but I can't really figure out if it's a
compiler bug or a 'feature' of java.

The execution of the following code generates a NullPointerException in a method
added by the compiler.

public class Test {
  abstract class Super {
    public Super() {
      hop();
    }
    abstract void hop();
  }

  class Inner extends Super {
    public Inner() {
      super();
    }
    public void hop() {
      System.out.println(fField);
    }
  }
  
  private Object fField;
  public static void main(String[] args) {
    Test3 test= new Test3("foo");
    Inner inn= test.new Inner();
    System.out.println("Test ...");
  }
  public Test3(Object field) {
    fField= field;
  }
}

The exception stack trace :

Exception in thread "main" java.lang.NullPointerException
  at Test3.access$0(Test3.java:23)
  at Test3$Inner.hop(Test3.java:19)
  at Test3$Super.<init>(Test3.java:6)
  at Test3$Inner.<init>(Test3.java:15)
  at Test3.main(Test3.java:27)

The problem is : the constructor of Inner calls the constructor of Super which
calls hop() on 'this' instance. By polymorphism, Inner#hop() is called. In
Inner#hop(), we try to access to fField, which in a field of Test. For get the
value of this field, the method access$0(..) is called. This method returns the
enclosing instance of the given object, the field this$0 of the given object.
But in this case, this field is not yet initialised. So the methode returns
null. For get the value of fField, a getField instruction is executed on the
return value. As the value is null, the VM throws an exception.

The question is: does this code is correct, and the compiler should manages the
case ? or is it a weakness in java ?

I tried to get an answer from the java specs, but I didn't find any good
information.
In the java lang spec, at
http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.html#229267,
there is something about anonymous classes in constructors, but it's not really
relevant.
The VM spec says to look at
http://java.sun.com/products/jdk/1.1/docs/guide/innerclasses/spec/innerclasses.doc.html
In this document, there is one note (end of the first bloc in 
http://java.sun.com/products/jdk/1.1/docs/guide/innerclasses/spec/innerclasses.doc2.html)
which says that some implementations delay the initialisation of this$0 after
the call of the super constructor, so the problem can occurs.

But I didn't find anything saying that the initialisation of this$0 can't be
done before the execution of the super constructor.


For info, javac from the Sun 1.4.1 sdk generates the same type of code and
throws the same NPE.