Parent constructor calls abstract method

Question

Hi everybody!
I'm wondering if there is something wrong with java or if the idea is just too ill?
Anyway, I think it would be great if this hierachy would work...
Two classes A and B.
Class A defines an astract method.
In A's constructor this abstract method is called.
Class B extends A and provides an implementation for the abstract method in A.
Class B also defines a member variable that is set in B's implementation of the abstract method.
In class' B constructor the parent constructor A() is called.
example:
public abstract class A {
  public A() {
    createComponents();
  public abstract void createComponents();
public class B extends A {
  private String string = null;
  public B() {
    super();
    System.out.println("B::B() " + string);
  public void createComponents() {
    System.out.println("B::createComponents() begin");
    string = new String("test");
    System.out.println("B::createComponents() " + string);
  public void describe() {
    System.out.println("B::describe() " + string);
  public static void main(String[] args) {
    B b = new B();
    b.describe();
}running the code above produces the following output:
B::createComponents() begin
B::createComponents() test
B::B() null
B::describe() null
why is the string member variable null in B's constructor??
thanks in advance
Peter Bachl
Polytechnic University of Upper Austria, Hagenberg
[email protected]

Answer 1

The answer is that the call of the super-constructor
is allways done before the initialization
of the member variable. That's all and that's the
normal behavior.
order :
- initialization of static variables
- call to the super-constructor
- initialization of the instance variables
- execution of the constructor
Since this is the advanced forum it is relevant to point out that that is not exactly true.
There is a step in java that 'initializes' member variables before any constructors are called, super or other wise.
From the JLS 12.5...
Otherwise, all the instance variables in the new object, including those declared in superclasses, are initialized to their default values (4.5.5)
Using the following code as an example
  class MyClass
     int i1;
     int i2 = 1;
.When creating an instance of the above there will be three 'initializations'
// Part of object creation...
i1 = 0; // The default value
i2 = 0; // The default value
// Part of construction...after super ctors called.
i2 = 1; // The variable initializer (see step 4 of JLS 12.5)
Unfortunately the descriptions are rather similar and so confusion can result as to when the assignment actually occurs.