Constructors invocation

Question

Hello,
Here is a problem I've experienced: The constructor of the abstract class is calling the abstract createComponent() method that is implemented in the child class.
The child class can initialize the Component in the constructor. But, this constructor is executed after the abstract class constructor! So it does add(null) . What is the solution.
Here is the simpified code:
abstract class AbtractPanel extends JPanel
  AbstractPanel()
    add(createComponent());
  abstract protected void createComponent();
class MyPanel extends AbstractPanel
  JButton button;
  MyPanel(JButton b)
    button = b;
  protected void createComponent()
    return button;
  }

Answer 1

Hello,
Here is a problem I've experienced: The constructor of
the abstract class is calling the abstract
createComponent() method that is implemented in the
child class.
The child class can initialize the Component in the
constructor. But, this constructor is executed
after the abstract class constructor! So it
does add(null) . What is the
solution.
Here is the simpified code:
abstract class AbtractPanel extends JPanel
AbstractPanel()
add(createComponent());
abstract protected void createComponent();
class MyPanel extends AbstractPanel
JButton button;
MyPanel(JButton b)
button = b;
protected void createComponent()
return button;
First of all the constructor AbstractPanel() will always be called FIRST from ANY of its subclasses. Therefore the construction must take place in the createComponent method NOT the MyPanel constructor. For instance
MyPanel(JButton b)
   super();
button = b;
}Note the call super() which is the first thing called by the MyPanel constructor. This is done automatically by java and you can NOT stop it. This call equates to calling the AbstractPanel() constructor which in turn calls the createComponent() method of MyPanel. This all takes place BEFORE button = b; So when createComponent is called, button == null.
The basic way to solve this is to add another constructor to the AbstractPanel class
abstract class AbtractPanel extends JPanel
AbstractPanel()
add(createComponent());
AbstractPanel(JButton b)
  add(b);
abstract protected void createComponent();
class MyPanel extends AbstractPanel
JButton button;
MyPanel(JButton b)
super(b);
}There are actually other ways to solve this if I knew your objective.