Re: Avoiding NPEs caused by indirect call

On Sun, 3 Aug 2008, Royan wrote:


As others have pointed out, there isn't a magic solution here. If you have
methods in the subclass which need an action to have been done in the
subclass constructor before they can be called, then you can't call them
from the superclass constructor.

Possible solutions:

- remove the dependency on the action, eg with an if
   (propertyChangeSupport != null) guard clause

- move the action to the superclass constructor: if you can make
   propertyChangeSupport a field in AbstractModel, and have it set in the
   AbstractModel constructor set it, you're fine - you could still have the
   subclass supply the value:

abstract class AbstractModel {
     protected final PropertyChangeSupport propertyChangeSupport;

     public AbstractModel(PropertyChangeSupport propertyChangeSupport) {
         this.propertyChangeSupport = propertyChangeSupport ;
     }
}

public class Model extends AbstractModel {
     public Model (Object source) {
  super(new PropertyChangeSupport(source)) ;
     }
}

- don't call the method - but then i don't know how you do the setting of
   the value

- call the method from some other place; i got mildly flamed (and quite
   fairly so!) for suggesting this recently, but:

abstract class AbstractModel {
     public AbstractModel() {
         // do nothing
     }
     protected void init() {
         setSomeValue(new Integer(1));
     }
     public void setSomeValue(Integer value) {
         firePropertyChange("someProperty", null, value);
     }
     protected void firePropertyChange(String propertyName, Object oldValue, Object newValue) {
         // some code
     }
}

public class Model extends AbstractModel {
     private final PropertyChangeSupport propertyChangeSupport;

     public Model (Object source) {
         propertyChangeSupport = new PropertyChangeSupport(source);
  init() ;
     }
     @Override
     public void firePropertyChange(String propertyName, Object oldValue, Object newValue) {
         propertyChangeSupport.firePropertyChange(propertyName, oldValue, newValue);
     }
}

tom

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