Good programming practice - Abstract class

Question

Hi all,
I have been trying to help another soul in this forum, and came to the conclusion that I don't know good
programming practice when it comes to abstract classes.
Is this correct?
You CAN implement methods in an abstract class, but it's not recommended.
I have NEVER done this...when is there possibly a need to?
Regards.
/ k

Answer 1

Yes, absolutely, you can implement methods in an abstract class. Any method that all subclasses will perform in the same way can be implemented in the abstract base class. If subclasses perform similiar functions depending on their type you declare those as abstract in the base class. Here is a contrived example that I have seen on job interviews.
Suppose your developing an application that draws on a panel. We want to provide some canned shapes such as a circle, a square and a triangle. We want to be able to draw the shape set or get its color and calculate its area.
Let's define an abstract base class Shape
public abstract class Shape{
    private Color myColor;
   //  since color has nothing to do with what kind of shape we're working with, create concrete implementation
   public Color getColor(){
        return myColor;
public void setColor(Color newColor){
   myColor = newColor;
// however, drawing the shape and calculation its area are depending on the actual shape.
public abstract void draw();
public abstract double getArea();
// so then Square would be something like
public class Square extends Shape{
   public double get Area()
      return sideLength * sideLength  // assumes somehow we know sideLength
public void draw(){
              // concrete implementation
}we can do the same things for Circle class and Triangle class.
And, if you think about it you'll notice that we could have made a Rectangle class and then Square would be a subclass of Rectangle where both dimensions are equal.
I hope that somewhat strained example helps answer your question.
DB