abstract class constructors

[preeti khane]

Can an abstract class have constructors?

I would have thought no , as they cannot be instantiated , so there is no real purpose... Am I right?

[Marlene Miller]

Yes, an abstract class can have constructors.
An abstract class may have instance variables and non-abstract methods (common to all concrete subclasses) that operate on those variables. A constructor could be used to initialize the instance variables.
A constructor of a concrete subclass would invoke the superclass constructor to initialize the variables of the abstract superclass.

[Marlene Miller]

abstract class Graph {
   private ArrayList nodes;
   private ArrayList edges;
   
   Graph() {
      nodes = new ArrayList();
      edges = new ArrayList();
   }
   //   
   //all kinds of methods that use nodes and edges
   //
   abstract Edge[] getEdgePrototypes();
   abstract Node[] getNodePrototypes();
}
//
class SimpleGraph extends Graph
{
   Edge[] getEdgePrototypes() {
      return ...
   }
   //
   Node[] getNodePrototypes() {
      return...
   }
}

Graph is part of a framework. It supplies nearly all of the methods to support a generic graph editor.
SimpleGraph is a client-defined extension that supplies the geometric shapes of the nodes and edges to be used in the graph.

Graph is an abstract class. It needs a constructor to initialize the variables to do its work.

(From Cay Horstmann�s Object Oriented Design & Patterns)
[ May 27, 2003: Message edited by: Marlene Miller ]