Visitor Design Pattern in Java: Double dispatch (within a single hierarchy)

Discussion. We would like to declare a function like: void process( virtual Base object1, virtual Base object2 ) that does the right thing based on the type of 2 objects that come from a single inheritance hierarchy. The only problem is that the keyword “virtual” may not be used to request dynamic binding for an object being passed as an argument. Java will only “discriminate” the type of an object being messaged, not the type of an object being passed.

So in order for the type of 2 objects to be discriminated, each object must be the receiver of a virtual function call. Here, when process1() is called on the first object, its type becomes “known” at runtime, but the type of the second is still UNknown. process2() is then called on the second object, and the identity (and type) of the first object is passed as an argument. Flow of control has now been vectored to the spot where the type (and identity) of both objects are known.

public class VisitorSingle {

interface Base {
   void process1( Base secondObject );
   void process2(  A   firstObject  );
   void process2(  B   firstObject  );
   void process2(  C   firstObject  );
}

static class A implements Base {
   public void process1( Base second ) { second.process2( this ); }
   public void process2( A first ) {
      System.out.println( "first is A, second is A" ); }
   public void process2( B first ) {
      System.out.println( "first is B, second is A" ); }
   public void process2( C first ) {
      System.out.println( "first is C, second is A" ); }
}

static class B implements Base {
   public void process1( Base second ) { second.process2( this ); }
   public void process2( A first ) {
      System.out.println( "first is A, second is B" ); }
   public void process2( B first ) {
      System.out.println( "first is B, second is B" ); }
   public void process2( C first ) {
      System.out.println( "first is C, second is B" ); }
}

static class C implements Base {
   public void process1( Base second ) { second.process2( this ); }
   public void process2( A first ) {
      System.out.println( "first is A, second is C" ); }
   public void process2( B first ) {
      System.out.println( "first is B, second is C" ); }
   public void process2( C first ) {
      System.out.println( "first is C, second is C" ); }
}

public static void main( String[] args ) {
   Base array[] = { new A(), new B(), new C() };
   for (int i=0; i < array.length; i++)
      for (int j=0; j < 3; j++)
         array[i].process1( array[j] );
}}