Visitor Pattern

IntList.java

/** The functional list type defined by IntList := EmptyIntList + ConsIntList(int, IntList) */
abstract class IntList {

  /** Visitor hook */
  abstract public Object visit(IntListVisitor iv);
  
  /** Adds i to the front of this. */
  public ConsIntList cons(int i) { return new ConsIntList(i, this); }
  /** Returns the unique empty list. */
  public static EmptyIntList empty() { return EmptyIntList.ONLY; }
  
}
 
/** Concrete empty list structure containing nothing. */
class EmptyIntList extends IntList {
  
  /** Singleton binding */
  public static final EmptyIntList ONLY = new EmptyIntList();
  
  /** Single pattern constructor */
  private EmptyIntList() { }
  
  /** Visitor hook */
  public Object visit(IntListVisitor v) { return  v.forEmptyIntList(this); }
  
}
 
/** Concrete non-empty list structure containing an int, called first, and an IntList called rest. */
class ConsIntList extends IntList {
  
  /** Cons node fields */
  int first;
  IntList rest;
  
  /** Data constructor for ConsIntList */
  ConsIntList(int f, IntList r) { first = f; rest = r; }
  
  /** Accessors */
  public int first() { return first; }
  public IntList rest() { return rest; }
  
  public Object visit(IntListVisitor v) { return v.forConsIntList(this); }
  
}  

/** Visitor interface for the IntList type. */
interface IntListVisitor {
  abstract Object forEmptyIntList(EmptyIntList host);
  abstract Object forConsIntList(ConsIntList host);
}

/**

IntListVisitor class that implements the length() method on IntLists.

length :: [Int] -> Int
length [] = 0
length xs = 1 + length (tail xs)

 */
 class LengthVisitor implements IntListVisitor {
  /* Singleton binding for LengthVisior because the method takes no parameters */
  public static final LengthVisitor ONLY = new LengthVisitor();  
  private LengthVisitor() { }
  
  public Object forEmptyIntList(EmptyIntList el) { return 0; }
  public Object forConsIntList(ConsIntList cil) { return 1 + (Integer) cil.rest().visit(this); }
}