/* an Operation is a command called execute mapping Entry 
   to String */

interface Operation {
     String execute(Entry e);
}

class NameOp implements Operation {
    String execute(Entry e) { return e.getName(); }
}

class AddressOp implements Operation {
    String execute(Entry e) { return e.getAddress(); }
}

class PhoneOp implements Operation {
    String execute(Entry e) { return e.getPhone(); }
}
/* a DeptDirectory is either:
   the empty directory new Empty(), or
   the non-empty directory new Cons(Entry,DeptDirectory)
*/

abstract class DeptDirectory {
  abstract String lookUp(String key, Operation m, Operation f);
    String NameToAddress(String key) { 
    return lookup(key, new NameOp, new AddressOp());
    }
    void static test() {
    DeptDirectory e = new Empty();
    DeptDirectory d1 = new Cons(new Entry("Corky","DH 3104","x 6042"), e);
    System.out.println(NameToAddress("Corky");
		       }    
    }

    class Empty extends DeptDirectory {
	String lookUp(String key, Operation m, Operation f) {
    return null;
	}
    }

    class Cons extends DeptDirectory {
  Entry first;
  DeptDirectory rest;

	/* constructor */
	Cons(Entry f, DeptDirectory r) {
    this.first = f;
    this.rest = r;
	}
 
	String lookUp(String key, Operation m, Operation f) {
    if (key.equals(m.execute(first)))
      return f.execute(first);
    else return rest.lookUp(key,m,f);
	}

	/* accessors */
	Entry getFirst() { return this.first; }
	DeptDirectory getRest() { return this.rest; } 
    }