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

interface Operation {
     String execute(Entry e);
}

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

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

class PhoneOp implements Operation {
    public 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());
  }
  String PhoneToName(String key) {
    return lookUp(key, new PhoneOp(), new NameOp());
  }
  static void test() {
    DeptDirectory e = new Empty();
    DeptDirectory d1 = new Cons(new Entry("Corky","DH 3104","x 6042"), e);
    DeptDirectory d2 = new Cons(new Entry("Matthias", "DH 3106", "x 5732"), d1);
    DeptDirectory d3 = new Cons(new Entry("Paul", "DH 3109", "x 3720"), d2);
    System.out.println(d1.NameToAddress("Paul"));
    System.out.println(d1.PhoneToName("DH 3104"));
  }    
  public static void main(String[] args) { test(); }
}

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; } 
}

class Entry {

  /* fields */
  private String name;	
  private String address;
  private String phone;

  /* constructor */
  Entry(String n, String a, String p) {
    this.name = n;
    this.address = a;
    this.phone = p;
  }	 	 

  /* accessors */
  String getName() { return this.name; }
  String getAddress() { return this.address; }
  String getPhone() { return this.phone; }
}