Java Primer Plus/Java Primer Plus (Waite Group Proess)(1996).iso/chapter17/linkedlist.java

Text File | 1995-12-31 | 6KB | 269 lines

import java.util.Enumeration;

/* Linked List class */
class linkedList {
    linkedListEntry beginning = null;
    linkedListEntry end = null;
    int numelements;

    linkedList() {}

/* Append a node to the end of the list */
 public void append(Object someobject) {

  if (someobject == null)
    throw new NullPointerException();

  linkedListEntry newelement = new linkedListEntry(someobject);
  numelements++;

 /* Handle if list happens to be empty */
  if (end == null) { end = newelement;
             beginning = newelement;
           }
  else {
    end.next = newelement;
    newelement.prev = end;
    end = newelement;
   }


   }

/* Insert after a specified object */
 public void insertAfter(Object afterobject,Object someobject) {

  if ((someobject == null) || (afterobject == null))
    throw new NullPointerException();

/* Find this object to insert after */
  linkedListEntry afterentry = getref(afterobject);

/* uhoh...trouble, object wasn't on the list */
  if (afterentry == null)
    throw new NullPointerException();

  numelements++;
  linkedListEntry newelement = new linkedListEntry(someobject);
  newelement.next = afterentry.next;
  newelement.prev = afterentry;

  if (afterentry == end) end = newelement;
  else afterentry.next.prev = newelement;

  afterentry.next = newelement;

 }


/* Insert before a specified object */
 public void insertBefore(Object beforeobject,Object someobject) {

  if ((someobject == null) || (beforeobject == null))
    throw new NullPointerException();

/* Find the object to insert before */
  linkedListEntry beforeentry = getref(beforeobject);

  if (beforeentry == null)
    throw new NullPointerException();

  numelements++;
  linkedListEntry newelement = new linkedListEntry(someobject);
  newelement.next = beforeentry;
  newelement.prev = beforeentry.prev;

  if (beforeentry == beginning) beginning = newelement;
  else beforeentry.prev.next = newelement;

  beforeentry.prev = newelement;

 }

/* Delete a node */
 public void delete(Object someobject) throws
            NullPointerException {

  if (someobject == null)
    throw new NullPointerException();

  linkedListEntry delentry = getref(someobject);

  if (delentry == null)
    throw new NullPointerException();

  numelements--;

 /* Reroute the pointers around this object */
 /* (let the garbage collector come along and actually destroy it */
  if (delentry != beginning) delentry.prev.next = delentry.next;
  else beginning = delentry.next;


  if (delentry != end) delentry.next.prev = delentry.prev;
  else end = delentry.prev;
 }



 public Object previous(Object someobject) {

  if (someobject == null)
    throw new NullPointerException();

  linkedListEntry preventry = getref(someobject);

  if (preventry == null)
    throw new NullPointerException();

  return preventry.prev.node;
 }

/* return the next object in the list */
 public Object next(Object someobject) {

  if (someobject == null)
    throw new NullPointerException();

  linkedListEntry nextentry = getref(someobject);

  if (nextentry == null)
    throw new NullPointerException();

  return nextentry.next.node;
 }



/* Find an entry in the list */
 private linkedListEntry getref(Object someobject) {

  if (beginning == null) return null;

  linkedListEntry runner = beginning;

  do {
   if (runner.node == someobject) return runner;
   runner = runner.next;
     } while (runner != null);

  return null;
  }

/* does a given node exist?? */
 public boolean isPresent(Object someobject) {

  if (someobject == null)
    throw new NullPointerException();

  return (getref(someobject) == null) ? false : true;
 }

/* Setup the enumeration */
public synchronized Enumeration elements() {
    return new linkedListEnumerator(this);
    }

}



/* Linked list enumerator class */
class linkedListEnumerator implements Enumeration {

 linkedListEntry position;

 linkedListEnumerator(linkedList L) {
    position = L.beginning;
    }

/* standard enumeration method */
 public boolean hasMoreElements() {
    return (position != null ) ? true : false;
    }

/* return next */
 public Object nextElement() {

    if (position == null) throw new NullPointerException();

    linkedListEntry retpos = position;

    position = position.next;

    return retpos.node;
    }
}





/* Linked List Entry class - i.e. a "node" in the list */
class linkedListEntry {

 Object  node = null;
 linkedListEntry  next = null;
 linkedListEntry  prev = null;

 linkedListEntry(Object someobject) {
    node = someobject;
    }



 }



/* Test classes */
class listme {

    String valname;
    listme(String v) {
        valname = v; }
    public void printmyval() {
        System.out.println(valname);
        }
    }


/* Test the classes out */
class test {

 static public void main(String args[]) {

    linkedList theList = new linkedList();
    listme T = new listme("five");
    listme S = new listme("three");
    listme U = new listme("two");
    listme V = new listme("seven");
    listme X = new listme("four");
    listme Y = new listme("six");
    theList.append(T);

    theList.append(S);
    theList.append(U);
    theList.insertAfter(T,V);
    theList.insertBefore(T,X);
    theList.append(Y);

    theList.delete(Y);

System.out.println("Done Addin");
T = (listme)theList.next(X);
T.printmyval();

System.out.println();
System.out.println();

 Enumeration E = theList.elements();

 while (E.hasMoreElements()) {
    T = (listme)E.nextElement();
    T.printmyval();
    }

    }
  }