A basic priority linked list: maintaining an individual LinkedList for each priority level. : LinkedList « Collections « Java Tutorial

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import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;

/**
 * A basic priority linked list
 * 
 * It implements this by maintaining an individual LinkedList for each priority
 * level.
 * 
 * @author <a href="mailto:tim.fox@jboss.com>Tim Fox</a>
 * @version <tt>$Revision: 1174 $</tt>
 * 
 * $Id: BasicPrioritizedDeque.java 1174 2006-08-02 14:14:32Z timfox $
 */
public class BasicPriorityLinkedList {

  protected LinkedList[] linkedLists;

  protected int priorities;

  protected int size;

  public BasicPriorityLinkedList(int priorities) {
    this.priorities = priorities;

    initDeques();
  }

  public void addFirst(Object obj, int priority) {
    linkedLists[priority].addFirst(obj);

    size++;
  }

  public void addLast(Object obj, int priority) {
    linkedLists[priority].addLast(obj);

    size++;
  }

  public Object removeFirst() {
    Object obj = null;

    // Initially we are just using a simple prioritization algorithm:
    // Highest priority refs always get returned first.
    // This could cause starvation of lower priority refs.

    // TODO - A better prioritization algorithm

    for (int i = priorities - 1; i >= 0; i--) {
      LinkedList ll = linkedLists[i];

      if (!ll.isEmpty()) {
        obj = ll.removeFirst();
        break;
      }

    }

    if (obj != null) {
      size--;
    }

    return obj;
  }

  public Object removeLast() {
    Object obj = null;

    // Initially we are just using a simple prioritization algorithm:
    // Lowest priority refs always get returned first.

    // TODO - A better prioritization algorithm

    for (int i = 0; i < priorities; i++) {
      LinkedList ll = linkedLists[i];
      if (!ll.isEmpty()) {
        obj = ll.removeLast();
      }
      if (obj != null) {
        break;
      }
    }

    if (obj != null) {
      size--;
    }

    return obj;
  }

  public Object peekFirst() {
    Object obj = null;

    // Initially we are just using a simple prioritization algorithm:
    // Highest priority refs always get returned first.
    // This could cause starvation of lower priority refs.

    // TODO - A better prioritization algorithm

    for (int i = priorities - 1; i >= 0; i--) {
      LinkedList ll = linkedLists[i];
      if (!ll.isEmpty()) {
        obj = ll.getFirst();
      }
      if (obj != null) {
        break;
      }
    }

    return obj;
  }

  public List getAll() {
    List all = new ArrayList();
    for (int i = priorities - 1; i >= 0; i--) {
      LinkedList deque = linkedLists[i];
      all.addAll(deque);
    }
    return all;
  }

  public void clear() {
    initDeques();
  }

  public int size() {
    return size;
  }

  public boolean isEmpty() {
    return size == 0;
  }

  public ListIterator iterator() {
    return new PriorityLinkedListIterator(linkedLists);
  }

  protected void initDeques() {
    linkedLists = new LinkedList[priorities];
    for (int i = 0; i < priorities; i++) {
      linkedLists[i] = new LinkedList();
    }

    size = 0;
  }

  class PriorityLinkedListIterator implements ListIterator {
    private LinkedList[] lists;

    private int index;

    private ListIterator currentIter;

    PriorityLinkedListIterator(LinkedList[] lists) {
      this.lists = lists;

      index = lists.length - 1;

      currentIter = lists[index].listIterator();
    }

    public void add(Object arg0) {
      throw new UnsupportedOperationException();
    }

    public boolean hasNext() {
      if (currentIter.hasNext()) {
        return true;
      }
      while (index >= 0) {
        if (index == 0 || currentIter.hasNext()) {
          break;
        }
        index--;
        currentIter = lists[index].listIterator();
      }
      return currentIter.hasNext();
    }

    public boolean hasPrevious() {
      throw new UnsupportedOperationException();
    }

    public Object next() {
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      return currentIter.next();
    }

    public int nextIndex() {
      throw new UnsupportedOperationException();
    }

    public Object previous() {
      throw new UnsupportedOperationException();
    }

    public int previousIndex() {
      throw new UnsupportedOperationException();
    }

    public void remove() {
      currentIter.remove();

      size--;
    }

    public void set(Object obj) {
      throw new UnsupportedOperationException();
    }
  }

}