linkedlist.java

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/*
 * @(#)LinkedList.java	1.46 03/01/23
 *
 * Copyright 2003 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util;

/**
 * Linked list implementation of the <tt>List</tt> interface.  Implements all
 * optional list operations, and permits all elements (including
 * <tt>null</tt>).  In addition to implementing the <tt>List</tt> interface,
 * the <tt>LinkedList</tt> class provides uniformly named methods to
 * <tt>get</tt>, <tt>remove</tt> and <tt>insert</tt> an element at the
 * beginning and end of the list.  These operations allow linked lists to be
 * used as a stack, queue, or double-ended queue (deque).<p>
 *
 * All of the stack/queue/deque operations could be easily recast in terms of
 * the standard list operations.  They're included here primarily for
 * convenience, though they may run slightly faster than the equivalent List
 * operations.<p>
 *
 * All of the operations perform as could be expected for a doubly-linked
 * list.  Operations that index into the list will traverse the list from
 * the begining or the end, whichever is closer to the specified index.<p>
 *
 * <b>Note that this implementation is not synchronized.</b> If multiple
 * threads access a list concurrently, and at least one of the threads
 * modifies the list structurally, it <i>must</i> be synchronized
 * externally.  (A structural modification is any operation that adds or
 * deletes one or more elements; merely setting the value of an element is not
 * a structural modification.)  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the list.  If no
 * such object exists, the list should be "wrapped" using the
 * Collections.synchronizedList method.  This is best done at creation time,
 * to prevent accidental unsynchronized access to the list: <pre>
 *     List list = Collections.synchronizedList(new LinkedList(...));
 * </pre><p>
 *
 * The iterators returned by the this class's <tt>iterator</tt> and
 * <tt>listIterator</tt> methods are <i>fail-fast</i>: if the list is
 * structurally modified at any time after the iterator is created, in any way
 * except through the Iterator's own <tt>remove</tt> or <tt>add</tt> methods,
 * the iterator will throw a <tt>ConcurrentModificationException</tt>.  Thus,
 * in the face of concurrent modification, the iterator fails quickly and
 * cleanly, rather than risking arbitrary, non-deterministic behavior at an
 * undetermined time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. 
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:   <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i><p>
 *
 * This class is a member of the 
 * <a href="{@docRoot}/../guide/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @version 1.46, 01/23/03 
 * @see	    List
 * @see	    ArrayList
 * @see	    Vector
 * @see	    Collections#synchronizedList(List)
 * @since 1.2
 */

public class LinkedList extends AbstractSequentialList
		        implements List, Cloneable, java.io.Serializable
{
    private transient Entry header = new Entry(null, null, null);
    private transient int size = 0;

    /**
     * Constructs an empty list.
     */
    public LinkedList() {
        header.next = header.previous = header;
    }

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param  c the collection whose elements are to be placed into this list.
     * @throws NullPointerException if the specified collection is null.
     */
     public LinkedList(Collection c) {
	 this();
	 addAll(c);
     }

    /**
     * Returns the first element in this list.
     *
     * @return the first element in this list.
     * @throws    NoSuchElementException if this list is empty.
     */
    public Object getFirst() {
	if (size==0)
	    throw new NoSuchElementException();

	return header.next.element;
    }

    /**
     * Returns the last element in this list.
     *
     * @return the last element in this list.
     * @throws    NoSuchElementException if this list is empty.
     */
    public Object getLast()  {
	if (size==0)
	    throw new NoSuchElementException();

	return header.previous.element;
    }

    /**
     * Removes and returns the first element from this list.
     *
     * @return the first element from this list.
     * @throws    NoSuchElementException if this list is empty.
     */
    public Object removeFirst() {
	Object first = header.next.element;
	remove(header.next);
	return first;
    }

    /**
     * Removes and returns the last element from this list.
     *
     * @return the last element from this list.
     * @throws    NoSuchElementException if this list is empty.
     */
    public Object removeLast() {
	Object last = header.previous.element;
	remove(header.previous);
	return last;
    }

    /**
     * Inserts the given element at the beginning of this list.
     * 
     * @param o the element to be inserted at the beginning of this list.
     */
    public void addFirst(Object o) {
	addBefore(o, header.next);
    }

    /**
     * Appends the given element to the end of this list.  (Identical in
     * function to the <tt>add</tt> method; included only for consistency.)
     * 
     * @param o the element to be inserted at the end of this list.
     */
    public void addLast(Object o) {
	addBefore(o, header);
    }

    /**
     * Returns <tt>true</tt> if this list contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this list contains
     * at least one element <tt>e</tt> such that <tt>(o==null ? e==null
     * : o.equals(e))</tt>.
     *
     * @param o element whose presence in this list is to be tested.
     * @return <tt>true</tt> if this list contains the specified element.
     */
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

    /**
     * Returns the number of elements in this list.
     *
     * @return the number of elements in this list.
     */
    public int size() {
	return size;
    }

    /**
     * Appends the specified element to the end of this list.
     *
     * @param o element to be appended to this list.
     * @return <tt>true</tt> (as per the general contract of
     * <tt>Collection.add</tt>).
     */
    public boolean add(Object o) {
	addBefore(o, header);
        return true;
    }

    /**
     * Removes the first occurrence of the specified element in this list.  If
     * the list does not contain the element, it is unchanged.  More formally,
     * removes the element with the lowest index <tt>i</tt> such that
     * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt> (if such an
     * element exists).
     *
     * @param o element to be removed from this list, if present.
     * @return <tt>true</tt> if the list contained the specified element.
     */
    public boolean remove(Object o) {
        if (o==null) {
            for (Entry e = header.next; e != header; e = e.next) {
                if (e.element==null) {
                    remove(e);
                    return true;
                }
            }
        } else {
            for (Entry e = header.next; e != header; e = e.next) {
                if (o.equals(e.element)) {
                    remove(e);
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the specified
     * collection's iterator.  The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in
     * progress.  (This implies that the behavior of this call is undefined if
     * the specified Collection is this list, and this list is nonempty.)
     *
     * @param c the elements to be inserted into this list.
     * @return <tt>true</tt> if this list changed as a result of the call.
     * @throws NullPointerException if the specified collection is null.
     */
    public boolean addAll(Collection c) {
        return addAll(size, c);
    }

    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in the list in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param index index at which to insert first element
     *		    from the specified collection.
     * @param c elements to be inserted into this list.
     * @return <tt>true</tt> if this list changed as a result of the call.
     * @throws IndexOutOfBoundsException if the specified index is out of
     *            range (<tt>index &lt; 0 || index &gt; size()</tt>).
     * @throws NullPointerException if the specified collection is null.
     */
    public boolean addAll(int index, Collection c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew==0)
            return false;
	modCount++;

        Entry successor = (index==size ? header : entry(index));
        Entry predecessor = successor.previous;
	for (int i=0; i<numNew; i++) {
            Entry e = new Entry(a[i], successor, predecessor);
            predecessor.next = e;
            predecessor = e;
        }
        successor.previous = predecessor;

        size += numNew;
        return true;
    }

    /**
     * Removes all of the elements from this list.
     */
    public void clear() {
	modCount++;
        header.next = header.previous = header;
	size = 0;
    }


    // Positional Access Operations

    /**
     * Returns the element at the specified position in this list.
     *
     * @param index index of element to return.
     * @return the element at the specified position in this list.
     * 
     * @throws IndexOutOfBoundsException if the specified index is is out of
     * range (<tt>index &lt; 0 || index &gt;= size()</tt>).
     */
    public Object get(int index) {
        return entry(index).element;
    }

    /**
     * Replaces the element at the specified position in this list with the
     * specified element.
     *
     * @param index index of element to replace.
     * @param element element to be stored at the specified position.
     * @return the element previously at the specified position.
     * @throws IndexOutOfBoundsException if the specified index is out of
     *		  range (<tt>index &lt; 0 || index &gt;= size()</tt>).
     */
    public Object set(int index, Object element) {
        Entry e = entry(index);
        Object oldVal = e.element;
        e.element = element;
        return oldVal;
    }

    /**
     * Inserts the specified element at the specified position in this list.
     * Shifts the element currently at that position (if any) and any
     * subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted.
     * @param element element to be inserted.
     * 
     * @throws IndexOutOfBoundsException if the specified index is out of
     *		  range (<tt>index &lt; 0 || index &gt; size()</tt>).
     */
    public void add(int index, Object element) {
        addBefore(element, (index==size ? header : entry(index)));
    }

    /**
     * Removes the element at the specified position in this list.  Shifts any
     * subsequent elements to the left (subtracts one from their indices).
     * Returns the element that was removed from the list.
     *
     * @param index the index of the element to removed.
     * @return the element previously at the specified position.
     * 
     * @throws IndexOutOfBoundsException if the specified index is out of
     * 		  range (<tt>index &lt; 0 || index &gt;= size()</tt>).
     */
    public Object remove(int index) {
        Entry e = entry(index);
        remove(e);
        return e.element;
    }

    /**