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Java LinkedList source code

September 2, 2016 Author: david

/*
 * Copyright 1997-2006 Sun Microsystems, Inc.  All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Sun designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Sun in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 */

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, {@linkplain Queue queue}, or {@linkplain Deque
 * double-ended queue}. <p>
 *
 * The class implements the <tt>Deque</tt> interface, providing
 * first-in-first-out queue operations for <tt>add</tt>,
 * <tt>poll</tt>, along with other stack and deque 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 beginning or the end, whichever is closer to the specified index.<p>
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access a linked 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
 * {@link Collections#synchronizedList 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 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 {@link
 * ConcurrentModificationException}.  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}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @see     List
 * @see     ArrayList
 * @see     Vector
 * @since 1.2
 * @param <E> the type of elements held in this collection
 */

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    private transient Entry<E> header = new Entry<E>(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<? extends E> 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 E 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 E 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 E removeFirst() {
        return remove(header.next);
    }

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

    /**
     * Inserts the specified element at the beginning of this list.
     *
     * @param e the element to add
     */
    public void addFirst(E e) {
        addBefore(e, header.next);
    }

    /**
     * Appends the specified element to the end of this list.
     *
     * <p>This method is equivalent to {@link #add}.
     *
     * @param e the element to add
     */
    public void addLast(E e) {
        addBefore(e, 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&nbsp;?&nbsp;e==null&nbsp;:&nbsp;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.
     *
     * <p>This method is equivalent to {@link #addLast}.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        addBefore(e, header);
        return true;
    }

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If this 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&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     */
    public boolean remove(Object o) {
        if (o==null) {
            for (Entry<E> e = header.next; e != header; e = e.next) {
                if (e.element==null) {
                    remove(e);
                    return true;
                }
            }
        } else {
            for (Entry<E> 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.  (Note that this will occur if the specified collection is
     * this list, and it's nonempty.)
     *
     * @param c collection containing elements to be added to 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<? extends E> 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 the first element
     *              from the specified collection
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index+
                                                ", Size: "+size);
        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew==0)
            return false;
        modCount++;

        Entry<E> successor = (index==size ? header : entry(index));
        Entry<E> predecessor = successor.previous;
        for (int i=0; i<numNew; i++) {
            Entry<E> e = new Entry<E>((E)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() {
        Entry<E> e = header.next;
        while (e != header) {
            Entry<E> next = e.next;
            e.next = e.previous = null;
            e.element = null;
            e = next;
        }
        header.next = header.previous = header;
        size = 0;
        modCount++;
    }


    // Positional Access Operations

    /**
     * Returns the element at the specified position in this list.
     *
     * @param index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E 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 the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
        Entry<E> e = entry(index);
        E 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 {@inheritDoc}
     */
    public void add(int index, E 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 be removed
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        return remove(entry(index));
    }

    /**
     * Returns the indexed entry.
     */
    private Entry<E> entry(int index) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException("Index: "+index+
                                                ", Size: "+size);
        Entry<E> e = header;
        if (index < (size >> 1)) {
            for (int i = 0; i <= index; i++)
                e = e.next;
        } else {
            for (int i = size; i > index; i--)
                e = e.previous;
        }
        return e;
    }


    // Search Operations

    /**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the first occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     */
    public int indexOf(Object o) {
        int index = 0;
        if (o==null) {
            for (Entry e = header.next; e != header; e = e.next) {
                if (e.element==null)
                    return index;
                index++;
            }
        } else {
            for (Entry e = header.next; e != header; e = e.next) {
                if (o.equals(e.element))
                    return index;
                index++;
            }
        }
        return -1;
    }

    /**
     * Returns the index of the last occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the highest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the last occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     */
    public int lastIndexOf(Object o) {
        int index = size;
        if (o==null) {
            for (Entry e = header.previous; e != header; e = e.previous) {
                index--;
                if (e.element==null)
                    return index;
            }
        } else {
            for (Entry e = header.previous; e != header; e = e.previous) {
                index--;
                if (o.equals(e.element))
                    return index;
            }
        }
        return -1;
    }

    // Queue operations.

    /**
     * Retrieves, but does not remove, the head (first element) of this list.
     * @return the head of this list, or <tt>null</tt> if this list is empty
     * @since 1.5
     */
    public E peek() {
        if (size==0)
            return null;
        return getFirst();
    }

    /**
     * Retrieves, but does not remove, the head (first element) of this list.
     * @return the head of this list
     * @throws NoSuchElementException if this list is empty
     * @since 1.5
     */
    public E element() {
        return getFirst();
    }

    /**
     * Retrieves and removes the head (first element) of this list
     * @return the head of this list, or <tt>null</tt> if this list is empty
     * @since 1.5
     */
    public E poll() {
        if (size==0)
            return null;
        return removeFirst();
    }

    /**
     * Retrieves and removes the head (first element) of this list.
     *
     * @return the head of this list
     * @throws NoSuchElementException if this list is empty
     * @since 1.5
     */
    public E remove() {
        return removeFirst();
    }

    /**
     * Adds the specified element as the tail (last element) of this list.
     *
     * @param e the element to add
     * @return <tt>true</tt> (as specified by {@link Queue#offer})
     * @since 1.5
     */
    public boolean offer(E e) {
        return add(e);
    }

    // Deque operations
    /**
     * Inserts the specified element at the front of this list.
     *
     * @param e the element to insert
     * @return <tt>true</tt> (as specified by {@link Deque#offerFirst})
     * @since 1.6
     */
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }

    /**
     * Inserts the specified element at the end of this list.
     *
     * @param e the element to insert
     * @return <tt>true</tt> (as specified by {@link Deque#offerLast})
     * @since 1.6
     */
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }

    /**
     * Retrieves, but does not remove, the first element of this list,
     * or returns <tt>null</tt> if this list is empty.
     *
     * @return the first element of this list, or <tt>null</tt>
     *         if this list is empty
     * @since 1.6
     */
    public E peekFirst() {
        if (size==0)
            return null;
        return getFirst();
    }

    /**
     * Retrieves, but does not remove, the last element of this list,
     * or returns <tt>null</tt> if this list is empty.
     *
     * @return the last element of this list, or <tt>null</tt>
     *         if this list is empty
     * @since 1.6
     */
    public E peekLast() {
        if (size==0)
            return null;
        return getLast();
    }

    /**
     * Retrieves and removes the first element of this list,
     * or returns <tt>null</tt> if this list is empty.
     *
     * @return the first element of this list, or <tt>null</tt> if
     *     this list is empty
     * @since 1.6
     */
    public E pollFirst() {
        if (size==0)
            return null;
        return removeFirst();
    }

    /**
     * Retrieves and removes the last element of this list,
     * or returns <tt>null</tt> if this list is empty.
     *
     * @return the last element of this list, or <tt>null</tt> if
     *     this list is empty
     * @since 1.6
     */
    public E pollLast() {
        if (size==0)
            return null;
        return removeLast();
    }

    /**
     * Pushes an element onto the stack represented by this list.  In other
     * words, inserts the element at the front of this list.
     *
     * <p>This method is equivalent to {@link #addFirst}.
     *
     * @param e the element to push
     * @since 1.6
     */
    public void push(E e) {
        addFirst(e);
    }

    /**
     * Pops an element from the stack represented by this list.  In other
     * words, removes and returns the first element of this list.
     *
     * <p>This method is equivalent to {@link #removeFirst()}.
     *
     * @return the element at the front of this list (which is the top
     *         of the stack represented by this list)
     * @throws NoSuchElementException if this list is empty
     * @since 1.6
     */
    public E pop() {
        return removeFirst();
    }

    /**
     * Removes the first occurrence of the specified element in this
     * list (when traversing the list from head to tail).  If the list
     * does not contain the element, it is unchanged.
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if the list contained the specified element
     * @since 1.6
     */
    public boolean removeFirstOccurrence(Object o) {
        return remove(o);
    }

    /**
     * Removes the last occurrence of the specified element in this
     * list (when traversing the list from head to tail).  If the list
     * does not contain the element, it is unchanged.
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if the list contained the specified element
     * @since 1.6
     */
    public boolean removeLastOccurrence(Object o) {
        if (o==null) {
            for (Entry<E> e = header.previous; e != header; e = e.previous) {
                if (e.element==null) {
                    remove(e);
                    return true;
                }
            }
        } else {
            for (Entry<E> e = header.previous; e != header; e = e.previous) {
                if (o.equals(e.element)) {
                    remove(e);
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Returns a list-iterator of the elements in this list (in proper
     * sequence), starting at the specified position in the list.
     * Obeys the general contract of <tt>List.listIterator(int)</tt>.<p>
     *
     * The list-iterator is <i>fail-fast</i>: if the list is structurally
     * modified at any time after the Iterator is created, in any way except
     * through the list-iterator's own <tt>remove</tt> or <tt>add</tt>
     * methods, the list-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.
     *
     * @param index index of the first element to be returned from the
     *              list-iterator (by a call to <tt>next</tt>)
     * @return a ListIterator of the elements in this list (in proper
     *         sequence), starting at the specified position in the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @see List#listIterator(int)
     */
    public ListIterator<E> listIterator(int index) {
        return new ListItr(index);
    }

    private class ListItr implements ListIterator<E> {
        private Entry<E> lastReturned = header;
        private Entry<E> next;
        private int nextIndex;
        private int expectedModCount = modCount;

        ListItr(int index) {
            if (index < 0 || index > size)
                throw new IndexOutOfBoundsException("Index: "+index+
                                                    ", Size: "+size);
            if (index < (size >> 1)) {
                next = header.next;
                for (nextIndex=0; nextIndex<index; nextIndex++)
                    next = next.next;
            } else {
                next = header;
                for (nextIndex=size; nextIndex>index; nextIndex--)
                    next = next.previous;
            }
        }

        public boolean hasNext() {
            return nextIndex != size;
        }

        public E next() {
            checkForComodification();
            if (nextIndex == size)
                throw new NoSuchElementException();

            lastReturned = next;
            next = next.next;
            nextIndex++;
            return lastReturned.element;
        }

        public boolean hasPrevious() {
            return nextIndex != 0;
        }

        public E previous() {
            if (nextIndex == 0)
                throw new NoSuchElementException();

            lastReturned = next = next.previous;
            nextIndex--;
            checkForComodification();
            return lastReturned.element;
        }

        public int nextIndex() {
            return nextIndex;
        }

        public int previousIndex() {
            return nextIndex-1;
        }

        public void remove() {
            checkForComodification();
            Entry<E> lastNext = lastReturned.next;
            try {
                LinkedList.this.remove(lastReturned);
            } catch (NoSuchElementException e) {
                throw new IllegalStateException();
            }
            if (next==lastReturned)
                next = lastNext;
            else
                nextIndex--;
            lastReturned = header;
            expectedModCount++;
        }

        public void set(E e) {
            if (lastReturned == header)
                throw new IllegalStateException();
            checkForComodification();
            lastReturned.element = e;
        }

        public void add(E e) {
            checkForComodification();
            lastReturned = header;
            addBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    private static class Entry<E> {
        E element;
        Entry<E> next;
        Entry<E> previous;

        Entry(E element, Entry<E> next, Entry<E> previous) {
            this.element = element;
            this.next = next;
            this.previous = previous;
        }
    }

    private Entry<E> addBefore(E e, Entry<E> entry) {
        Entry<E> newEntry = new Entry<E>(e, entry, entry.previous);
        newEntry.previous.next = newEntry;
        newEntry.next.previous = newEntry;
        size++;
        modCount++;
        return newEntry;
    }

    private E remove(Entry<E> e) {
        if (e == header)
            throw new NoSuchElementException();

        E result = e.element;
        e.previous.next = e.next;
        e.next.previous = e.previous;
        e.next = e.previous = null;
        e.element = null;
        size--;
        modCount++;
        return result;
    }

    /**
     * @since 1.6
     */
    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }

    /** Adapter to provide descending iterators via ListItr.previous */
    private class DescendingIterator implements Iterator {
        final ListItr itr = new ListItr(size());
        public boolean hasNext() {
            return itr.hasPrevious();
        }
        public E next() {
            return itr.previous();
        }
        public void remove() {
            itr.remove();
        }
    }

    /**
     * Returns a shallow copy of this <tt>LinkedList</tt>. (The elements
     * themselves are not cloned.)
     *
     * @return a shallow copy of this <tt>LinkedList</tt> instance
     */
    public Object clone() {
        LinkedList<E> clone = null;
        try {
            clone = (LinkedList<E>) super.clone();
        } catch (CloneNotSupportedException e) {
            throw new InternalError();
        }

        // Put clone into "virgin" state
        clone.header = new Entry<E>(null, null, null);
        clone.header.next = clone.header.previous = clone.header;
        clone.size = 0;
        clone.modCount = 0;

        // Initialize clone with our elements
        for (Entry<E> e = header.next; e != header; e = e.next)
            clone.add(e.element);

        return clone;
    }

    /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this list
     *         in proper sequence
     */
    public Object[] toArray() {
        Object[] result = new Object[size];
        int i = 0;
        for (Entry<E> e = header.next; e != header; e = e.next)
            result[i++] = e.element;
        return result;
    }

    /**
     * Returns an array containing all of the elements in this list in
     * proper sequence (from first to last element); the runtime type of
     * the returned array is that of the specified array.  If the list fits
     * in the specified array, it is returned therein.  Otherwise, a new
     * array is allocated with the runtime type of the specified array and
     * the size of this list.
     *
     * <p>If the list fits in the specified array with room to spare (i.e.,
     * the array has more elements than the list), the element in the array
     * immediately following the end of the list is set to <tt>null</tt>.
     * (This is useful in determining the length of the list <i>only</i> if
     * the caller knows that the list does not contain any null elements.)
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose <tt>x</tt> is a list known to contain only strings.
     * The following code can be used to dump the list into a newly
     * allocated array of <tt>String</tt>:
     *
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     *
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @param a the array into which the elements of the list are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose.
     * @return an array containing the elements of the list
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this list
     * @throws NullPointerException if the specified array is null
     */
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            a = (T[])java.lang.reflect.Array.newInstance(
                                a.getClass().getComponentType(), size);
        int i = 0;
        Object[] result = a;
        for (Entry<E> e = header.next; e != header; e = e.next)
            result[i++] = e.element;

        if (a.length > size)
            a[size] = null;

        return a;
    }

    private static final long serialVersionUID = 876323262645176354L;

    /**
     * Save the state of this <tt>LinkedList</tt> instance to a stream (that
     * is, serialize it).
     *
     * @serialData The size of the list (the number of elements it
     *             contains) is emitted (int), followed by all of its
     *             elements (each an Object) in the proper order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out any hidden serialization magic
        s.defaultWriteObject();

        // Write out size
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (Entry e = header.next; e != header; e = e.next)
            s.writeObject(e.element);
    }

    /**
     * Reconstitute this <tt>LinkedList</tt> instance from a stream (that is
     * deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in any hidden serialization magic
        s.defaultReadObject();

        // Read in size
        int size = s.readInt();

        // Initialize header
        header = new Entry<E>(null, null, null);
        header.next = header.previous = header;

        // Read in all elements in the proper order.
        for (int i=0; i<size; i++)
            addBefore((E)s.readObject(), header);
    }
}

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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

* This code is free software; you can redistribute it and/or modify it

* under the terms of the GNU General Public License version 2 only, as

* published by the Free Software Foundation. Sun designates this

* particular file as subject to the "Classpath" exception as provided

* by Sun in the LICENSE file that accompanied this code.

* This code is distributed in the hope that it will be useful, but WITHOUT

* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License

* version 2 for more details (a copy is included in the LICENSE file that

* accompanied this code).

* You should have received a copy of the GNU General Public License version

* 2 along with this work; if not, write to the Free Software Foundation,

* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

* Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

* CA 95054 USA or visit www.sun.com if you need additional information or

* have any questions.

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, {@linkplain Queue queue}, or {@linkplain Deque

* double-ended queue}.

* The class implements the <tt>Deque</tt> interface, providing

* first-in-first-out queue operations for <tt>add</tt>,

* <tt>poll</tt>, along with other stack and deque operations.

* 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 beginning or the end, whichever is closer to the specified index.

* Note that this implementation is not synchronized.

* If multiple threads access a linked list concurrently, and at least

* one of the threads modifies the list structurally, it must 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

* {@link Collections#synchronizedList 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>

* The iterators returned by this class's <tt>iterator</tt> and

* <tt>listIterator</tt> methods are fail-fast: 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 {@link

* ConcurrentModificationException}. 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.

* 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: the fail-fast behavior of iterators

* should be used only to detect bugs.

* This class is a member of the

* <a href="{@docRoot}/../technotes/guides/collections/index.html">

* Java Collections Framework</a>.

* @author Josh Bloch

* @see List

* @see ArrayList

* @see Vector

* @since 1.2

* @param <E> the type of elements held in this collection

public class LinkedList<E>

extends AbstractSequentialList<E>

implements List<E>, Deque<E>, Cloneable, java.io.Serializable

{

private transient Entry<E> header = new Entry<E>(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<? extends E> 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 E 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 E 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 E removeFirst() {

return remove(header.next);

}

/**

* Removes and returns the last element from this list.

* @return the last element from this list

* @throws NoSuchElementException if this list is empty

public E removeLast() {

return remove(header.previous);

}

/**

* Inserts the specified element at the beginning of this list.

* @param e the element to add

public void addFirst(E e) {

addBefore(e, header.next);

}

/**

* Appends the specified element to the end of this list.

* This method is equivalent to {@link #add}.

* @param e the element to add

public void addLast(E e) {

addBefore(e, 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.

* This method is equivalent to {@link #addLast}.

* @param e element to be appended to this list

* @return <tt>true</tt> (as specified by {@link Collection#add})

public boolean add(E e) {

addBefore(e, header);

return true;

}

/**

* Removes the first occurrence of the specified element from this list,

* if it is present. If this 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). Returns <tt>true</tt> if this list

* contained the specified element (or equivalently, if this list

* changed as a result of the call).

* @param o element to be removed from this list, if present

* @return <tt>true</tt> if this list contained the specified element

public boolean remove(Object o) {

if (o==null) {

for (Entry<E> e = header.next; e != header; e = e.next) {

if (e.element==null) {

remove(e);

return true;

}

} else {

for (Entry<E> 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. (Note that this will occur if the specified collection is

* this list, and it's nonempty.)

* @param c collection containing elements to be added to 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<? extends E> 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 the first element

* from the specified collection

* @param c collection containing elements to be added to this list

* @return <tt>true</tt> if this list changed as a result of the call

* @throws IndexOutOfBoundsException {@inheritDoc}

* @throws NullPointerException if the specified collection is null

public boolean addAll(int index, Collection<? extends E> c) {

if (index < 0 || index > size)

throw new IndexOutOfBoundsException("Index: "+index+

", Size: "+size);

Object[] a = c.toArray();

int numNew = a.length;

if (numNew==0)

return false;

modCount++;

Entry<E> successor = (index==size ? header : entry(index));

Entry<E> predecessor = successor.previous;

for (int i=0; i<numNew; i++) {

Entry<E> e = new Entry<E>((E)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() {

Entry<E> e = header.next;

while (e != header) {

Entry<E> next = e.next;

e.next = e.previous = null;

e.element = null;

e = next;

}

header.next = header.previous = header;

size = 0;

modCount++;

}

// Positional Access Operations

/**

* Returns the element at the specified position in this list.

* @param index index of the element to return

* @return the element at the specified position in this list

* @throws IndexOutOfBoundsException {@inheritDoc}

public E 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 the element to replace

* @param element element to be stored at the specified position

* @return the element previously at the specified position

* @throws IndexOutOfBoundsException {@inheritDoc}

public E set(int index, E element) {

Entry<E> e = entry(index);

E 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 {@inheritDoc}

public void add(int index, E 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 be removed

* @return the element previously at the specified position

* @throws IndexOutOfBoundsException {@inheritDoc}

public E remove(int index) {

return remove(entry(index));

}

/**

* Returns the indexed entry.

private Entry<E> entry(int index) {

if (index < 0 || index >= size)

throw new IndexOutOfBoundsException("Index: "+index+

", Size: "+size);

Entry<E> e = header;

if (index < (size >> 1)) {

for (int i = 0; i <= index; i++)

e = e.next;

} else {

for (int i = size; i > index; i--)

e = e.previous;

}

return e;

}

// Search Operations

/**

* Returns the index of the first occurrence of the specified element

* in this list, or -1 if this list does not contain the element.

* More formally, returns the lowest index <tt>i</tt> such that

* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,

* or -1 if there is no such index.

* @param o element to search for

* @return the index of the first occurrence of the specified element in

* this list, or -1 if this list does not contain the element

public int indexOf(Object o) {

int index = 0;

if (o==null) {

for (Entry e = header.next; e != header; e = e.next) {

if (e.element==null)

return index;

index++;

}

} else {

for (Entry e = header.next; e != header; e = e.next) {

if (o.equals(e.element))

return index;

index++;

}

return -1;

}

/**

* Returns the index of the last occurrence of the specified element

* in this list, or -1 if this list does not contain the element.

* More formally, returns the highest index <tt>i</tt> such that

* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,

* or -1 if there is no such index.

* @param o element to search for

* @return the index of the last occurrence of the specified element in

* this list, or -1 if this list does not contain the element

public int lastIndexOf(Object o) {

int index = size;

if (o==null) {

for (Entry e = header.previous; e != header; e = e.previous) {

index--;

if (e.element==null)

return index;

}

} else {

for (Entry e = header.previous; e != header; e = e.previous) {

index--;

if (o.equals(e.element))

return index;

}

return -1;

}

// Queue operations.

/**

* Retrieves, but does not remove, the head (first element) of this list.

* @return the head of this list, or <tt>null</tt> if this list is empty

* @since 1.5

public E peek() {

if (size==0)

return null;

return getFirst();

}

/**

* Retrieves, but does not remove, the head (first element) of this list.

* @return the head of this list

* @throws NoSuchElementException if this list is empty

* @since 1.5

public E element() {

return getFirst();

}

/**

* Retrieves and removes the head (first element) of this list

* @return the head of this list, or <tt>null</tt> if this list is empty

* @since 1.5

public E poll() {

if (size==0)

return null;

return removeFirst();

}

/**

* Retrieves and removes the head (first element) of this list.

* @return the head of this list

* @throws NoSuchElementException if this list is empty

* @since 1.5

public E remove() {

return removeFirst();

}

/**

* Adds the specified element as the tail (last element) of this list.

* @param e the element to add

* @return <tt>true</tt> (as specified by {@link Queue#offer})

* @since 1.5

public boolean offer(E e) {

return add(e);

}

// Deque operations

/**

* Inserts the specified element at the front of this list.

* @param e the element to insert

* @return <tt>true</tt> (as specified by {@link Deque#offerFirst})

* @since 1.6

public boolean offerFirst(E e) {

addFirst(e);

return true;

}

/**

* Inserts the specified element at the end of this list.

* @param e the element to insert

* @return <tt>true</tt> (as specified by {@link Deque#offerLast})

* @since 1.6

public boolean offerLast(E e) {

addLast(e);

return true;

}

/**

* Retrieves, but does not remove, the first element of this list,

* or returns <tt>null</tt> if this list is empty.

* @return the first element of this list, or <tt>null</tt>

* if this list is empty

* @since 1.6

public E peekFirst() {

if (size==0)

return null;

return getFirst();

}

/**

* Retrieves, but does not remove, the last element of this list,

* or returns <tt>null</tt> if this list is empty.

* @return the last element of this list, or <tt>null</tt>

* if this list is empty

* @since 1.6

public E peekLast() {

if (size==0)

return null;

return getLast();

}

/**

* Retrieves and removes the first element of this list,

* or returns <tt>null</tt> if this list is empty.

* @return the first element of this list, or <tt>null</tt> if

* this list is empty

* @since 1.6

public E pollFirst() {

if (size==0)

return null;

return removeFirst();

}

/**

* Retrieves and removes the last element of this list,

* or returns <tt>null</tt> if this list is empty.

* @return the last element of this list, or <tt>null</tt> if

* this list is empty

* @since 1.6

public E pollLast() {

if (size==0)

return null;

return removeLast();

}

/**

* Pushes an element onto the stack represented by this list. In other

* words, inserts the element at the front of this list.

* This method is equivalent to {@link #addFirst}.

* @param e the element to push

* @since 1.6

public void push(E e) {

addFirst(e);

}

/**

* Pops an element from the stack represented by this list. In other

* words, removes and returns the first element of this list.

* This method is equivalent to {@link #removeFirst()}.

* @return the element at the front of this list (which is the top

* of the stack represented by this list)

* @throws NoSuchElementException if this list is empty

* @since 1.6

public E pop() {

return removeFirst();

}

/**

* Removes the first occurrence of the specified element in this

* list (when traversing the list from head to tail). If the list

* does not contain the element, it is unchanged.

* @param o element to be removed from this list, if present

* @return <tt>true</tt> if the list contained the specified element

* @since 1.6

public boolean removeFirstOccurrence(Object o) {

return remove(o);

}

/**

* Removes the last occurrence of the specified element in this

* list (when traversing the list from head to tail). If the list

* does not contain the element, it is unchanged.

* @param o element to be removed from this list, if present

* @return <tt>true</tt> if the list contained the specified element

* @since 1.6

public boolean removeLastOccurrence(Object o) {

if (o==null) {

for (Entry<E> e = header.previous; e != header; e = e.previous) {

if (e.element==null) {

remove(e);

return true;

}

} else {

for (Entry<E> e = header.previous; e != header; e = e.previous) {

if (o.equals(e.element)) {

remove(e);

return true;

}

return false;

}

/**

* Returns a list-iterator of the elements in this list (in proper

* sequence), starting at the specified position in the list.

* Obeys the general contract of <tt>List.listIterator(int)</tt>.

* The list-iterator is fail-fast: if the list is structurally

* modified at any time after the Iterator is created, in any way except

* through the list-iterator's own <tt>remove</tt> or <tt>add</tt>

* methods, the list-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.

* @param index index of the first element to be returned from the

* list-iterator (by a call to <tt>next</tt>)

* @return a ListIterator of the elements in this list (in proper

* sequence), starting at the specified position in the list

* @throws IndexOutOfBoundsException {@inheritDoc}

* @see List#listIterator(int)

public ListIterator<E> listIterator(int index) {

return new ListItr(index);

}

private class ListItr implements ListIterator<E> {

private Entry<E> lastReturned = header;

private Entry<E> next;

private int nextIndex;

private int expectedModCount = modCount;

ListItr(int index) {

if (index < 0 || index > size)

throw new IndexOutOfBoundsException("Index: "+index+

", Size: "+size);

if (index < (size >> 1)) {

next = header.next;

for (nextIndex=0; nextIndex<index; nextIndex++)

next = next.next;

} else {

next = header;

for (nextIndex=size; nextIndex>index; nextIndex--)

next = next.previous;

}

public boolean hasNext() {

return nextIndex != size;

}

public E next() {

checkForComodification();

if (nextIndex == size)

throw new NoSuchElementException();

lastReturned = next;

next = next.next;

nextIndex++;

return lastReturned.element;

}

public boolean hasPrevious() {

return nextIndex != 0;

}

public E previous() {

if (nextIndex == 0)

throw new NoSuchElementException();

lastReturned = next = next.previous;

nextIndex--;

checkForComodification();

return lastReturned.element;

}

public int nextIndex() {

return nextIndex;

}

public int previousIndex() {

return nextIndex-1;

}

public void remove() {

checkForComodification();

Entry<E> lastNext = lastReturned.next;

try {

LinkedList.this.remove(lastReturned);

} catch (NoSuchElementException e) {

throw new IllegalStateException();

}

if (next==lastReturned)

next = lastNext;

else

nextIndex--;

lastReturned = header;

expectedModCount++;

}

public void set(E e) {

if (lastReturned == header)

throw new IllegalStateException();

checkForComodification();

lastReturned.element = e;

}

public void add(E e) {

checkForComodification();

lastReturned = header;

addBefore(e, next);

nextIndex++;

expectedModCount++;

}

final void checkForComodification() {

if (modCount != expectedModCount)

throw new ConcurrentModificationException();

}

private static class Entry<E> {

E element;

Entry<E> next;

Entry<E> previous;

Entry(E element, Entry<E> next, Entry<E> previous) {

this.element = element;

this.next = next;

this.previous = previous;

}

private Entry<E> addBefore(E e, Entry<E> entry) {

Entry<E> newEntry = new Entry<E>(e, entry, entry.previous);

newEntry.previous.next = newEntry;

newEntry.next.previous = newEntry;

size++;

modCount++;

return newEntry;

}

private E remove(Entry<E> e) {

if (e == header)

throw new NoSuchElementException();

E result = e.element;

e.previous.next = e.next;

e.next.previous = e.previous;

e.next = e.previous = null;

e.element = null;

size--;

modCount++;

return result;

}

/**

* @since 1.6

public Iterator<E> descendingIterator() {

return new DescendingIterator();

}

/** Adapter to provide descending iterators via ListItr.previous */

private class DescendingIterator implements Iterator {

final ListItr itr = new ListItr(size());

public boolean hasNext() {

return itr.hasPrevious();

}

public E next() {

return itr.previous();

}

public void remove() {

itr.remove();

}

/**

* Returns a shallow copy of this <tt>LinkedList</tt>. (The elements

* themselves are not cloned.)

* @return a shallow copy of this <tt>LinkedList</tt> instance

public Object clone() {

LinkedList<E> clone = null;

try {

clone = (LinkedList<E>) super.clone();

} catch (CloneNotSupportedException e) {

throw new InternalError();

}

// Put clone into "virgin" state

clone.header = new Entry<E>(null, null, null);

clone.header.next = clone.header.previous = clone.header;

clone.size = 0;

clone.modCount = 0;

// Initialize clone with our elements

for (Entry<E> e = header.next; e != header; e = e.next)

clone.add(e.element);

return clone;

}

/**

* Returns an array containing all of the elements in this list

* in proper sequence (from first to last element).

* The returned array will be "safe" in that no references to it are

* maintained by this list. (In other words, this method must allocate

* a new array). The caller is thus free to modify the returned array.

* This method acts as bridge between array-based and collection-based

* APIs.

* @return an array containing all of the elements in this list

* in proper sequence

public Object[] toArray() {

Object[] result = new Object[size];

int i = 0;

for (Entry<E> e = header.next; e != header; e = e.next)

result[i++] = e.element;

return result;

}

/**

* Returns an array containing all of the elements in this list in

* proper sequence (from first to last element); the runtime type of

* the returned array is that of the specified array. If the list fits

* in the specified array, it is returned therein. Otherwise, a new

* array is allocated with the runtime type of the specified array and

* the size of this list.

* If the list fits in the specified array with room to spare (i.e.,

* the array has more elements than the list), the element in the array

* immediately following the end of the list is set to <tt>null</tt>.

* (This is useful in determining the length of the list only if

* the caller knows that the list does not contain any null elements.)

* Like the {@link #toArray()} method, this method acts as bridge between

* array-based and collection-based APIs. Further, this method allows

* precise control over the runtime type of the output array, and may,

* under certain circumstances, be used to save allocation costs.

* Suppose <tt>x</tt> is a list known to contain only strings.

* The following code can be used to dump the list into a newly

* allocated array of <tt>String</tt>:

* <pre>

* String[] y = x.toArray(new String[0]);</pre>

* Note that <tt>toArray(new Object[0])</tt> is identical in function to

* <tt>toArray()</tt>.

* @param a the array into which the elements of the list are to

* be stored, if it is big enough; otherwise, a new array of the

* same runtime type is allocated for this purpose.

* @return an array containing the elements of the list

* @throws ArrayStoreException if the runtime type of the specified array

* is not a supertype of the runtime type of every element in

* this list

* @throws NullPointerException if the specified array is null

public <T> T[] toArray(T[] a) {

if (a.length < size)

a = (T[])java.lang.reflect.Array.newInstance(

a.getClass().getComponentType(), size);

int i = 0;

Object[] result = a;

for (Entry<E> e = header.next; e != header; e = e.next)

result[i++] = e.element;

if (a.length > size)

a[size] = null;

return a;

}

private static final long serialVersionUID = 876323262645176354L;

/**

* Save the state of this <tt>LinkedList</tt> instance to a stream (that

* is, serialize it).

* @serialData The size of the list (the number of elements it

* contains) is emitted (int), followed by all of its

* elements (each an Object) in the proper order.

private void writeObject(java.io.ObjectOutputStream s)

throws java.io.IOException {

// Write out any hidden serialization magic

s.defaultWriteObject();

// Write out size

s.writeInt(size);

// Write out all elements in the proper order.

for (Entry e = header.next; e != header; e = e.next)

s.writeObject(e.element);

}

/**

* Reconstitute this <tt>LinkedList</tt> instance from a stream (that is

* deserialize it).

private void readObject(java.io.ObjectInputStream s)

throws java.io.IOException, ClassNotFoundException {

// Read in any hidden serialization magic

s.defaultReadObject();

// Read in size

int size = s.readInt();

// Initialize header

header = new Entry<E>(null, null, null);

header.next = header.previous = header;

// Read in all elements in the proper order.

for (int i=0; i<size; i++)

addBefore((E)s.readObject(), header);

}

Java LinkedList documentation:

https://docs.oracle.com/javase/7/docs/api/java/util/LinkedList.html

[Read More...]

Best websites to learn java programming

August 31, 2016 Author: david
No comment yet
Here are some good resources to learn Java programming.
1. Oracle Technology Network – java.sun.com
  This is the official Java developers’ website. It has all the information you need. It provides some interesting sections such as developers’ spot light, blogs, information about various Java technologies top downloads, and links for community. It is useful to get technical updates on Java, different IDEs and libraries.
2. en.wikipedia.org/wiki/Java_programming_language
  Wikipedia has always been a first-stop for learning something new, and it is also a hub for Java programmers too! Whether you are looking for some basic Java concepts or looking for the list of the Java libraries,
[Read More...]
LeetCode – Flip Game II

August 7, 2016 Author: david
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You are playing the following Flip Game with your friend: Given a string that contains only these two characters: + and -, you and your friend take turns to flip two consecutive “++” into “–“. The game ends when a person can no longer make a move and therefore the other person will be the winner.

Write a function to determine if the starting player can guarantee a win.

For example, given s = “++++”, return true. The starting player can guarantee a win by flipping the middle “++” to become “+–+”.

Follow up:
Derive your algorithm’s runtime complexity.
[Read More...]
LeetCode – Moving Average from Data Stream

August 6, 2016 Author: david
No comment yet

Given a stream of integers and a window size, calculate the moving average of all integers in the sliding window.

For example,
MovingAverage m = new MovingAverage(3);
m.next(1) = 1
m.next(10) = (1 + 10) / 2
m.next(3) = (1 + 10 + 3) / 3
m.next(5) = (10 + 3 + 5) / 3

Analysis

We can use a bounded queue to record the last n integers in the window, and define a variable sumN to record the sum of the numbers in the window.

Each time when a new number is registered,
[Read More...]
Java HashMap Examples

August 1, 2016 Author: david
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In this post, I walk through the concept of Java HashMap, and use examples to describe its usage cases.

What is HashMap

Java HashMap is a Hash table based implementation of the Java Map interface. It provides all of the optional map operations, and permits null values and the null key. The order of Keys in a HashMap is not guaranteed. So we should not reply on the order of the keys in a HashMap.

HashMap vs Hashtable

Based on java doc, Java HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.
[Read More...]
Java Map vs HashMap vs TreeMap vs LinkedHashMap

August 1, 2016 Author: david
No comment yet
Java Map

Java Map is an interface with the following signature.

public interface Map<K,V>

Here are some properties of Java Map:
1. It defines an operation to map keys to values.
2. A map cannot contain duplicate keys; each key can map to at most one value.
3. The Map interface provides three collection views, which allow a map’s contents to be viewed as a set of keys, collection of values, or set of key-value mappings.
4. The order of a map is dependent on its implementations.
[Read More...]
Java SortedMap Tutorial

August 1, 2016 Author: david
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In this post, I will use examples to show how to use Java Sorted Map. A SortedMap is a Map that sort its entries in ascending order according to the keys’ natural ordering, or according to a Comparator provided at the time of building a SortedMap. SortedMap is useful if you want to quickly locate an entry that is larger than a specified key. For example, it can be used to implement a Consistent hash.

Since the entries in SortedMap are sorted by keys, all the keys in a sortedMap must implement the Comparable interface or provided a comparator when creating the SortedMap.
[Read More...]
Java Sort using Comparable or Comparator

August 1, 2016 Author: david
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In Java, it is straightforward to sort objects of the predefined class such as Integer, Double, Float and String. This is because these classes have been implemented the Comparable interface. Comparable defines a natural ordering, as when you’re defining it, you specify which one is considered “less than” or “greater than” the another.

How can you sort custom Objects? For example, suppose you have a list of objects of Student class, which has name, age, and score attributes. You may want to sort the students by their scores. Another example is to sort a list strings by their length.
[Read More...]

Five ways to implement Singleton pattern in Java

July 31, 2016 Author: david

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In this post, I will describe five ways to implement the Singleton pattern in Java. They are Synchronization the getInstane() method,static final variable, using double checking lock with volatile keyword, using SingletonHolder, and Enum.

1. Classic Java Singleton pattern

The following code is the simplest implementation of Singleton Pattern in Java. However it is not multiple thread-safe.

public class Singleton {
    private static Singleton instance;
    private Singleton (){}

    public static Singleton getInstance() {
	if (instance == null) {
	    instance = new Singleton();
	}
	return instance;
    }
}

public class Singleton {

private static Singleton instance;

private Singleton (){}

public static Singleton getInstance() {

if (instance == null) {

instance = new Singleton();

}

return instance;

}

The singleton pattern implemented in the above example is easy to understand. We maintain a static reference to the singleton instance and returns that reference from the static getInstance() method. We also make the Constructor private,

[Read More...]

Count word frequency

July 30, 2016 Author: david

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Count word frequency is a popular task for text analysis. In this post, I describe how to count word frequency using Java HashMap, python dictionary, and Spark.

Use Java HashMap to Count Word frequency

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class JavaHashMapExample {
    public Map<String, Integer> countFreq(List<String> list){
        Map<String, Integer> histogram = new HashMap<>();
        for(String word: list) {

            if (!histogram.containsKey(word)) {
                histogram.put(word, 1);
            } else {
                histogram.put(word, histogram.get(word) + 1);
            }
        }
        return histogram;
    }

    public static void main(String[] args) {
        String[] a = {"a", "b", "c", "d", "a", "b", "a", "d", "a", "c", "c", "d", "a", "c", "c", "c"};
        List<String> wordList = Arrays.asList(a);
        System.out.println(new JavaHashMapExample().countFreq(wordList));
     }
}

import java.util.Arrays;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

public class JavaHashMapExample {

public Map<String, Integer> countFreq(List<String> list){

Map<String, Integer> histogram = new HashMap<>();

for(String word: list) {

if (!histogram.containsKey(word)) {

histogram.put(word, 1);

} else {

histogram.put(word, histogram.get(word) + 1);

}

return histogram;

}

public static void main(String[] args) {

String[] a = {"a", "b", "c", "d", "a", "b", "a", "d", "a", "c", "c", "d", "a", "c", "c", "c"};

List<String> wordList = Arrays.asList(a);

System.out.println(new JavaHashMapExample().countFreq(wordList));

}

{a=5, b=2, c=6, d=3}

Use Python Dict to count word frequency

words = ["a", "b", "c", "d", "a", "b", "a", "d", "a", "c", "c", "d", "a", "c", "c", "c"]

freq = {}

for w in words:
    if w in freq:
        freq[w] += 1
    else:
        freq[w] = 1

print freq

words = ["a", "b", "c", "d", "a", "b", "a", "d", "a", "c", "c", "d", "a", "c", "c", "c"]

freq = {}

for w in words:

if w in freq:

freq[w] += 1

else:

freq[w] = 1

print freq

The output:

{‘a’: 5, ‘c’: 6, ‘b’: 2, ‘d’: 3}

Use Spark to count word Frequency

The above method works well for small dataset. However, if you have a huge dataset, the hashTable based method will not work. You will need to develop a distributed program to accomplish this task.

[Read More...]