LinkedList源碼分析

package java.util;

import java.util.function.Consumer;

public class LinkedList<E> extends AbstractSequentialList<E> implements List<E>, Deque<E>, Cloneable, java.io.Serializable {
	
	// **************************** 1.鏈表的結構 ****************************
	transient int size = 0;
	
	// 頭節點
	transient Node<E> first;

	// 尾節點
	transient Node<E> last;

	// Constructs an empty list.
	public LinkedList() {
	}
	
	/**
	 * 鏈表中節點的數據結構，根據節點的數據結構能夠看出：該鏈表是一個雙向鏈表。
	 */
	private static class Node<E> {
		E item;
		Node<E> next;
		Node<E> prev;

		Node(Node<E> prev, E element, Node<E> next) {
			this.item = element;
			this.next = next;
			this.prev = prev;
		}
	}
	// **************************** 鏈表的結構 ****************************
	
	
	// **************************** 2.add方法 ****************************
	/**
	 * 在鏈表的尾部添加一個元素。
	 * Appends the specified element to the end of this list.
	 * This method is equivalent to {[@link](https://my.oschina.net/u/393) #addLast}.
	 */
	public boolean add(E e) {
		linkLast(e);
		return true;
	}
	
	/**
	 * Links e as last element.
	 */
	void linkLast(E e) {
		final Node<E> l = last;
		final Node<E> newNode = new Node<>(l, e, null);
		last = newNode;
		if (l == null)
			first = newNode;
		else
			l.next = newNode;
		size++;
		modCount++;
	}
	
	/**
	 * 將一個元素插入到指定的位置。
	 * Inserts the specified element at the specified position in this list.
	 */
	public void add(int index, E element) {
		checkPositionIndex(index);

		if (index == size)
			linkLast(element);
		else
			linkBefore(element, node(index));
	}
	
	/**
	 * Inserts element e before non-null Node succ.
	 */
	void linkBefore(E e, Node<E> succ) {
		// assert succ != null;
		final Node<E> pred = succ.prev;
		final Node<E> newNode = new Node<>(pred, e, succ);
		succ.prev = newNode;
		if (pred == null)
			first = newNode;
		else
			pred.next = newNode;
		size++;
		modCount++;
	}
	
	// **************************** add方法 ****************************
	
	
	// **************************** 3.get、set方法 ****************************
	/**
	 * 獲取指定下標的元素。
	 * Returns the element at the specified position in this list.
	 */
	public E get(int index) {
		checkElementIndex(index);
		return node(index).item;
	}
	
	/**
	 * Returns the (non-null) Node at the specified element index.
	 */
	Node<E> node(int index) {
		// assert isElementIndex(index);

		if (index < (size >> 1)) {
			Node<E> x = first;
			for (int i = 0; i < index; i++)
				x = x.next;
			return x;
		} else {
			Node<E> x = last;
			for (int i = size - 1; i > index; i--)
				x = x.prev;
			return x;
		}
	}
	

	/**
	 * Replaces the element at the specified position in this list with the specified element.
	 */
	public E set(int index, E element) {
		checkElementIndex(index);
		Node<E> x = node(index);
		E oldVal = x.item;
		x.item = element;
		return oldVal;
	}
	
	// **************************** get、set方法 ****************************
	
	
	// **************************** 4.remove、clear方法 ****************************
	/**
	 * 刪除一個指定的元素。注：若是該元素在鏈表中有多個，則只會刪除第一個出現的元素。
	 * 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 {[@code](https://my.oschina.net/codeo) i} such that 
	 * o==null ? get(i)==null : o.equals(get(i))
	 */
	public boolean remove(Object o) {
		if (o == null) {
			for (Node<E> x = first; x != null; x = x.next) {
				if (x.item == null) {
					unlink(x);
					return true;
				}
			}
		} else {
			for (Node<E> x = first; x != null; x = x.next) {
				if (o.equals(x.item)) {
					unlink(x);
					return true;
				}
			}
		}
		return false;
	}
	
	/**
	 * 刪除指定下標的元素。
	 * Removes the element at the specified position in this list. 
	 */
	public E remove(int index) {
		checkElementIndex(index);
		return unlink(node(index));
	}
	
	/**
	 * Unlinks non-null node x.
	 */
	E unlink(Node<E> x) {
		// assert x != null;
		final E element = x.item;
		final Node<E> next = x.next;
		final Node<E> prev = x.prev;

		if (prev == null) {
			first = next;
		} else {
			prev.next = next;
			x.prev = null;
		}

		if (next == null) {
			last = prev;
		} else {
			next.prev = prev;
			x.next = null;
		}

		x.item = null;
		size--;
		modCount++;
		return element;
	}
	
	/**
	 * Removes all of the elements from this list.
	 */
	public void clear() {
		// Clearing all of the links between nodes is "unnecessary", but:
		// - helps a generational GC if the discarded nodes inhabit
		//   more than one generation
		// - is sure to free memory even if there is a reachable Iterator
		for (Node<E> x = first; x != null; ) {
			Node<E> next = x.next;
			x.item = null;
			x.next = null;
			x.prev = null;
			x = next;
		}
		first = last = null;
		size = 0;
		modCount++;
	}
	// **************************** remove、clear方法 ****************************
	
	
	/**
	 * Returns the index of the first occurrence of the specified elemen 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 (Node<E> x = first; x != null; x = x.next) {
				if (x.item == null)
					return index;
				index++;
			}
		} else {
			for (Node<E> x = first; x != null; x = x.next) {
				if (o.equals(x.item))
					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.
	 */
	public int lastIndexOf(Object o) {
		int index = size;
		if (o == null) {
			for (Node<E> x = last; x != null; x = x.prev) {
				index--;
				if (x.item == null)
					return index;
			}
		} else {
			for (Node<E> x = last; x != null; x = x.prev) {
				index--;
				if (o.equals(x.item))
					return index;
			}
		}
		return -1;
	}
	

	
	/**
	 * 判斷元素的下標是否合法。
	 */
	private void checkElementIndex(int index) {
		if (!isElementIndex(index))
			throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
	}
	
	/**
	 * 判斷元素的下標是否合法。
	 */
	private void checkPositionIndex(int index) {
		if (!isPositionIndex(index))
			throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
	}
	
	/**
	 * Tells if the argument is the index of an existing element.
	 */
	private boolean isElementIndex(int index) {
		return index >= 0 && index < size;
	}
	
	/**
	 * Tells if the argument is the index of a valid position for an iterator or an add operation.
	 */
	private boolean isPositionIndex(int index) {
		return index >= 0 && index <= size;
	}
	

	/**
	 * 將指定的元素添加到鏈表的頭部。
	 * Links e as first element.
	 */
	private void linkFirst(E e) {
		final Node<E> f = first;
		final Node<E> newNode = new Node<>(null, e, f);
		first = newNode;
		if (f == null)
			last = newNode;
		else
			f.prev = newNode;
		size++;
		modCount++;
	}

	/**
	 * 刪除鏈表的頭元素f。
	 * Unlinks non-null first node f.
	 */
	private E unlinkFirst(Node<E> f) {
		// assert f == first && f != null;
		final E element = f.item;
		final Node<E> next = f.next;
		f.item = null;
		f.next = null; // help GC
		first = next;
		if (next == null)
			last = null;
		else
			next.prev = null;
		size--;
		modCount++;
		return element;
	}

	/**
	 * Unlinks non-null last node l.
	 */
	private E unlinkLast(Node<E> l) {
		// assert l == last && l != null;
		final E element = l.item;
		final Node<E> prev = l.prev;
		l.item = null;
		l.prev = null; // help GC
		last = prev;
		if (prev == null)
			first = null;
		else
			prev.next = null;
		size--;
		modCount++;
		return element;
	}

	/**
	 * Returns the first element in this list.
	 *
	 * [@return](https://my.oschina.net/u/556800) the first element in this list
	 * [@throws](https://my.oschina.net/throws) NoSuchElementException if this list is empty
	 */
	public E getFirst() {
		final Node<E> f = first;
		if (f == null)
			throw new NoSuchElementException();
		return f.item;
	}

	/**
	 * Returns the last element in this list.
	 *
	 * [@return](https://my.oschina.net/u/556800) the last element in this list
	 * @throws NoSuchElementException if this list is empty
	 */
	public E getLast() {
		final Node<E> l = last;
		if (l == null)
			throw new NoSuchElementException();
		return l.item;
	}

	/**
	 * 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() {
		final Node<E> f = first;
		if (f == null)
			throw new NoSuchElementException();
		return unlinkFirst(f);
	}

	/**
	 * 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() {
		final Node<E> l = last;
		if (l == null)
			throw new NoSuchElementException();
		return unlinkLast(l);
	}

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

	/**
	 * Appends the specified element to the end of this list.
	 */
	public void addLast(E e) {
		linkLast(e);
	}

	public boolean contains(Object o) {
		return indexOf(o) != -1;
	}

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

	public boolean addAll(Collection<? extends E> c) {
		return addAll(size, c);
	}


	// Queue operations. 將鏈表用做隊列時的相關操做：

	/**
	 * Retrieves, but does not remove, the head (first element) of this list.
	 *
	 * @return the head of this list, or {@code null} if this list is empty
	 * @since 1.5
	 */
	public E peek() {
		final Node<E> f = first;
		return (f == null) ? null : f.item;
	}

	/**
	 * 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 {@code null} if this list is empty
	 * @since 1.5
	 */
	public E poll() {
		final Node<E> f = first;
		return (f == null) ? null : unlinkFirst(f);
	}

	/**
	 * 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 {@code true} (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 {@code true} (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 {@code true} (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 {@code null} if this list is empty.
	 *
	 * @return the first element of this list, or {@code null} if this list is empty
	 * @since 1.6
	 */
	public E peekFirst() {
		final Node<E> f = first;
		return (f == null) ? null : f.item;
	 }

	/**
	 * Retrieves, but does not remove, the last element of this list, or returns {@code null} if this list is empty.
	 *
	 * @return the last element of this list, or {@code null} if this list is empty
	 * @since 1.6
	 */
	public E peekLast() {
		final Node<E> l = last;
		return (l == null) ? null : l.item;
	}

	/**
	 * Retrieves and removes the first element of this list, or returns {@code null} if this list is empty.
	 *
	 * @return the first element of this list, or {@code null} if this list is empty
	 * @since 1.6
	 */
	public E pollFirst() {
		final Node<E> f = first;
		return (f == null) ? null : unlinkFirst(f);
	}

	/**
	 * Retrieves and removes the last element of this list, or returns {@code null} if this list is empty.
	 *
	 * @return the last element of this list, or {@code null} if this list is empty
	 * @since 1.6
	 */
	public E pollLast() {
		final Node<E> l = last;
		return (l == null) ? null : unlinkLast(l);
	}

	/**
	 * 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.
	 * @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).
	 * @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 {@code true} if the list contained the specified element
	 * @since 1.6
	 */
	public boolean removeLastOccurrence(Object o) {
		if (o == null) {
			for (Node<E> x = last; x != null; x = x.prev) {
				if (x.item == null) {
					unlink(x);
					return true;
				}
			}
		} else {
			for (Node<E> x = last; x != null; x = x.prev) {
				if (o.equals(x.item)) {
					unlink(x);
					return true;
				}
			}
		}
		return false;
	}

	/**
	 * Returns a shallow copy of this {@code LinkedList}. (The elements themselves are not cloned.)
	 */
	public Object clone() {
		LinkedList<E> clone = superClone();

		// Put clone into "virgin" state
		clone.first = clone.last = null;
		clone.size = 0;
		clone.modCount = 0;

		// Initialize clone with our elements
		for (Node<E> x = first; x != null; x = x.next)
			clone.add(x.item);

		return clone;
	}

	/**
	 * Returns an array containing all of the elements in this list in proper sequence (from first to last element).
	 */
	public Object[] toArray() {
		Object[] result = new Object[size];
		int i = 0;
		for (Node<E> x = first; x != null; x = x.next)
			result[i++] = x.item;
		return result;
	}

	/**
	 * Returns an array containing all of the elements in this list in proper sequence (from first to last element); 
	 */
	@SuppressWarnings("unchecked")
	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 (Node<E> x = first; x != null; x = x.next)
			result[i++] = x.item;

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

		return a;
	}

}