Java源碼閱讀之LinkedList

Summary：

• public class LinkedList<E>  extends AbstractSequentialList<E> implements List<E>, Deque<E>, Cloneable, java.io.Serializable
• LinkedList通過鏈表結構實現的，對象中存有一個首一個尾引用；
• 本類也提供了一些類似隨機操作的方法。如get(index)，但是這些方法都是是僞隨機的！！

Fields：

transient int size = 0; //當前存儲的數據的個數
transient Node<E> first;
transient Node<E> last;

Constructor：

//默認構造空表
public LinkedList() {
}

getXX():

public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
}
public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
}

addXX():

public void addFirst(E e) {
        linkFirst(e);
}
public boolean add(E e) {
        linkLast(e);
        return true;
}
public void addLast(E e) {
        linkLast(e);
}

removeXX():

public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
}
public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
 }
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;
}

indexOf():

public boolean contains(Object o) {
        return indexOf(o) != -1;
}
//從first向last方向一個一個比較，最差O(n)
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;
}

模擬索引的隨機操作

public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
}
public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
}
public void add(int index, E element) {
        checkPositionIndex(index);

        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
}
public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
}

//下面方法是上面方法都會使用的一個方法，根據index返回對應的節點
//從後往前或者從前往後移動min(index,size-index)個位置得到Node
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;
        }
}

LinkedList底層實現

linkFirst()

//將元素插入鏈表的第一個位置處
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++;
}

linkLast()

//將元素插入到鏈表的末尾
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++;
}

unlinkFirst()

//刪掉第一個節點的數據
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;
}

unlinkLast()

//刪掉最後一個節點的數據
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;
}

unlink():

//刪掉指定節點
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;
}

內部類

//擁有前繼和後繼引用
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;
        }
}