Collection series Queue (ten): LinkedList

Before it comes to our List collection have said the LinkedList. But more than just a LinkedList List collection implementation, which is a two-way queue implementation.

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable

LinkedList implements List interface not only, but also to achieve the Deque interface. So this section we will talk about the two-way queue feature LinkedList.

principle

To understand the principles of LinkedList, we will class member variables, constructors, two core methods introduced one by one.

Class member variables

// 链表大小
transient int size = 0;
// 首节点
transient Node<E> first;
// 尾节点
transient Node<E> last;
// Node节点
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;
    }
}

LinkedList embodiment can be seen using the implementation list node, and each node has a predecessor and successor nodes.

Construction method

There are two LinkedList constructor:

public LinkedList() {
}
public LinkedList(Collection<? extends E> c) {
    this();
    addAll(c);
}

Construction method is relatively simple, there is no in-depth introduction.

The core method

Several methods LinkedList with double-ended queue associated to: offerFirst, offerLast, pollFirst, pollLast.

offerFirst

public boolean offerFirst(E e) {
    addFirst(e);
    return true;
}
    
    public void addFirst(E e) {
    linkFirst(e);
}

// 将e节点作为头结点插入
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++;
}

offerLast

public boolean offerLast(E e) {
    addLast(e);
    return true;
}
    
public void addLast(E e) {
    linkLast(e);
}
// 将e节点作为末尾节点插入
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++;
}

pollFirst

public E pollFirst() {
    final Node<E> f = first;
    return (f == null) ? null : unlinkFirst(f);
}
// 删除头结点
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;
}

pollLast

public E pollLast() {
    final Node<E> l = last;
    return (l == null) ? null : unlinkLast(l);
}
// 删除尾节点
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;
}

It can be seen either inserted or deleted, poll and offer operations are relatively simple, with emphasis on modification and maintenance of reference.

to sum up

LinkedList is not just a simple implementation of List, which is also a two-way queue implementation.