Java collection – LinkedList source analysis

The basic use

Create a new Java class and walk through how ArrayList works in basic usage

package demo;

import java.util.LinkedList;

public class LinkedListDemo {
    public static void main(String[] args) {
        LinkedList<String> list = new LinkedList<>();
        list.add("demo");
        String s = list.get(0);
        list.remove(0); }}Copy the code

There are 4 steps

  1. Declare a LinkedList
  2. Add an element
  3. Get an element
  4. Delete an element
Start analyzing the source code step by step

  • Declare a LinkedList

    Enter the constructor of LinkedList

    public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable.java.io.Serializable{
    
    transient int size = 0;

    transient Node<E> first;

    transient Node<E> last;
    
    public LinkedList(a) {}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; }}}Copy the code

    As you can see, nothing is actually done to the LinkedList constructor.

    However, we need to pay attention to the inner class Node

    . We can see that LinkedList is actually a bidirectional LinkedList. For the nodes in it, Node stores Pointers to forward and backward nodes respectively, and its general structure is as follows

  • Add an element

    The trace code goes into the add function and finds that the add function looks like this

    public boolean add(E e) {
    linkLast(e);
    return true;
}

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++;
}
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    As you can see, adding an element directly adds a node to the end of the list and returns true

    Enter the linkLast(E E) function, you can see that there are three steps:

    • Records the current last node
    • Object that declares a new node and assigns the current last node to the new nodeprev
    • Will the currentlastPoint to a new node

    Like the add of ArrayList, the current operation count is recorded by modCount

  • Get an element

    public E get(int index) {
    checkElementIndex(index);
    return node(index).item;
}
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    First check whether the index passed in is valid

    Then go to the node(index) function to find the element

    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;
        returnx; }}Copy the code

    As you can see, the elements are found using a split lookup, starting from fisRT from front to back if less than half, or from last to back if greater

  • Delete an element

    public E remove(int index) {
    checkElementIndex(index);
    return unlink(node(index));
}

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;
}
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    As you can see, remove actually goes into the unlink function. This operation can be divided into four steps:

    • Record the prev and next nodes of the current node
    • ifprevIf it is empty, the first node of the current node willfisrtPoint to the next node of the current node; Otherwise the former node willnextPoints to the next node after the current node
    • ifnextIf it is null, the current node is the tail node and willlastPoint to the previous node of the current node; Otherwise the current node’sprevPoints to the previous node of the current node
    • The current node is empty,sizeThe length of the minus one

    Finally return the deleted element and increment operand

    The whole process can be summarized in the following figure