ArrayList 与 LinkedList

摘要

• 与vector(线程安全) 一起都是list的子类
• ArrayList基于数组的数据结构 LinkedList基于双向链表的数据结构
• ArrayList 查询时直接返回数据，LinkedList需要遍历链表，所以在随机查询中ArrayList效率比较高
• ArrayList 在增删时需要移动元素的位置，需要进行数组的copy,LinkedList需要根据位置查到插入位置的元素，然后再进行添加指针，在数据量比较小时(10000以下)时性能差不多，但是数据量较大时LinkedList效率比较高

1.与vector(线程安全) 一起都是list的子类

List JDK文档描述

2.ArrayList基于数组的数据结构 LinkedList基于双向链表的数据结构

   // ArrayList
   public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
            initialCapacity);
        }
    }
    //LinkedList
    public boolean addAll(int index, Collection<? extends E> c) {
    	checkPositionIndex(index);
    
    	Object[] a = c.toArray();
    	int numNew = a.length;
    	if (numNew == 0)
    		return false;
    
    	Node<E> pred, succ;
    	if (index == size) {
    		succ = null;
    		pred = last;
    	} else {
    		succ = node(index);
    		pred = succ.prev;
    	}
    
    	for (Object o : a) {
    		@SuppressWarnings("unchecked") E e = (E) o;
    		Node<E> newNode = new Node<>(pred, e, null);
    		if (pred == null)
    			first = newNode;
    		else
    			pred.next = newNode;
    		pred = newNode;
    	}
    
    	if (succ == null) {
    		last = pred;
    	} else {
    		pred.next = succ;
    		succ.prev = pred;
    	}
    
    	size += numNew;
    	modCount++;
    	return true;
    }

3.ArrayList 查询时直接返回数据，LinkedList需要遍历链表，所以在随机查询中ArrayList效率比较高

    //ArrayList
   public E get(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

        return (E) elementData[index];
    }
    //LinkedList
     public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }
    
    
    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;
        }
    }

4.ArrayList 在增删时需要移动元素的位置，需要进行数组的copy,LinkedList需要根据位置查到插入位置的元素，然后再进行添加指针，在数据量比较小时(10000以下)时性能差不多，但是数据量较大时LinkedList效率比较高

    //ArrayList
    public void add(int index, E element) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

        ensureCapacityInternal(size + 1); // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }
    
    //LinkedList
    public void add(int index, E element) {
        checkPositionIndex(index);

        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }
    
    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++;
    }

5.测试程序

public class Test1 {
    static List<Integer> arrayData =new ArrayList<Integer>();
    static List<Integer> linkedData =new LinkedList<Integer>();
    public static  int  maxCount = 30000;

    public static void main(String[] args) {

        for(int i=0;i<maxCount;i++){
            arrayData.add(i);
            linkedData.add(i);
        }
        System.out.println("maxCount:"+maxCount);
        //获得两者随机访问的时间
        System.out.println("arrayData get time:"+getTime(arrayData));
        System.out.println("linkedData get time:"+getTime(linkedData));
        //获得两者插入数据的时间
        System.out.println("arrayData insert time:"+insertTime(arrayData));
        System.out.println("linkedData insert time:"+insertTime(linkedData));

    }
    //获取数据
    public static long getTime(List<Integer> list){
        long time=System.currentTimeMillis();
        for(int i = 0; i < maxCount; i++){
            int getData = list.get(i);
        }
        return System.currentTimeMillis()-time;
    }

    //插入数据
    public static long insertTime(List<Integer> list){
        long num = maxCount;
        int index = 1000;
        long time=System.currentTimeMillis();
        for(int i = 1; i < num; i++){
            list.add(index, i);
        }
        return System.currentTimeMillis()-time;
    }

}

6.运行结果

maxCount:10000
arrayData get time:1
linkedData get time:82
arrayData insert time:24
linkedData insert time:27

maxCount:30000
arrayData get time:2
linkedData get time:658
arrayData insert time:127
linkedData insert time:58