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Java(List接口)集合ArrayList源码分析

概述

ArrayList本质上就是一个动态数组，所以通过下标访问的效率高，但是在增删操作时，需要消耗的性能较大。

类关系结构图

相关的接口抽象类的介绍

关键字段

  /**
	* Default initial capacity.
	* 默认的数组的长度
	*/
	private static final int DEFAULT_CAPACITY = 10;

  /**
	* Shared empty array instance used for empty instances.
	* 空数组
	*/
	private static final Object[] EMPTY_ELEMENTDATA = {};

  /**
	* Shared empty array instance used for default sized empty instances. We
	* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
	* first element is added.
	*/
	private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

  /**
	* The array buffer into which the elements of the ArrayList are stored.
	* The capacity of the ArrayList is the length of this array buffer. Any
	* empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
	* will be expanded to DEFAULT_CAPACITY when the first element is added.
	* 集合中存储数据的 数组对象
	*/
	transient Object[] elementData; // non-private to simplify nested class access

  /**
	* The size of the ArrayList (the number of elements it contains).
	* 集合中元素的个数
	* @serial
	*/
	private int size;

初始操作

无参构造
elementData=null;无参构造去并不会真实的创建数组，数组会在add方法中去创建，有助于性能的提升，懒加载的方式。

public ArrayList() {
	this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
	// this.elementData = {}
}

有参构造

public ArrayList(int initialCapacity) {
    if (initialCapacity > 0) {
    // 初始长度大于0 就创建一个指定大小的数组
         this.elementData = new Object[initialCapacity];
    } else if (initialCapacity == 0) {
        // {}数组赋值给 this.elementData
        this.elementData = EMPTY_ELEMENTDATA;
    } else {
        throw new IllegalArgumentException("Illegal Capacity: "+
            initialCapacity);
    }
}

add 方法

第一次添加

  /**
	* 将元素添加到集合的尾部
	*
	* @param e element to be appended to this list
	* @return <tt>true</tt> (as specified by {@link Collection#add})
	*/
	public boolean add(E e) {
	    // 确定容量 动态扩容 size 初始 0
	    ensureCapacityInternal(size + 1); // Increments modCount!!
	    // 将要添加的元素 添加到数组中 elementData[0] = 1 --> size = 1
	    elementData[size++] = e;
	    return true;
	}

private void ensureCapacityInternal(int minCapacity) {
    // ensureExplicitCapacity(10)
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

/**
 * elementData {}
 minCapacity 1
 */
private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        // 10 1 return 10
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    // 5
    return minCapacity;
}

private void ensureExplicitCapacity(int minCapacity) {
    modCount++; // 增长 操作次数

    // minCapacity 10    
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

private void grow(int minCapacity) { // 10
    // 记录原来的容量
    int oldCapacity = elementData.length; // 0
    // newCapacity = 0
    // 计算新的容量 新容量为老容量的1.5倍，第一次为0
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    if (newCapacity - minCapacity < 0)
        // newCapacity = 10
        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
        // {} {,,,,,,,,,} 返回一个新的数组 长度为10
        // 把原来的数组中的内容拷贝到一个新建的指定容量为newCapacity的数组中，扩容
    elementData = Arrays.copyOf(elementData, newCapacity);
}

第二次添加

elementData = {1,,,,,,,,,};
size = 1;

public boolean add(E e) {
    // 2
    ensureCapacityInternal(size + 1); // Increments modCount!!
    elementData[size++] = e; // elementData[1] = 2 size = 2
    return true;
}

private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    // 2
    return minCapacity;
}

private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // overflow-conscious code 2 - 10
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

第十一次添加

elementData = {1,2,3,4,5,6,7,8,9,10};
size = 10;

public boolean add(E e) {
    ensureCapacityInternal(size + 1); // Increments modCount!!
    elementData[size++] = e;
    return true;
}

private void ensureCapacityInternal(int minCapacity) {
    // ensureExplicitCapacity(11)
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // 11 - 10 > 0
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

private void grow(int minCapacity) { // 11
    // 10
    int oldCapacity = elementData.length;
    // 15 newCapacity 是oldCapacity的1.5倍
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    // minCapacity is usually close to size, so this is a win:
    // {1,2,3,4,5,6,7,8,9,10} -- > {1,2,3,4,5,6,7,8,9,10,,,,,}
    elementData = Arrays.copyOf(elementData, newCapacity);
}

get方法

public E get(int index) {
	// 检查下标是否合法
	rangeCheck(index);
	// 通过下标获取数组对应的元素
	return elementData(index);
}

set方法

public E set(int index, E element) {
    rangeCheck(index); // 检查下标
    // 获取下标原来的值
    E oldValue = elementData(index);
    elementData[index] = element;
    return oldValue;
}

remove方法

public E remove(int index) {
    // 检查下标
    rangeCheck(index);    

    modCount++;
    E oldValue = elementData(index);
    // 获取要移动的元素的个数 {1,2,3,4,5,6,7,8,9} // 3 size=9 index=3
    // {1,2,3,5,6,7,8,9,null}
    int numMoved = size - index - 1; // 5
    if (numMoved > 0)
    // 源数组 开始下标 目标数组 开始下标 长度
        System.arraycopy(elementData, index+1, elementData, index,
            numMoved);
    elementData[--size] = null; // clear to let GC do its work
    // 删除的节点对应的信息
    return oldValue;
}

FailFast机制

快速失败机制，也就是在多线程操作一个List数据的时候如果出现数据安全问题会直接抛异常（java.util.ConcurrentModificationException）。

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