JDK1.8 ArrayList 源码解析

源码的解读逻辑按照程序运行的轨迹展开

1. Arraylist的继承&实现关系
   
   打开ArrayList源码，会看到有如下的属性定义，

2. ArrayList中定义的属性

    /**
     * Default initial capacity. 
     * 初始容量
     */
    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.
     * 默认容量的空数组
     */
    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.
     * 真正存放对象的数组
     */
    transient Object[] elementData; 

     /**
     * The size of the ArrayList (the number of elements it contains).
     * 实际数据的数量
     * @serial
     */
    private int size;

    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     * Integer 最大值
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

当运行 ArrayList list = new ArrayList<>() ; ，因为它没有指定初始容量，所以它调用的是它的无参构造

//无参构造，
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
// 指定初始容量
public ArrayList(int initialCapacity) {
    if (initialCapacity > 0) {
        this.elementData = new Object[initialCapacity];//创建数组
    } else if (initialCapacity == 0) {
        this.elementData = EMPTY_ELEMENTDATA;
    } else {
        throw new IllegalArgumentException("Illegal Capacity: "+                                     initialCapacity);
    }
}

当我们仅仅new出一个ArrayList时，它仅仅只会创建一个空数组，由此我们可以得知它的初始化操作被延迟到了第一次add()

    //添加一个元素
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }
    public static int max(int a, int b) {
        return (a >= b) ? a : b;
    }
//判断是否要扩容,minCapacity的值大于add数据之前的大小，就调用grow方法，进行扩容，否则什么也不做
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);//扩充capacity,将其向右一位再加上原来的数，实际上是扩充了1.5倍
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)//确保数组的容量不大于Integer的最大值
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);//复制
    }

对源码阅读有问题的可以把以下代码复制自行运行，这是一个简版的ArrayList,是我从JDK源码中抽取出来的，理解下面的代码再去看JDK的源码相信就很简单了

package com.tanoak.list.arraylist;

import java.io.Serializable;
import java.util.Arrays;
import java.util.Collection;

/**
 * @Desc  自定义ArrayList集合类， 基于数组实现
 */
public class TkArrayList<E> implements Serializable {


	/**
	 *
	 * 初始容量
	 */
	private static final int DEFAULT_CAPACITY = 10;

	/**
	 * 空数组
	 */
	private static final Object[] EMPTY_ELEMENTDATA = {};

	/**
	 * 默认容量的空数组
	 */
	private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

	/**
	 * 真正存放数据的数组
	 */
	 transient Object[] elementData;

	/**
	 * 实际数据的数量
	 */
	private int size;

	/**
	 * 记录了ArrayList结构性变化的次数
	 */
	protected transient int modCount = 0;


	/**
	 * Integer 最大值
	 */
	private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;


	public TkArrayList() {
		this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
	}


	/**
	 * 指定数组大小
	 * @param initialCapacity
	 */
	public TkArrayList(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);
		}
	}

	/**
	 * 构造一个包含指定元素的list，这些元素的是按照Collection的迭代器返回的顺序排列的
	 * @param c
	 */
	public TkArrayList(Collection<? extends E> c) {
		elementData = c.toArray();
		if ((size = elementData.length) != 0) {
			// c.toArray might (incorrectly) not return Object[] (see 6260652)
			if (elementData.getClass() != Object[].class){
				elementData = Arrays.copyOf(elementData, size, Object[].class);
			}
		} else {
			// replace with empty array.
			this.elementData = EMPTY_ELEMENTDATA;
		}
	}
	//增
	/**
	 *  新增元素
	 * @param e
	 * @return
	 */
	public boolean add(E e) {
		// Increments modCount!!
		ensureCapacityInternal(size + 1);
		elementData[size++] = e;
		return true;
	}

	/**
	 *
	 * @param minCapacity
	 */
	private void ensureCapacityInternal(int minCapacity) {
		if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
			minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
		}

		ensureExplicitCapacity(minCapacity);
	}

	/**
	 * 判断是否扩容
	 * @param minCapacity
	 */
	private void ensureExplicitCapacity(int minCapacity) {
		modCount++;

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

	}
   //进行扩容
	private void grow(int minCapacity) {
		// overflow-conscious code
		int oldCapacity = elementData.length;

		//扩充capacity,将其向右一位再加上原来的数，实际上是扩充了1.5倍
		int newCapacity = oldCapacity + (oldCapacity >> 1);
		if (newCapacity - minCapacity < 0){
			newCapacity = minCapacity;
		}
		//确保数组的容量不大于Integer类型最大值
		if (newCapacity - MAX_ARRAY_SIZE > 0){
			newCapacity = hugeCapacity(minCapacity);
		}
		// //复制数据
		elementData = Arrays.copyOf(elementData, newCapacity);
	}

	private static int hugeCapacity(int minCapacity) {
		if (minCapacity < 0) {
			// overflow
			throw new OutOfMemoryError();
		}
		return (minCapacity > MAX_ARRAY_SIZE) ?
				Integer.MAX_VALUE :
				MAX_ARRAY_SIZE;
	}
	//查
	/**
	 * 根据索引 调用 elementData 返回值
	 * @param index
	 * @return
	 */
	public E get(int index) {
		rangeCheck(index);

		return elementData(index);
	}

	/**
	 * 根据索引取出值
	 * @param index
	 * @return
	 */
	E elementData(int index) {
		return (E) elementData[index];
	}

	private void rangeCheck(int index) {
		if (index >= size){
			throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
		}
	}

	/**
	 * 越界信息
	 * @param index
	 * @return
	 */
	private String outOfBoundsMsg(int index) {
		return "Index: "+index+", Size: "+size;
	}

	//删
	/**
	 *
	 * @param index
	 * @return
	 */
	public E remove(int index) {
		rangeCheck(index);
		modCount++;
		E oldValue = elementData(index);

		int numMoved = size - index - 1;
		if (numMoved > 0){
			System.arraycopy(elementData, index+1, elementData, index, numMoved);
		}
		// clear to let GC do its work
		elementData[--size] = null;

		return oldValue;
	}
	
	//改
	public E set(int index, E element) {
		rangeCheck(index);

		E oldValue = elementData(index);
		elementData[index] = element;
		return oldValue;
	}
}

ArrayList比较难理解的就是扩容，思路首先理清楚，但是只要理清楚几个属性在方法中所做的判断，然后运行上面简版的源码，多熟悉几次就不成问题了

• 如理解有误，请指正