ArrayList源码

ArrayList

• 本质是数组
• 初始容量DEFAULT_CAPACITY = 10
• get ， set操作比较有效率，是直接根据索引来操作
• add，remove操作涉及到index时，需要对数组拷贝，消耗大

代码块

成员变量：

/**
 * 默认数组容量
 */
private static final int DEFAULT_CAPACITY = 10;

/**
 * 有参构造器初始容纳数组
 */
private static final Object[] EMPTY_ELEMENTDATA = {};

/**
 * 无参构造器初始容纳数组
 */
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

/**
 * 存放数据的缓冲区
 */
transient Object[] elementData; // non-private to simplify nested class access

/**
 * 容纳数据数
 */
private int size;

构造器：

三种构造方式：无参数，int参数，Collection参数

/**
  * 无参构造器，默认以初始容量DEFAULT_CAPACITY = 10 构造
  */
 public ArrayList() {
     this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
 }
  
/**
  * 参数为int的构造器
  */
 public ArrayList(int initialCapacity) {
     if (initialCapacity > 0) {  //initialCapacity参数大于0，直接构建对象
         this.elementData = new Object[initialCapacity];
     } else if (initialCapacity == 0) {//initialCapacity参数等于0，用EMPTY_ELEMENTDATA
         this.elementData = EMPTY_ELEMENTDATA;
     } else {
         throw new IllegalArgumentException("Illegal Capacity: "+
                                            initialCapacity);
     }
 }
 
 /**
  * 参数为Collection的构造器
  */
 public ArrayList(Collection<? extends E> c) {
     elementData = c.toArray();//将Collection参数转数组
     if ((size = elementData.length) != 0) {//Collection参数中存在数据时（非空Collection）
         // 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;
     }
 }

辅助方法：

 
/**
  * 增加Array容量
  */
private void ensureExplicitCapacity(int minCapacity) {
     modCount++;
     // overflow-conscious code
     if (minCapacity - elementData.length > 0)// 要求的容量大于此时数组容量，触发grow（）
         grow(minCapacity);
 }
 
 /**
  * 最大容量
  * 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 
  */
 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

 private void grow(int minCapacity) {
     // overflow-conscious code
     int oldCapacity = elementData.length;//当前数组容量
     int newCapacity = oldCapacity + (oldCapacity >> 1);//新数组容量（右移一位代表除2^1位，新容量是旧容量的1.5倍）
     if (newCapacity - minCapacity < 0)//新容量仍然不满足要求的容量
         newCapacity = minCapacity;
     if (newCapacity - MAX_ARRAY_SIZE > 0)//新容量大于最大容量
         newCapacity = hugeCapacity(minCapacity);
     // minCapacity is usually close to size, so this is a win:
     elementData = Arrays.copyOf(elementData, newCapacity);
 }
 
 private static int hugeCapacity(int minCapacity) {
     if (minCapacity < 0) // overflow  ？？？？为什么会出现小于0情况
         throw new OutOfMemoryError();
     return (minCapacity > MAX_ARRAY_SIZE) ?   //要求的容量大于最大容量时用Integer.MAX_VALUE尝试 ，否则用MAX_ARRAY_SIZE（也就是不可以按1.5倍来扩容了，数组实在太大）
         Integer.MAX_VALUE :
         MAX_ARRAY_SIZE;
 }

常用API :

  
   /**
     * 正序返回index
     */
    public int indexOf(Object o) {
        if (o == null) {
            for (int i = 0; i < size; i++)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = 0; i < size; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

    /**
     * 倒序返回index
     */
    public int lastIndexOf(Object o) {
        if (o == null) {
            for (int i = size-1; i >= 0; i--)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = size-1; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

    /**
     * 根据index获取数据元素，对数组直接操作，比较高效
     */
    public E get(int index) {
        rangeCheck(index);
        return elementData(index);
    }

    /**
     * 根据index放入数据元素，对数组直接操作，比较高效
     */
    public E set(int index, E element) {
        rangeCheck(index);
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

    /**
     * 不要求index次序的添加，对数组直接操作，也比较高效
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    /**
     * 要求index次序的添加，需要拷贝数组，比较耗费资源
     */
    public void add(int index, E element) {
        rangeCheckForAdd(index);
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }
    /**
     * 移除指定位置元素，需要拷贝数组，比较耗费资源
     */
    public E remove(int index) {
        rangeCheck(index);
        modCount++;
        E oldValue = elementData(index);//被移除的元素
        int numMoved = size - index - 1;//index后的元素数目
        if (numMoved > 0)//index后存在元素（也就是被移除的这个元素不是最后一个）
            System.arraycopy(elementData, index+1, elementData, index, numMoved);//index后存在元素（也就是被移除的这个元素不是最后一个）将index后的元素复制过来
                            
        elementData[--size] = null; // （最后一个数组位置将其变成null,因为复制完后面的元素均前移一步，最后位置此时无值）clear to let GC do its work 
        return oldValue;
    }
    /**
     * 移除指定的元素，需要拷贝数组，比较耗费资源
     */
    public boolean remove(Object o) {
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }
  

/**
     * 与public remove(int index)方法差不多,只不过没有返回值
     */
    private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }
    /**
     * 清除所有元素
     */
    public void clear() {
        modCount++;
        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;
        size = 0;
    }
    /**
     * 将Collection添加进ArrayList中，需要拷贝数组，比较耗费资源
     */
    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }
    /**
     * 根据index来添加Collection，需要拷贝数组，比较耗费资源
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount
        int numMoved = size - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);
        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }

---

三种循环效率

public class testArrayList {
    public static void main(String[] args) {
        List list = new ArrayList();
        for (int i = 0; i <10000 ; i++) {
            list.add(i);
        }
        testArrayList.iterateByForEach(list);
        testArrayList.iterateByiterator(list);
        testArrayList.iterateByRandomAccess(list);
    }

    private static void iterateByRandomAccess (List list) {
        long startTime;
        long endTime;
        startTime = System.currentTimeMillis();
        for (int i = 0; i <list.size() ; i++) {
            list.get(i);
        }
        endTime = System.currentTimeMillis();
        System.out.println("iterateByRandomAccess Time is :" + (endTime - startTime));
    }

    private static void iterateByForEach (List list) {
        long startTime;
        long endTime;
        startTime = System.currentTimeMillis();
        for (Object obj : list) {

        }
        endTime = System.currentTimeMillis();
        System.out.println("iterateByForEach Time is :" + (endTime - startTime));
    }

    private static void iterateByiterator (List list) {
        long startTime;
        long endTime;
        startTime = System.currentTimeMillis();
        for (Iterator iterator = list.iterator(); iterator.hasNext();) {
            iterator.next();
        }
        endTime = System.currentTimeMillis();
        System.out.println("iterateByiterator Time is :" + (endTime - startTime));
    }

}

• 随机访问 iterateByRandomAccess 最快
• iforEach循环 中等速度
• 迭代器 iterateByiterator 最慢