ArrayList 源码分析

ArrayList 是平时常用的数据存储结构，它基于动态数组，支持随机访问，值得一提的是：它是不安全的。造成这种不安全的原因主要是：在在多线程环境下，向 ArrayList 添加或移除时，会产生数组越界等问题。推荐使用 Vector、CopyOnWriteArrayList 代替。

ArrayList 与 Array 区别：

Array 包含的是基本类型和对象类型，而ArrayList 只能包含对象类型
Array 的大小的是固定的，所以在定义数组的时候尽量确定需要的数组大小，而ArrayList 的大小是动态变化的，当容量不足时能自动扩容
ArrayList 可以看作Array的增强版，提供了更多的方法和特性，在查找等操作上更方便

定义

1 2	public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable

ArrayList 支持泛型，继承了 AbstractList，在实现了List接口的同时，实现了一下三个接口：

RandomAccess — 随机访问，即通过元素的序号快速获取对象
Cloneable — 覆盖了 clone()，能被克隆
Serializable — 支持序列化

属性

private static final long serialVersionUID = 8683452581122892189L;

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.
     */
    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.
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

其中，size 指 ArrayList 的大小，DEFAULT_CAPACITY 指在未定义 ArrayList大小的情况下，初始化 elementData 的长度为10
elementData 是 ArrayList 在存储数据时的 buffer，通过数组存储数据。同时，elementData 被 transient 修饰，所以在 ArrayList 序列化的时候，elementData 并不参与，而序列化的过程主要通过 writeObject()，readObject()实现。

构造方法

/**
 * 通过传入参数设置 ArrayList的容量
 */
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);
    }
}

/**
 * 默认的构造方法，默认容量为10的空数组
 */
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

/**
 * 使用集合c 初始化 ArrayList
 */
public ArrayList(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;
    }
}

具体方法

查找

/**
 * 正序查找
 * 判断查询对象是否为null, 若是则正序查找第一个值为 null 的对象，返回索引号
 */
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;
}

/**
 * 逆序查找
 * 判断查询对象是否为null, 若是则逆序查找第一个值为 null 的对象，返回索引号
 */
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;
}

拷贝

/**
 * 通过super.clone(), 获取 ArrayList 的一个副本，
 * 再设置 ArrayList 的 elementData 和 modCount 属性。因为拷贝的是新的副本，将新副本的 modCount 设为0。
 */
public Object clone() {
    try {
        ArrayList<?> v = (ArrayList<?>) super.clone();
        v.elementData = Arrays.copyOf(elementData, size);
        v.modCount = 0;
        return v;
    } catch (CloneNotSupportedException e) {
        // this shouldn't happen, since we are Cloneable
        throw new InternalError(e);
    }
}

在拷贝的过程中，发现它调用了Arrays.copoOf(original, lenght)，而Arrays.copy()又调用了System.arraycopy()

@SuppressWarnings("unchecked")
public static <T> T[] copyOf(T[] original, int newLength) {
    return (T[]) copyOf(original, newLength, original.getClass());
}


/**
 *  这里的又调用了System.arraycopy()
 */ 
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
    @SuppressWarnings("unchecked")
    T[] copy = ((Object)newType == (Object)Object[].class)
        ? (T[]) new Object[newLength]
        : (T[]) Array.newInstance(newType.getComponentType(), newLength);
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}

System.arraycopy()是个本地方法，底层调用了 c++ 方法实现，使其的效率更高

1
2
3

public static native void arraycopy(Object src,  int  srcPos,
                                        Object dest, int destPos,
                                        int length);

转化为数组

/**
 * 返回 elementData 的副本
 */
public Object[] toArray() {
    return Arrays.copyOf(elementData, size);
}

@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
    if (a.length < size)
        // Make a new array of a's runtime type, but my contents:
        return (T[]) Arrays.copyOf(elementData, size, a.getClass());
    System.arraycopy(elementData, 0, a, 0, size);
    if (a.length > size)
        a[size] = null;
    return a;
}

获取，修改元素

/**
 * 通过索引，返回查询elementData 中的值，便于其他方法的操作
 */
@SuppressWarnings("unchecked")
E elementData(int index) {
    return (E) elementData[index];
}

// 数组越界检查
private void rangeCheck(int index) {
    if (index >= size)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

/**
 *  先进行越界异常检查，判断 index < size 时,返回查询结果
 */
public E get(int index) {
    rangeCheck(index);

    return elementData(index);
}

/**
 * 一样，先进行越界检查，将新值替代为旧值，返回旧值
 */
 public E set(int index, E element) {
    rangeCheck(index);

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

移除

/**
 *  通过索引值删除
 *  System.arraycopy()将 elementData 的索引值后的元素向前移动，最后删除最后重复的元素值，size -1
 */
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);
    elementData[--size] = null; // clear to let GC do its work

    return oldValue;
}

/**
 *  通过对象值删除
 *  正序删除，当查找到要删除的值时，调用 fastRemove() 实现删除的具体操作
 */
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;
}

/**
 *  fastRemove() 相比于 remove(): 跳过 rangeCheck() 越界检查 和 不返回删除值，这样可以更快得删除元素
 */
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

/**
 * clear() 并不是将删除整个 elementData,而是清空的意思，所以将所有值赋予null
 */
 public void clear() {
    modCount++;

    // clear to let GC do its work
    for (int i = 0; i < size; i++)
        elementData[i] = null;

    size = 0;
}

添加

/**
 *  先容量判断，在数组尾部插入元素
 */
public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

/**
 *  先越界检查 和 容量判断，将待插入后的元素全部后移，再插入指定元素
 */
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++;
}

/**
 *  和add()方法类似，先将数组后移待插入的集合长度，再插入数组
 */
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;
}

扩容机制 (扩容机制在jdk 1.7开始做出了改动，主要改动：采用 位运算 替代之前的操作符, 加入 最大数组容量 的判断)

/**
 *  当需要的 容量空间 大于 默认容量，执行ensureExplicitCapacity(minCapacity)
 */
public void ensureCapacity(int minCapacity) {
    int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
        // any size if not default element table
        ? 0
        // larger than default for default empty table. It's already
        // supposed to be at default size.
        : DEFAULT_CAPACITY;

    if (minCapacity > minExpand) {
        ensureExplicitCapacity(minCapacity);
    }
}

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

    ensureExplicitCapacity(minCapacity);
}

/**
 *   modcount+1
 *   需求容量 大于 数组的大小时，执行grow()
 */
private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

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

/**
 *  扩容的具体方法
 *  oldCapacity + （ oldCapacity >> 1) 位运算向右移一位，使得oldCapacity 为原来的1.5倍
 *  当 newCapacity 大于数组的最大长度时，调用hugeCapacity()调整
 *  拷贝新的 elementData 副本
 */
private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    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:
    elementData = Arrays.copyOf(elementData, newCapacity);
}

/**
 *  调整容量
 */
private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
        throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
        MAX_ARRAY_SIZE;
}

序列化（序列化的过程是将size, elementData中的所有对象写入或者读取），不采用将整个elementData写入是因为节省空间

/**
 *  首先执行 defaultWriteObject()，目的是将 non-transient 和 non-final 对象写入读取。
 *  写入 size 和 对象
 */
private void writeObject(java.io.ObjectOutputStream s)
    throws java.io.IOException{
    // Write out element count, and any hidden stuff
    int expectedModCount = modCount;
    s.defaultWriteObject();

    // Write out size as capacity for behavioural compatibility with clone()
    s.writeInt(size);

    // Write out all elements in the proper order.
    for (int i=0; i<size; i++) {
        s.writeObject(elementData[i]);
    }

    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
}

private void readObject(java.io.ObjectInputStream s)
    throws java.io.IOException, ClassNotFoundException {
    elementData = EMPTY_ELEMENTDATA;

    // Read in size, and any hidden stuff
    s.defaultReadObject();

    // Read in capacity
    s.readInt(); // ignored

    if (size > 0) {
        // be like clone(), allocate array based upon size not capacity
        ensureCapacityInternal(size);

        Object[] a = elementData;
        // Read in all elements in the proper order.
        for (int i=0; i<size; i++) {
            a[i] = s.readObject();
        }
    }

排序 (ArrayList的排序采用的是Array的并归排序)

/**
 *  采用array的排序方式：并归排序
 */
@Override
@SuppressWarnings("unchecked")
public void sort(Comparator<? super E> c) {
    final int expectedModCount = modCount;
    Arrays.sort((E[]) elementData, 0, size, c);
    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
    modCount++;
}


public static <T> void sort(T[] a, int fromIndex, int toIndex,
                            Comparator<? super T> c) {
    if (c == null) {
        sort(a, fromIndex, toIndex);
    } else {
        rangeCheck(a.length, fromIndex, toIndex);
        if (LegacyMergeSort.userRequested)
            legacyMergeSort(a, fromIndex, toIndex, c);
        else
            TimSort.sort(a, fromIndex, toIndex, c, null, 0, 0);
    }
}

private static <T> void legacyMergeSort(T[] a, int fromIndex, int toIndex,
                                        Comparator<? super T> c) {
    T[] aux = copyOfRange(a, fromIndex, toIndex);
    if (c==null)
        mergeSort(aux, a, fromIndex, toIndex, -fromIndex);
    else
        mergeSort(aux, a, fromIndex, toIndex, -fromIndex, c);
}


@SuppressWarnings({"unchecked", "rawtypes"})
private static void mergeSort(Object[] src,
                              Object[] dest,
                              int low,
                              int high,
                              int off) {
    int length = high - low;

    // Insertion sort on smallest arrays
    if (length < INSERTIONSORT_THRESHOLD) {
        for (int i=low; i<high; i++)
            for (int j=i; j>low &&
                     ((Comparable) dest[j-1]).compareTo(dest[j])>0; j--)
                swap(dest, j, j-1);
        return;
    }

    // Recursively sort halves of dest into src
    int destLow  = low;
    int destHigh = high;
    low  += off;
    high += off;
    int mid = (low + high) >>> 1;
    mergeSort(dest, src, low, mid, -off);
    mergeSort(dest, src, mid, high, -off);

    // If list is already sorted, just copy from src to dest.  This is an
    // optimization that results in faster sorts for nearly ordered lists.
    if (((Comparable)src[mid-1]).compareTo(src[mid]) <= 0) {
        System.arraycopy(src, low, dest, destLow, length);
        return;
    }

    // Merge sorted halves (now in src) into dest
    for(int i = destLow, p = low, q = mid; i < destHigh; i++) {
        if (q >= high || p < mid && ((Comparable)src[p]).compareTo(src[q])<=0)
            dest[i] = src[p++];
        else
            dest[i] = src[q++];
    }
}

发布于 2018-04-15

tags:

{ Java }

{ 源码分析 }

{ ArrayList }