第1天 ArrayList源码分析

ArrayList是一个有序列表。

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

• RandomAccess 标识接口，无内部方法，表示可以随机访问内部元素
• Cloneable 标识接口，无内部方法，表示可以进行负责
• java.io.Serializable 标识接口，无内部方法，表示支持序列化

底层数据结构是一个对象数组

transient Object[] elementData; 

ArrayList对象有存储上限

private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

缩容，当buffer分配的空间容量大于实际存储的元素数量时，通过缩容释放对于的资源。

public void trimToSize() {
    modCount++;
    if (size < elementData.length) {
        elementData = (size == 0)
          ? EMPTY_ELEMENTDATA
          : Arrays.copyOf(elementData, size);
    }
}

扩容，步长为原容量的 50%， 扩容后将原buffer复制到新创建的buffer中。所以ArrayList如果存在频繁扩容的情况，会引起程序性能下降。

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);
}

插入或删除指定位置的Element，都会引起底层数据的批量复制

插入

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++;
    }

删除

 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;
}


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
}

清空数据时，并不释放底层数组分配的空间，只是将数组中的对象持有复制成null,方便虚拟机在GC时，直接将对象内存释放。

public void clear() {
    modCount++;

    // clear to let GC do its work
    // 释放句柄，告诉GC对象可以被回收，但并不是立即回收
    for (int i = 0; i < size; i++)
        elementData[i] = null;

    size = 0;
}

在数据计算中存在集合求交集运算，在ArrayList中同样提供了该方法。

求交集

public boolean retainAll(Collection<?> c) {
    Objects.requireNonNull(c);
    return batchRemove(c, true);
}

private boolean batchRemove(Collection<?> c, boolean complement) {
    final Object[] elementData = this.elementData;
    int r = 0, w = 0;
    boolean modified = false;
    try {
        for (; r < size; r++)
            if (c.contains(elementData[r]) == complement)
                elementData[w++] = elementData[r];
    } finally {
        // Preserve behavioral compatibility with AbstractCollection,
        // even if c.contains() throws.
        if (r != size) {
            System.arraycopy(elementData, r,
                             elementData, w,
                             size - r);
            w += size - r;
        }
        if (w != size) {
            // clear to let GC do its work
            for (int i = w; i < size; i++)
                elementData[i] = null;
            modCount += size - w;
            size = w;
            modified = true;
        }
    }
    return modified;
}

ArrayList实现了Serializable接口，表示它可以被序列化，在其内部提供了jdk标准的序列化和反序列号的方法。通过序列化和发序列化方法可以发现，JDK并不是把ArrayList所有属性都写入文件，只是将size，以及数组中的每一个对象进行了序列化。

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();
        }
    }
}