java集合之ArrayList源码解读

源自：jdk1.8.0_121

ArrayList继承自AbstractList，实现了List、RandomAccess、Cloneable、Serializable。

ArrayList内部是通过数组及数组的扩容来实现

变量

    // 默认容量为10
    private static final int DEFAULT_CAPACITY = 10;

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

    // 默认空数组
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    // 存放数据的数组，被transient修饰的参数不会被序列化
    transient Object[] elementData;

    // 实际元素的大小
    private int size;

构造方法

    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);
        }
    }

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

    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;
        }
    }

toArray()实现方式的不同

注：ArrayList(Collection<? extends E> c) 之所以要判断是否为Object类型，是因为调用toArray()方法的实现方式不同。

在java.util.ArrayList中，toArray()返回的是Object数组。

    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

在java.util.Arrays中，有个名为ArrayList的内部类，当调用Arrays.asList()时，返回的是java.util.Arrays$ArrayList内部类对象，而并非java.util.ArrayList。java.util.Arrays$ArrayList中的toArray()返回的虽然是Object数组，但它是有真是类型的数组。

    private final E[] a;

    public Object[] toArray() {
        return a.clone();
    }

ArrayList扩容

    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);
    }

    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);
        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);
    }

从源码可以看出，如果1.5倍的elementData.length小于10，会将elementData的大小扩容成默认的10，反之，则会以1.5倍的elementData.length进行扩容。

将elementData设置为实际容量，动态扩充的多余容量将被删除

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