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
     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);        }    }