public boolean add(E e) {//确保容量够用，内部会尝试扩容，如果需要ensureCapacityInternal(size + 1);  // Increments modCount!!elementData[size++] = e;return true;}
//在未指定容量的情况下，容量为DEFAULT_CAPACITY = 10//并且在第一次使用时创建容器数组，在存储过一次数据后，数组的真实容量至少DEFAULT_CAPACITYprivate void ensureCapacityInternal(int minCapacity) {//判断当前的元素容器是否是初始的空数组if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {//如果是默认的空数组，则 minCapacity 至少为DEFAULT_CAPACITYminCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);}ensureExplicitCapacity(minCapacity);}//通过该方法进行真实准确扩容尝试的操作private void ensureExplicitCapacity(int minCapacity) {modCount++;//记录List的结构修改的次数//需要扩容if (minCapacity - elementData.length > 0)grow(minCapacity);}private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;//扩容操作private void grow(int minCapacity) {//原来的容量int oldCapacity = elementData.length;//新的容量 = 原来的容量 + （原来的容量的一半）int newCapacity = oldCapacity + (oldCapacity >> 1);//如果计算的新的容量比指定的扩容容量小，那么就使用指定的容量if (newCapacity - minCapacity < 0)newCapacity = minCapacity;//如果新的容量大于MAX_ARRAY_SIZE(Integer.MAX_VALUE - 8)//那么就使用hugeCapacity进行容量分配if (newCapacity - MAX_ARRAY_SIZE > 0)newCapacity = hugeCapacity(minCapacity);// minCapacity is usually close to size, so this is a win://创建长度为newCapacity的数组，并复制原来的元素到新的容器，完成ArrayList的内部扩容elementData = Arrays.copyOf(elementData, newCapacity);}private static int hugeCapacity(int minCapacity) {if (minCapacity < 0) // overflowthrow new OutOfMemoryError();return (minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE :MAX_ARRAY_SIZE;}

        ArrayList<Integer> list = new ArrayList<Integer>();list.add(1);list.add(2);list.add(3);Integer[] array1 = new Integer[3];list.toArray(array1);Integer[] array2 = list.toArray(new Integer[0]);System.out.println(Arrays.equals(array1, array2));// 1 结果是什么？为什么？
Integer[] array = { 1, 2, 3 };List<Integer> list =  Arrays.asList(array);list.add(4);// 2 结果是什么？为什么？
Integer[] array = { 1, 2, 3 };List<Integer> list = new ArrayList<Integer>(Arrays.asList(array));list.add(4);// 3 结果是什么？为什么？

    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;}

    public static <T> List<T> asList(T... a) {return new ArrayList<>(a);}private static class ArrayList<E> extends AbstractList<E>implements RandomAccess, java.io.Serializable{private static final long serialVersionUID = -2764017481108945198L;private final E[] a;ArrayList(E[] array) {a = Objects.requireNonNull(array);}@Overridepublic int size() {return a.length;}@Overridepublic Object[] toArray() {return a.clone();}@Override@SuppressWarnings("unchecked")public <T> T[] toArray(T[] a) {int size = size();if (a.length < size)return Arrays.copyOf(this.a, size,(Class<? extends T[]>) a.getClass());System.arraycopy(this.a, 0, a, 0, size);if (a.length > size)a[size] = null;return a;}@Overridepublic E get(int index) {return a[index];}@Overridepublic E set(int index, E element) {E oldValue = a[index];a[index] = element;return oldValue;}@Overridepublic int indexOf(Object o) {E[] a = this.a;if (o == null) {for (int i = 0; i < a.length; i++)if (a[i] == null)return i;} else {for (int i = 0; i < a.length; i++)if (o.equals(a[i]))return i;}return -1;}@Overridepublic boolean contains(Object o) {return indexOf(o) != -1;}@Overridepublic Spliterator<E> spliterator() {return Spliterators.spliterator(a, Spliterator.ORDERED);}@Overridepublic void forEach(Consumer<? super E> action) {Objects.requireNonNull(action);for (E e : a) {action.accept(e);}}@Overridepublic void replaceAll(UnaryOperator<E> operator) {Objects.requireNonNull(operator);E[] a = this.a;for (int i = 0; i < a.length; i++) {a[i] = operator.apply(a[i]);}}@Overridepublic void sort(Comparator<? super E> c) {Arrays.sort(a, c);}}