Java 集合 ArrayList 需要知道的几个问题

 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_CAPACITY
    private 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_CAPACITY
            minCapacity = 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) // overflow
            throw 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);
        }

        @Override
        public int size() {
            return a.length;
        }

        @Override
        public 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;
        }

        @Override
        public E get(int index) {
            return a[index];
        }

        @Override
        public E set(int index, E element) {
            E oldValue = a[index];
            a[index] = element;
            return oldValue;
        }

        @Override
        public 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;
        }

        @Override
        public boolean contains(Object o) {
            return indexOf(o) != -1;
        }

        @Override
        public Spliterator<E> spliterator() {
            return Spliterators.spliterator(a, Spliterator.ORDERED);
        }

        @Override
        public void forEach(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            for (E e : a) {
                action.accept(e);
            }
        }

        @Override
        public 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]);
            }
        }

        @Override
        public void sort(Comparator<? super E> c) {
            Arrays.sort(a, c);
        }
    }