动态数组(代码实现)

public class ArrayList<E> {
    /***
     * 元素的数量
     */
    private int size;
    /**
     * 所有的元素
     */
    private E[] elements;
    private static final int DEFAULT_CAPACITY = 10;
    private static final int ELEMENT_NOT_FOUND = -1;

    public ArrayList(int capacticy) {
        capacticy = (capacticy < DEFAULT_CAPACITY) ? DEFAULT_CAPACITY : capacticy;
        elements = (E[])new Object[capacticy];
    }

    public ArrayList() { this(DEFAULT_CAPACITY); }

    /**
     * 清除所有的元素
     */
    public void clear() {
        for (int i = 0; i < size; i++) {
            elements[i] = null;
        }
        size = 0;
    }
    /**
     * 元素的数量
     */
    public int size() { return size; }

    /***
     * 添加到数组尾部
     * @param element
     */
    public void add(E element) {
        add(size, element);
    }
    /**
     * 获得index位置处的元素
     * @param index
     * @return
     */
    public E get(int index) {
        rangeCheck(index);
        return elements[index];
    }

    /**
     * 设置index位置的元素
     * @param index
     * @param element
     * @return
     */
    public E set(int index, E element) {
        rangeCheck(index);
        E old = elements[index];
        elements[index] = element;
        return old;
    }
    public E remove(int index) {
        rangeCheck(index);
        E old = elements[index];
        for (int i = index + 1; i < size; i++) {
            elements[i - 1] = elements[i];
        }
        elements[--size] = null;
        return old;
    }
    /***
     * 判断是否为空
     * @return
     */
    public boolean isEmpty() { return size == 0; }
    public boolean contains(E elements) {
        return indexOf(elements) != ELEMENT_NOT_FOUND;
    }
    public int indexOf(E element) {
        if (element == null) {
            for (int i = 0; i < size; i++) {
                if (elements[i] == null) { return i; }
            }
        } else {
            for (int i = 0; i < size; i++) {
                if (element.equals(elements[i])) { return i; }
            }
        }
        return ELEMENT_NOT_FOUND;
    }
    /**
     * 在index处插入元素
     * @param index
     * @param element
     */
    public void add(int index, E element) {
        rangeCheckForAdd(index);
        ensureCapacity(size + 1);
        for (int i = size; i > index ; i--) {
            elements[i] = elements[i - 1];
        }
        elements[index] = element;
        size++;
    }
    private void rangeCheck(int index){
        if (index < 0 || index >= size) {
            outOfBounds(index);
        }
    }
    private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size) {
            outOfBounds(index);
        }
    }
    private void outOfBounds(int index) {
        throw new IndexOutOfBoundsException("Index" + index + "Size" + size);
    }

    /**
     * 保证capacity的容量
     * @param capacity
     */
    private void ensureCapacity(int capacity) {
        int oldCapacity = elements.length;
        if (oldCapacity > capacity) return;
        // 扩容 新容量扩容1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        E[] newElements = (E[]) new Object[newCapacity];
        for (int i = 0; i < size; i++) {
            newElements[i] = elements[i];
        }
        elements = newElements;

        System.out.println(oldCapacity + "扩容为" + newCapacity);
    }

    @Override
    public String toString() {
        StringBuilder string = new StringBuilder();
        string.append("size=").append(size).append(",[");
        for (int i = 0; i < size; i++) {
            if (i != 0) {
                string.append(", ");
            }
            string.append(elements[i]);
        }
        string.append("]");

        return string.toString();
    }
}