动态数组的实现

public class MyArrayList<E> {

    private int size=0;
    private E[] elements;
    private static final int DEFAULT_CAPACITY=10;
    private static final int ELEMENT_NOTFOUND=-1;

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

    public MyArrayList() {
        //elements=new int[DEFAULT_CAPACITY];
        this(DEFAULT_CAPACITY); //在无参数构造方法中调用有参的构造方法
    }

    /**
     * 清楚所有的元素
     * size 为0 无法访问之前存储的元素
     * 动态覆盖之前的元素 size从0开始
     */
    public void clear(){
        //确保堆内存中不能循环利用的对象被销毁(重点)
        for (int i=0;i<size;i++){
            elements[i]=null;
        }
        size=0;
    }

    public int size(){
        return size;
    }

    public boolean isEmpty(){
        return size==0;
    }

    public boolean contains(E element){
        return indexOf(element)!=ELEMENT_NOTFOUND;
    }

    public void add(E element){
        add(size,element);
    }

    public void add(int index,E element){
        rangeCheckforAdd(index);
        ensureCapacity(size+1);//作用是确保容量值可以达到size+1
        //注意扩容，最右侧的元素向右位移动，不能被覆盖
        for (int i=size-1;i>=index;i--){
            elements[i+1] = elements[i];
        }
        elements[index]=element;
        size++;
    }

    public E get(int index){
        if (index<0||index>=size){
            throw new IndexOutOfBoundsException("Index:"+index+" size "+size);
        }
        return elements[index];
}

    /**
     *
     * @param index
     * @param element
     * @return 返回原来存放的元素
     */
    public E set(int index,E element){
        if (index<0||index>=size){
            throw new IndexOutOfBoundsException("Index:"+index+" size "+size);
        }
        E old = elements[index];
        elements[index] = element;
        return old;
    }

    /**
     *
     * @param index
     * @return 返回被删除的元素
     */
    public E remove(int index){
        rangeCheck(index); //范围是0~size-1
        // 有bug,最后一个元素的删除是通过size减一去去除
        E old=elements[index];
        for (int i=index;i<size-1;i++){
            elements[index]=elements[index+1];
        }
        //上面的i取index-1；也就是删除最后一个元素，不进入for
//        size--;//去掉最后一个元素了
//        elements[size]=null;
        //上面两句可以合成成一句
        elements[--size]=null; //使用累减后的值进行运算
        return old;
    }

    /**
     * @param element
     * @return 返回元素对应的下标位置
     * 实现时需要注意空指针异常
     */
    public int indexOf(E element){
        if (element==null){
            for (int i=0;i<size;i++){
                if (elements[i]==null){
                    return i;
                }
            }
        }
        //这里要重写equals
        for (int i=0;i<size;i++){
            if (element.equals(elements[i])){
                return i;
            }
        }
        return ELEMENT_NOTFOUND;
    }
    @Override
    public String toString() {
        StringBuilder stringBuilder = new StringBuilder();
        stringBuilder.append("size: ").append(size).append(" [");
        for (int i = 0; i < size; i++) {
            stringBuilder.append(elements[i]);
            if (i != size - 1) {
                stringBuilder.append(",");
            }
        }
        stringBuilder.append("]");
        return stringBuilder.toString();
    }
    //封装错误信息
    private void outOfBounds(int index){
        throw new IndexOutOfBoundsException("Index:"+index+" size "+size);
    }
    //封装indexCheck
    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 ensureCapacity(int capacity){
        int oldCapacity = elements.length;
        if (oldCapacity>=capacity){
            return;
        }
        int newCapacity=oldCapacity+(oldCapacity>>1); //右移操作一定要加括号
        System.out.println(newCapacity);
        E[] newElements=(E[])new Object[newCapacity];
        for (int i=0;i<size;i++){
            newElements[i]=elements[i];
        }
        elements=newElements;
    }
}