数组的各种插入/删除

public class MyArrayList implements IArrayList{
    private int[] array;    //保存数据的空间
    private int size;       //保存有效数据个数
    MyArrayList(int capacity){
        this.array = new int[capacity];
        this.size = 0;
    }

    //O(n)
    @Override
    public void pushFront(int item) {
        ensureCapacity();
        //将数据中已有的数据后移一格
        for (int i = this.size;i>=1;i--){
            this.array[i] = this.array[i-1];
        }
        this.array[0] = item;
        this.size++;
    }

    //O(1)
    @Override
    public void pushBack(int item) {
        ensureCapacity();
        this.array[this.size] = item;
        this.size++;
    }


//最坏时间复杂度O(n)
    @Override
    public void add(int item, int index){
        if(index < 0||index > this.size){
            throw new Error();
        }
        ensureCapacity();
        //i代表的是数据所在的下标
        for (int i = this.size - 1; i >= index; i--) {
            this.array[i + 1] = this.array[i];
        }
        this.array[index] = item;
        this.size--;
    }

    /**
     * i代表循环的次数
     * for(int i=0;i<this.size-index;i++){
     *     this.array[this.size-i]=this.array[this.size-i-1];
     * }
     */


    //O(n)
    @Override
    public void popFront() {
        if(this.size == 0){
            throw new Error();
        }
        for(int i=1;i<this.size;i++){
            this.array[i-1]=this.array[i];
        }
        /**
        for(int j=0;j<this.size-1;j++){
            this.array[j]=this.array[j+1];
        }
         */
        this.size--;
    }

    //O(1)
    @Override
    public void popBack() {
        if(this.size == 0){
            throw new Error();
        }
        //this.array[--this.size] = 0;
        this.size--;
    }

    //O(n)
    @Override
    public void remove(int index) {
        if(index < 0||index > this.size){
            throw new Error();
        }
        if(this.size == 0){
            throw new Error();
        }
        for(int i=index;i<this.size-2;i++){
            this.array[i]=this.array[i+1];
        }
        size--;
    }

    //O(n)
    /**
     * 保证数组容量够用
     */
    private void ensureCapacity() {
        if (this.size < this.array.length) {
            return;
        }

        //1、找新房子，找原来的2倍大小
        int capacity = this.array.length * 2;
        int[] newArray = new int[capacity];

        //2、搬家
        for(int i = 0;i<this.size;i++){
            newArray[i] = this.array[i];
        }
        //3、去学校登记新房子的位置，退掉新房子
        this.array = newArray;
    }
}