手写ArrayList

java基础之简单手写ArrayList，闲话休提，直接上代码：

import java.util.Arrays;

/**
 * @Author: gerald
 * @Description: 手写ArrayList
 * @Date: Created in 2020/12/16 18:23
 * @Modified By:
 **/
public class MyArrayList {
    //存储任意类型元素
    private Object[] elements = null;
    //数组中存储多少元素
    private int size = 0;

    private static final int DEFAULT_INITIAL_CAPACITY = 10;

    public MyArrayList() {
        this(10);
    }

    public MyArrayList(int initialCapacity) {
        if (initialCapacity < 0) {
            throw new IllegalArgumentException("容量不能为负");
        }
        elements = new Object[initialCapacity];
    }

    //保存新的元素
    public void add(Object ele) {
        //判断和扩容
        if (size == elements.length) {
            Object[] temp = Arrays.copyOf(elements, elements.length * 2);
            elements = temp;
        }
        elements[size] = ele;
        size++;
    }

    //查询指定索引位置元素
    public Object get(int index) {
        if (index < 0 || index > size) {
            throw new IllegalArgumentException("容量不能为负");
        }
        return elements[index];
    }

    //替换指定位置元素
    public void set(int index, Object newEle) {
        if (index < 0 || index > size) {
            throw new IllegalArgumentException("容量不能为负");
        }
        elements[index] = newEle;
    }

    //删除指定索引位置元素
    public void remove(int index) {
        if (index < 0 || index > size) {
            throw new IllegalArgumentException("容量不能为负");
        }
        for (int i = index; i < size - 1; i++) {
            //把index+1位置移动到index位置
            elements[i] = elements[i + 1];
        }
        //把最后一个位置设置为null
        elements[size - 1] = null;
        //size-1
        size--;
    }

    //返回数组元素个数
    public int size() {
        return size;
    }

    //判断数组中元素个数是否为0
    public boolean isEmpty() {
        return size == 0;
    }

    //清空元素
    public void clear() {
        this.elements = new Object[DEFAULT_INITIAL_CAPACITY];
        size = 0;
    }

    @Override
    public String toString() {
        if (elements == null) {
            return "null";
        }
        if (size == 0) {
            return "[]";
        }
        StringBuilder sb = new StringBuilder(size * 2 + 1);
        sb.append("[");
        for (int i = 0; i < size; i++) {
            sb.append(elements[i]);
            if (i != size - 1) {    //判断是否最后一个元素
                sb.append(",");
            } else {
                sb.append("]");
            }
        }
        return sb.toString();
    }
}