ArrayList底层

package listDemo;

import java.util.Arrays;

public class MyArrayList {
    //存储任意数据类型
    private Object[] elements = null;
    //数组中存储多少个元素
    private int size = 0;
    private static final int DEFAULT = 10;
    
    public MyArrayList(int a) {
        if(a<0) {
            throw new IllegalArgumentException("容量不能为负数");
        }
        elements = new Object[DEFAULT];
    }
    public MyArrayList() {
        this(10);
        //默认调用有参数的那个构造器，初始容量为10
    }
    public void add(Object ele) {
        if(size==elements.length) {
            Object[] temp = Arrays.copyOf(elements, elements.length * 2);
        }
        elements[size] = ele;
        size++;
    }
    
    //查找指定索引位置的索引
    
    public Object get(int index) {
        if(index<0||index>=size) {
            throw new IllegalArgumentException("索引越界");
        }
        return elements[index];
    }
    //替换指定索引位置的元素值
    public void set(int index,Object newEle) {
        if(index<0||index>=size) {
            throw new IllegalArgumentException("索引越界");
        }
        elements[index] = newEle;
    }
    //删除指定索引位置的元素
    public void remove(int index) {
        if(index<0||index>=size) {
            throw new IllegalArgumentException("索引越界");
        }
        for(int i=index; i<size-1;i++) {
            elements[i] = elements[i+1];
            
        }
        //最后一个位置的元素置空
        elements[size - 1] = null;
//        删除之后元素的长度肯定要减去1
        size--;
    }
    
    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) {
                sb.append(",");
            }
            else {
                sb.append("]");
            }
        }
        return sb.toString();
    }
    public int size() {
        return size;
    }
    public boolean isEmpty() {
        return size==0;
    }
    public void clear() {
        //再这个的基础上重新赋值，相当于清空。再把长度变成0
        this.elements = new Object[DEFAULT];
        size=0;
    }
    
}