【面试专栏】ArrayList&HashMap&HashSet是

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写时复制：CopyOnWriteArrayList

CopyOnWrite容器即写时复制的容器，往一个容器添加元素的时候，不直接往当前容器Object[]添加，而是先将当前容器Object[]进行Copy,复制出一个新的容器Object[] newElements,然后新的容器Object[] newElwmwnts里添加元素，添加元素之后，再将原容器的引用指向新的容器setArrat(newElements);这样做的好处是可以CopyOnWrite容器进行并发的读，而不需要加锁，因为当前容器不会添加任何元素。
所以CopyOnWrite容器也是一种读写分离的思想，读和写不同的容器。

案例

package com.zixin;

import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.CopyOnWriteArraySet;

/**
 * @ClassName ContainerNotSafeDemo
 * @Description TODO
 * @Author zixin
 * @Date 2019/12/27 12:37
 * @Version 1.0
 **/
public class ContainerNotSafeDemo {

    /**
     * HashMap线程不安全
     * 解决方案：
     *        new ConcurrentHashMap<>();
     * java.util.ConcurrentModificationException
     * @param args
     */
    public static void main(String[] args) {
        Map<String,String> map = new HashMap<>();
        new ConcurrentHashMap<>();
        for (int i = 0; i <100 ; i++) {
            new Thread(()->{
                        map.put(Thread.currentThread().getName(),UUID.randomUUID().toString().substring(0,8));
                        System.out.println(map);
                    },String.valueOf(i)
            ).start();
        }
    }

    /**
     * HashSet线程不安全
     * 解决方案：
     *        1.Collections.synchronizedSet(new HashSet<String>())
     *        2、new CopyOnWriteArraySet<String>();
     * @param args
     */
    public static void setNotSafe(String[] args) {
        Set<String> set = Collections.synchronizedSet(new HashSet<String>());
        new CopyOnWriteArraySet<String>();
        for (int i=1;i<=500;i++) {
            new Thread(()->{
                set.add(UUID.randomUUID().toString().substring(0,8));
                System.out.println(set);
            },String.valueOf(i)).start();
        }
    }

    public static void listNotSafe(String[] args) {
        //Constructs an empty list with an initial capacity of ten.
        new ArrayList<Integer>().add(1);
        List<String> list = new ArrayList<String>();
        for (int i=1;i<=500;i++) {
            new Thread(()->{
               list.add(UUID.randomUUID().toString().substring(0,8));
               System.out.println(list);
            },String.valueOf(i)).start();
        }
        //java.util.ConcurrentModificationException

        /**
         * 1、故障现象：java.util.ConcurrentModificationException
         * 2、导致原因
         *          并发争抢修改导致，一个线程正在写，另外一个线程过来抢夺，导致数据不一致，出现数据修改异常
         * 3、解决方法
         *     3.1、使用Vector（Vector出现在ArrayList之前）
         *     3.2、Collections.synchronizedList(new ArrayList<String>());
         *     3.3、new CopyOnWriteArrayList<String>();
         * 4、优化建议
         */
        Collections.synchronizedList(new ArrayList<String>());
        new CopyOnWriteArrayList<String>();
    }
}

1. ArrayList实现
   jdk1.9

public boolean add(E e) {
        synchronized (lock) {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            newElements[len] = e;
            setArray(newElements);
            return true;
        }
    }

jdk1.8:

public boolean add(E e) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            newElements[len] = e;
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }

2. HashSet底层是HashMap

/**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * default initial capacity (16) and load factor (0.75).
     */
    public HashSet() {
        map = new HashMap<>();
    }

3. set的add参数只有一个而map是kV为什么?

/**
     * Adds the specified element to this set if it is not already present.
     * More formally, adds the specified element <tt>e</tt> to this set if
     * this set contains no element <tt>e2</tt> such that
     * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns <tt>false</tt>.
     *
     * @param e element to be added to this set
     * @return <tt>true</tt> if this set did not already contain the specified
     * element
     */
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }