java集合 list、set、map能否存儲null

java集合能否存儲null

package com.idea.test.nulltest;

import jxl.common.Assert;

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

public class NullTest {
    public static void main(String[] args) {
        List list = new ArrayList();
        list.add(null);
        list.add(null);
        System.out.println("ArrayList :"+list.size());   //2


        List linkedList = new LinkedList();
        linkedList.add(null);
        linkedList.add(null);
        System.out.println("LinkedList :"+linkedList.size());  //2



        Set set=new HashSet();
        set.add(null);
        set.add(null);
       System.out.println("HashSet :"+set.size());// 1

        Set linkedHashSet=new LinkedHashSet();
        linkedHashSet.add(null);
        linkedHashSet.add(null);
        System.out.println("LinkedHashSet :"+linkedHashSet.size()); //1

        Set treeSet=new TreeSet();
        //treeSet.add(null);   //treeSet key不能null ava.lang.NullPointerException
        //treeSet.add(null);
       // System.out.println("TreeSet : "+treeSet.size());

        HashMap<String,String> map = new HashMap<>();
        map.put(null,null);
        map.put("liubei",null);
        System.out.println("HashMap :"+map.size());// 2

        LinkedHashMap linkedHashMap = new LinkedHashMap();
        linkedHashMap.put(null,null);
        linkedHashMap.put("liubei",null);
        System.out.println("LinkedHashMap :"+linkedHashMap.size()); //2



        TreeMap treeMap= new TreeMap<>();
        treeMap.put("liu",null);   //treeMap key  不能為空，value 可以為空
        //treeMap.put(null,"liubei");
        //treeMap.put(null,null);
        System.out.println("TreeMap :"+treeMap.size());


        Hashtable table=new Hashtable();
        //table.put("liu",null);  
        //table.put(null,"關羽");    //hashtable key與value均不能為空
        table.put(null,null);
        System.out.println("Hashtable"+table.size());


        ConcurrentHashMap concurrentHashMap=new ConcurrentHashMap();
        //concurrentHashMap.put("liu",null);    //concurrentHashmap key 與 value 均不可以為空
        //concurrentHashMap.put(null,"zhangfei");
        System.out.println("ConcurrentHashMap :"+concurrentHashMap.size());
    }

}

 

綜上所述可以得出結論：

1、vector、arraylist、linkedlist可以存儲多個null.

2、hashset、linkedset可以存儲一個null. treeset不能存儲null.

3、hashmap 、linkedhashmap  key與value均可以為null.  treemap  key不可以為null,value可以為null.  hashtable、concurrenthashmap key與value均不能為null.