hashmap可以用null为键值

import java.util.HashMap;

import java.util.Map;

import java.util.TreeMap;

 

public class TestMain {

    public static void main(String[] args) {

 

        // HashMap可以的键值可以是null, "".

        Map<String, String> strMap1 = new HashMap<String, String>();

        strMap1.put(null, "1");

        strMap1.put("", "2");

        strMap1.put(" ", "3");

        strMap1.put(null, "4");

        System.out.println(strMap1.get(null));

        for (String s : strMap1.keySet()) {

            System.out.println(s);

        }

        for (String s : strMap1.values()) {

            System.out.println(s);

        }

         

        // TreeMap的键值不能是null

        Map<String, String> strMap2 = new TreeMap<String, String>();

        strMap2.put(null, "1");

        //strMap2.put("", "2");

        //strMap2.put(" ", "3");

        //strMap2.put(null, "4");

        //System.out.println(strMap2.get(null));

        for (String s : strMap2.keySet()) {

            System.out.println(s);

        }

        for (String s : strMap2.values()) {

            System.out.println(s);

        }

    }

}