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);

}

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