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);
}
}
}