Map 的 key、value 是否允许为null

Map的key和value是否允许null？

直接写程序验证一下：

import java.util.HashMap;
import java.util.Hashtable;
import java.util.Map;
import java.util.TreeMap;
import java.util.concurrent.ConcurrentHashMap;

/**
 * @author robin
 */
public class MyTest {

    public static void main(String[] args) {
        Map<Object, Object> m1 = new HashMap<>();
        Map<Object, Object> m2 = new TreeMap<>();
        Map<Object, Object> m3 = new Hashtable<>();
        Map<Object, Object> m4 = new ConcurrentHashMap<>();

        try {
            m1.put("zzz", null);
        } catch (Exception e) {
            System.out.println("m1-a：" + e.getMessage());
        }
        try {
            m1.put(null, null);
        } catch (Exception e) {
            System.out.println("m1-b：" + e.getMessage());
        }
        try {
            m2.put("zzz", null);
        } catch (Exception e) {
            System.out.println("m2-a：" + e.getMessage());
        }
        try {
            m2.put(null, null);
        } catch (Exception e) {
            System.out.println("m2-b：" + e.getMessage());
        }
        try {
            m3.put("zzz", null);
        } catch (Exception e) {
            System.out.println("m3-a：" + e.getMessage());
        }
        try {
            m3.put(null, null);
        } catch (Exception e) {
            System.out.println("m3-b：" + e.getMessage());
        }
        try {
            m4.put("zzz", null);
        } catch (Exception e) {
            System.out.println("m4-a：" + e.getMessage());
        }
        try {
            m4.put(null, null);
        } catch (Exception e) {
            System.out.println("m4-b：" + e.getMessage());
        }
    }

}

执行结果：

m2-b：null
m3-a：null
m3-b：null
m4-a：null
m4-b：null

结论如下表：