Map 的 key、value 是否允许为null

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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());
}
}}