What is the difference between HashMap, Hashtable and ConcurrentHashMap?

Point	HashMap	Hashtable	ConcurrentHashMap
Thread safe	No	Yes (locks the whole table)	Yes (fine grained locks)
Null key	One allowed	Not allowed	Not allowed
Null values	Allowed	Not allowed	Not allowed
Speed	Fastest (one thread)	Slow	Fast with many threads
Use	Normal code	Old code only	Shared data between threads

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

public class Main {
    static void tryNull(String name, Map<String, String> map) {
        try {
            map.put(null, "x");
            System.out.println(name + ": null key allowed");
        } catch (NullPointerException e) {
            System.out.println(name + ": null key NOT allowed");
        }
    }

    public static void main(String[] args) {
        tryNull("HashMap", new HashMap<>());
        tryNull("Hashtable", new Hashtable<>());
        tryNull("ConcurrentHashMap", new ConcurrentHashMap<>());
    }
}

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