Null keys and values

Definition

HashMap allows one null key and any number of null values. TreeMap and ConcurrentHashMap do not allow a null key, and ConcurrentHashMap does not allow null values either, because a null result from get would then be ambiguous.

Example code

Which maps accept null

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

class Demo {
    static void tryPut(String name, Map<String, String> map) {
        try {
            map.put(null, "value");
            System.out.println(name + ": null key accepted");
        } catch (NullPointerException e) {
            System.out.println(name + ": null key rejected");
        }
    }

    public static void main(String[] args) {
        tryPut("HashMap", new HashMap<>());
        tryPut("TreeMap", new TreeMap<>());
        tryPut("ConcurrentHashMap", new ConcurrentHashMap<>());

        Map<String, String> map = new HashMap<>();
        map.put("a", null);                                       // a null value is fine here
        System.out.println(map.get("a") + " / " + map.containsKey("a"));
    }
}

// Output:
// HashMap: null key accepted
// TreeMap: null key rejected
// ConcurrentHashMap: null key rejected
// null / true