No null keys or values

Definition

ConcurrentHashMap rejects null keys and null values. In a concurrent map, a null returned from get must mean "no such key". If null values were allowed you could not tell the two cases apart without another call that another thread may change.

Example code

null is rejected on purpose

import java.util.concurrent.ConcurrentHashMap;

class Demo {
    public static void main(String[] args) {
        ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();

        try {
            map.put("key", null);
        } catch (NullPointerException e) {
            System.out.println("null value rejected");
        }
        try {
            map.put(null, "value");
        } catch (NullPointerException e) {
            System.out.println("null key rejected");
        }

        // use a clear "empty" marker instead of null
        map.put("key", "");
        System.out.println(map.get("missing") == null);      // null here always means "not in the map"
    }
}

// Output:
// null value rejected
// null key rejected
// true