@Override
    public void putAll(Map<? extends K, ? extends V> m) {
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }
@Test
    public void testPutAll() {
        HashMap<Object, Object> hashMap = new HashMap<>();
        hashMap.put(null, "value");
        hashMap.put("key", null);
        hashMap.put("key1", "value");
        Assert.assertEquals(3, hashMap.size());
        SafeConcurrentHashMap<Object, Object> safeConcurrentHashMap = new SafeConcurrentHashMap<>();
        safeConcurrentHashMap.putAll(hashMap);
        Assert.assertEquals(1, safeConcurrentHashMap.size());
        Assert.assertEquals("value", safeConcurrentHashMap.get("key1"));
    }