### Question:
ObjectId implements Comparable<ObjectId>, Serializable { public boolean isNull() { return NULL.equals(this); } ObjectId(byte[] raw); private  ObjectId(int h1, long h2, long h3); static ObjectId create(byte[] raw); static ObjectId create(int h1, long h2, long h3); boolean isNull(); @Override boolean equals(Object o); @Override int hashCode(); @Override String toString(); static ObjectId valueOf(final @NonNull String hash); static byte[] toRaw(final @NonNull String hash); int compareTo(final ObjectId o); byte[] getRawValue(); void getRawValue(byte[] target); void getRawValue(byte[] target, final int length); int byteN(final int index); static ObjectId readFrom(DataInput in); void writeTo(DataOutput out); static final Ordering<ObjectId> NATURAL_ORDER; static final ObjectId NULL; static final HashFunction HASH_FUNCTION; static final int NUM_BYTES; }### Answer:
@Test public void testIsNull() { assertTrue(ObjectId.NULL.isNull()); }