### Question:
ObjectId implements Comparable<ObjectId>, Serializable { public boolean isNull() { return NULL.equals(this); } ObjectId();  ObjectId(byte[] raw); private  ObjectId(byte[] raw, boolean cloneArg); static ObjectId createNoClone(byte[] rawHash); boolean isNull(); @Override boolean equals(Object o); @Override int hashCode(); @Override String toString(); static ObjectId valueOf(final String hash); static byte[] toRaw(final String hash); int compareTo(final ObjectId o); static int compare(byte[] left, byte[] right); byte[] getRawValue(); void getRawValue(byte[] target); void getRawValue(byte[] target, int size); static ObjectId forString(final String strToHash); int byteN(int index); 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()); }
@Test public void testDefaultConstructor() { ObjectId oid = new ObjectId(); assertTrue(oid.isNull()); }