### Question:
LinearId implements Id { @Override public LinearId parent() { int n = getCount(); if (n <= 1) { return new LinearId(null, aggregator_, null); } String[] ids = Arrays.copyOf(data_, n - 1); return new LinearId(ids, aggregator_, null); } LinearId();  LinearId(String id);  LinearId(String parent, String tail);  LinearId(String... id);  LinearId(String[] ids, IdAggregator aggregator); private  LinearId(String[] ids, IdAggregator aggregator, Void private_constructor); @NonNull static LinearId of(@NonNull Id id); @Override String get(int index); @Override LinearId extend(String tail); @Override LinearId parent(); @Override int getCount(); @Override boolean equals(Object obj); @Override int hashCode(); @Override String toString(); @Override String[] toArray();  }### Answer:
@Test public void testParent() { assertThat(empty.parent()).isEqualTo(empty); assertThat(single.parent()).isEqualTo(empty); assertThat(dual.parent()).isEqualTo(new LinearId("hello")); }