### Question:
ClassToInstanceStore { public boolean isEmpty() { return backingMap.isEmpty(); }  @SafeVarargs static ClassToInstanceStore<T> create(T... instances); Optional<U> getInstance(Class<U> clazz); Optional<U> putInstance(Class<? extends U> clazz, U instance); Optional<U> removeInstance(Class<U> clazz); void putAll(ClassToInstanceStore<? extends T> toCopy); boolean containsKey(Class<? extends T> clazz); boolean isEmpty(); @SuppressWarnings("unchecked") Optional<U> putInstance(U instance); Collection<T> values(); Collection<Class<? extends T>> keys(); @Override String toString();  }### Answer:
@Test public void testIsEmpty() { ClassToInstanceStore<CharSequence> ctis = new ClassToInstanceStore<>(); assertThat(ctis.isEmpty(), is(true)); ctis.putInstance(String.class, "test"); assertThat(ctis.isEmpty(), is(false)); }