### Question:
HashBag implements MutableBag<T> { @Override public boolean add(T element) { this.backingMap.merge(element, 1, (existingValue, newValue) -> existingValue + 1); this.size++; return true; }  @Override int sizeDistinct(); @Override int size(); @Override boolean isEmpty(); @Override boolean contains(Object o); @Override Object[] toArray(); void forEachWithIndex(BiConsumer<? super T, Integer> biConsumer); @Override T1[] toArray(T1[] array); @Override boolean add(T element); @Override boolean remove(Object o); @Override boolean containsAll(Collection<?> c); @Override boolean addAll(Collection<? extends T> c); @Override boolean removeAll(Collection<?> c); @Override boolean retainAll(Collection<?> c); @Override void clear(); @Override Iterator<T> iterator(); @Override int getOccurrences(T element); @Override boolean addOccurrence(T element); @Override boolean addOccurrences(T element, int occurrences); @Override boolean removeOccurrence(T element); @Override boolean removeOccurrences(T element, int occurrences); @Override void forEachWithOccurrences(BiConsumer<? super T, Integer> biConsumer); @Override void forEach(Consumer<? super T> consumer); @Override int hashCode(); @Override boolean equals(Object other);  }### Answer:
@Test public void add() { this.testObj.add("1"); this.testObj.add("2"); this.testObj.add("2"); Assert.assertEquals(MutableBag.of("1", "2", "2"), this.testObj); }