### Question:
ListBoxSelectionModeMulti implements ListBoxSelectionMode<T> { @Override public void add(@Nonnull final T item) { selection.add(item); }  @Override void clear(); @Nonnull @Override List<T> getSelection(); @Override void remove(@Nonnull final T item); @Override void removeForced(@Nonnull final T item); @Override void add(@Nonnull final T item); @Override void enableRequiresSelection(final boolean enabled); @Override boolean requiresAutoSelection();  }### Answer:
@Test public void testAdd() { selectionMode.add(o1); assertSelection(o1); }
@Test public void testAddTwice() { selectionMode.add(o1); selectionMode.add(o2); assertSelection(o1, o2); }