### Question:
LongSummingWalker extends DefaultWalker<Long> implements SummingWalker<Long> { @Override public Long getValue() { return sum; }  @Override boolean onNext(Long node); @Override Long getValue();  }### Answer:
@Test public void sum() { final SummingWalker<Long> walker = new LongSummingWalker(); TREES .treeWalkers() .walkElementsInOrder(tree, walker); assertEquals(27L, walker.getValue().longValue()); }