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