### Question:
DoubleSummingWalker extends DefaultWalker<Double> implements SummingWalker<Double> { @Override public Double getValue() { return sum; }  @Override boolean onNext(Double node); @Override Double getValue();  }### Answer:
@Test public void sum() { final SummingWalker<Double> walker = new DoubleSummingWalker(); TREES .treeWalkers() .walkElementsInOrder(tree, walker); assertEquals(28.9, walker.getValue().doubleValue(), 0.02); }