### Question:
DoubleWrapper extends AbstractWrapper<Double> implements NumberWrapper<Double> { @Override public void increment() { Double value = doGet(); if (value != null) { doSet(value + 1d); } } DoubleWrapper();  DoubleWrapper(Double value); @Override void set(Number v); @Override void increment(); @Override void decrement(); @Override R accept(WrapperVisitor<R, P, Q, TH> visitor, P p, Q q);  }### Answer:
@Test public void testIncrement() { DoubleWrapper wrapper = new DoubleWrapper(10d); wrapper.increment(); assertEquals(new Double(11d), wrapper.get()); }