### Question:
RealValue extends Value { @Override Value add (Value v) { return new RealValue (mValue + v.toRealValue ().mValue); } RealValue(double value);   RealValue(String value); @Override String toString();  }### Answer:
@Test public void testAdd () { assertEquals ("0", mPositiveValue.add (mNegativeValue).toString ()); assertEquals ("1,23", mPositiveValue.add (mZeroValue).toString ()); assertEquals ("-1,23", mZeroValue.add (mNegativeValue).toString ()); }