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