Domain-Driven Design: Tackling Complexity in the Heart of Software

Example
Refactoring: A Paint-Mixing Application
A program for paint stores can show a customer the result of mixing standard paints. Here is the
initial design, which has a single domain class.
Figure 10.2.
The only way to even guess what the 
paint(Paint)
method does is to read the code.
public void paint(Paint paint) {
v = v + paint.getV(); //After mixing, volume is summed
// Omitted many lines of complicated color mixing logic
// ending with the assignment of new r, b, and y values.
}
OK, so it looks like this method combines two 
Paints
together, the result having a larger volume
and a mixed color.
To shift our perspective, let's write a test for this method. (This code is based on the JUnit test
framework.)
public void testPaint() {
// Create a pure yellow paint with volume=100
Paint yellow = new Paint(100.0, 0, 50, 0);
// Create a pure blue paint with volume=100
Paint blue = new Paint(100.0, 0, 0, 50);
// Mix the blue into the yellow
yellow.paint(blue);
// Result should be volume of 200.0 of green paint
assertEquals(200.0, yellow.getV(), 0.01);
assertEquals(25, yellow.getB());

---

assertEquals(25, yellow.getY());
assertEquals(0, yellow.getR());
}
The passing test is the starting point. It is unsatisfying at this point because the code in the test
doesn't tell us what it is doing. Let's rewrite the test to reflect the way we would 
like
to use the
Paint
objects if we were writing a client application. Initially, this test will fail. In fact, it won't even
compile. We are writing it to explore the interface design of the 
Paint
object from the client
developer's point of view.
public void testPaint() {
// Start with a pure yellow paint with volume=100
Paint ourPaint = new Paint(100.0, 0, 50, 0);
// Take a pure blue paint with volume=100
Paint blue = new Paint(100.0, 0, 0, 50);
// Mix the blue into the yellow
ourPaint.mixIn(blue);
// Result should be volume of 200.0 of green paint
assertEquals(200.0, ourPaint.getVolume(), 0.01);
assertEquals(25, ourPaint.getBlue());
assertEquals(25, ourPaint.getYellow());
assertEquals(0, ourPaint.getRed());
}
We should take our time to write a test that reflects the way we would like to talk to these objects.
After that, we refactor the