Strategy

The problem it solves

A checkout applies a discount. At first there is one rule. Then marketing adds more, and the code becomes this:

if (type.equals("PERCENT")) { ... }
else if (type.equals("FLAT")) { ... }
else if (type.equals("CAPPED")) { ... }

Problems with that:

• Every new rule means editing the same method, and an edit can break the old rules.
• The chain is hard to test, because you must set up the type to reach each branch.
• The same chain gets copied into other places (reports, refunds).

Strategy moves each rule into its own class that implements one small interface. The checkout holds a strategy and calls it. Which one it holds can change while the program runs.