From the Gang of Four [gof], Strategy and Visitor immediately leap to mind. Kent Beck’s Self Delegation [Beck SBPP] is another. Use these to combat the divergent change smell. The fundamental rule of thumb is to put things together that change together. Data and the behavior that references that data usually change together—but there are exceptions. When the exceptions occur, we move the behavior to keep changes in one place. Strategy and Visitor allow you to change behavior easily because they isolate the small amount of behavior that needs to be overridden, at the cost of further
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