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The first refactoring we're going to see is the collapse hierarchy

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refactoring. When a software system undergoes a number of changes, over

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time the collapse hierarchy may become, let's say, sub-optimal. There are

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several refactorings that address this

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issue for example, refactorings that allow

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you to move methods and fields up and down the class

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hierarchy. So what happens when you apply a number of these

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refactorings, is that a subclass might become too similar to its

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superclass and might not be adding much value to the system.

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In this case, it is a good idea to merge

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the classes together. That's exactly what the Collapse Hierarchy refactoring

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does. Imagine, for instance, that we have two classes: employee

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and salesman. And that salesman is just so similar to

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employee that it does not make sense to keep them

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separated. In this case, you could merge the two classes,

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so that at the end of the refactoring, only employee

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is left. And the resulting structure of the system is improved.