See http://www.refactoring.com/catalog/collapseHierarchy.html

This refactoring merges a superclass and subclass into a single class.

It is used when one of those classes provides little value. This condition occurs when a developer pre-emptively designs a deep class hierarchy or after other refactorings have moved most of the functionality out of one of the classes.

This refactoring uses the refactorings PullUpField, PullUpMethod, PushDownMethod, PushDownField.

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See MaxThreeLayersOfInheritance

CategoryRefactoring