abstract_class_abstract_class

1.Offering both an interface and an abstract class is redundant, but I find it convenient: in most cases, I only need the abstract class, but the interface enables multiple inheritance.
同时提供接口和抽象类是多余的，但我发现这样很方便：在大多数情况下，我只需要抽象类，但接口支持多继承性。
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2.It isn't always possible to define an interface in an abstract class, but in the light of evolution you should consider whether an abstract class is sufficient.
尽管并不是总能在抽象类中定义接口，但是在编程过程中总是免不了要考虑下一个抽象类够不够呢。
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3.Typically this is implemented by having an abstract class that has a static create method that returns a concrete object implementing the abstract class.
这种模式的典型实现方式是创建一个抽象类，其中具有静态方法create，这个方法返回的是实现了这个抽象类的一个具体对象。
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