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。

youdao

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。

youdao

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。

youdao

This is not just some abstract thing going on in a class in Yale.

这不仅仅是耶鲁课堂上的抽象概念

耶鲁公开课 - 博弈论课程节选