抽象类是一种特殊的类，不能实例化.所以问题是为什么我们需要一个不能实例化?抽象类只能被子类化(继承)从).换句话说，它仅允许其他类从中继承但无法实例化.好处是可以强制执行所有子类的层次结构.简单来说，这是一种强制所有子类进行相同的合同层次结构或标准.
An abstract class is a special kind of class that cannot be
instantiated. So the question is why we need a class that cannot be
instantiated? An abstract class is only to be sub-classed (inherited
from). In other words, it only allows other classes to inherit from it
but cannot be instantiated. The advantage is that it enforces certain
hierarchies for all the subclasses. In simple words, it is a kind of
contract that forces all the subclasses to carry on the same
hierarchies or standards.
接口不是类.它是由字接口.接口没有实现；它只有签名或换句话说，只是方法的定义没有身体.作为与Abstract类的相似之处之一，它是用于定义所有子类的层次结构的合同定义一组特定的方法及其参数.
An interface is not a class. It is an entity that is defined by the
word Interface. An interface has no implementation; it only has the
signature or in other words, just the definition of the methods
without the body. As one of the similarities to Abstract class, it is
a contract that is used to define hierarchies for all subclasses or it
defines specific set of methods and their arguments.
