抽象类型

在编程语言中，抽象类型是指名義型別系統中不能直接实例化的类型；而非抽象的类型，即可以实例化的类型，則被称为具体类型^[1]。

一个抽象类型可以不提供实现，或者不完整的实现。在一些语言中，没有实现的抽象类型被称为协议、接口。在基于类编程以及面向对象程序设计中，抽象类型被实现为抽象类（也被称为抽象基类），而具体类型被实现为具体类。

示例（Java）

//By default, all methods in all classes are concrete, unless the abstract keyword is used.
abstract class Demo {
    // An abstract class may include abstract methods, which have no implementation.
    abstract public int sum(int x, int y);

    // An abstract class may also include concrete methods.
    public int product(int x, int y) { return x*y; }
}

//By default, all methods in all interfaces are abstract, unless the default keyword is used.
interface DemoInterface {
    [abstract] int getLength(); //Abstract can be used here, though is completely useless
    
    //The default keyword can be used in this context to specify a concrete method in an interface
    default int product(int x, int y) {
        return x * y;
    }
}

参考文献

1. Mitchell, John C.; Plotkin, Gordon D.; Abstract Types Have Existential Type （页面存档备份，存于互联网档案馆）, ACM Transactions on Programming Languages and Systems, Vol. 10, No. 3, July 1988, pp. 470–502