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Java generic programming

在Theory\Programming-paradigm\Generic-programming章节中阐述了generic programming的核心思想，其中非常重要的一点是实现concept，并且基于concept来设计algorithm。后来无意中，阅读了Java的"interface as method parameter"相关的文章，对Java generic programming有了一个新的认知，下面首先描述这两篇文章，然后对Java generic programming进行总结。

Interface as method parameter

stackoverflow interface as a method parameter in Java

I had an interview days ago and was thrown a question like this.

Q: Reverse a linked list. Following code is given:

public class ReverseList { 
    interface NodeList {
        int getItem();
        NodeList nextNode();
    }
    void reverse(NodeList node) {

    }
    public static void main(String[] args) {

    }
}

NOTE: NodeList是一个interface，void reverse(NodeList node)方法接收一个interface parameter。

I was confused because I did not know an interface object could be used as a method parameter. The interviewer explained a little bit but I am still not sure about this. Could somebody enlighten(启发) me?

A

NOTE:

1、interface is a contact

This is in fact one of the most common and useful ways to use an interface. The interface defines a contract, and your code can work with any class that implements the interface, without having to know the concrete class - it can even work with classes that didn't exist yet when the code was written.

There are many examples in the Java standard API, especially in the collections framework. For example, Collections.sort() can sort anything that implements the List interface (not just ArrayList or LinkedList, though implementing your own List is uncommon) and whose contents implement the Comparable interface (not just String or the numerical wrapper classes - and having your own class implement Comparable for that purpose is quite common).

A

NOTE: 这是容易让人混淆的

It's not the interface "object" being passed to the method, still just a regular object. It's just a way of saying "this parameter will accept any object that supports this interface". It's equivalent to accepting some object of a base class type, even if you're passing in a subclass.

stackoverflow Java Interface as argument to a method

A

Yes - if an interface type is required as an argument, you can pass any object whose class implements this interface.

Example:

// the method
void myMethod(MyInterface object) { ... }

// the interface
interface MyInterface {

    void interfaceMethod();

}

// class implementing the interface
class MyImplementation implements MyInterface {

    void interfaceMethod() {
        // ...
    }

}

You can now do this

MyInterface object = new MyImplementation();
myMethod(object);

Hope this helps!

Java generic programming

Java interface

我是通过C++ template来学习generic programming的，在阅读了"Interface as method parameter"章节的example后，我的想法是:

1、如果使用C++ template来实现上述"Interface as method parameter"章节的example，会发现Java interface相当于C++ template parameter，如果我们使用类比思维来进行分析的话，可以发现: 上述example也是符合generic programming的，因此:

a、Java interface特性让programmer能够explicit、formal描述interface

b、Java通过interface来进行generic programming，java是interface是完全behavior-based的；

c、Java interface能够让programmer来实现concept。

d、相比于C++ template behavior based的implicit，java interface是explicit。

关于我的这个观点，在 wikipedia Concept (generic programming) 中有证据:

As generics in Java and C# have some similarities to C++'s templates, the role of concepts there is played by interfaces.

NOTE: 上面这段话中的concept指的是Concept (generic programming) 。

关于Java interface，参见:

1、wikipedia Interface (Java)

Java Generics

仅仅使用Java interface还不足以完全地实现generic programming，Java还有generics特性，借助这个特性，Java能够更加完整的实现generic programming。

关于Java generics，参见:

1、wikipedia Generics in Java

2、tutorialspoint Java - Generics

C++ VS Java: generic programming

1、Java的generic programming可以使用Java interface，Java interface是对behavior的formal description，Java interface是完全behavior-based的；C++ template也是behavior based的，但是它的interface是implicit，java interface是explicit。C++ concept相比之下，不似JAVA那般直观、容易理解