集合&lt ;?扩展T> vs Collection T [英] Collection<? extends T> vs Collection<T>

问题描述

尝试理解 Spring的概念后MVC ，我遇到了以前从未见过的表达式Collection<? extends Book>.我试图自己弄清楚它，但是我发现使用Collection<? extends Book>和Collection<Book>之间没有区别.我猜想它只允许Book的扩展，但是它也允许Book的扩展.所以抓.我曾尝试使用Google，但自那以后呢?是Google中的通配符，它​​几乎无法搜索.我已经在stackoverflow上搜索了答案，但是有关此问题的所有问题(例如 List< ;?扩展了MyType> 和< ;?扩展> Java语法)已经假定具备Collection<? extends T>的知识.这是最初引起我兴趣的代码:

After trying to understand the concepts at Spring MVC, I came across the expression Collection<? extends Book> which I have never seen before. I have tried to figure it out on my own, but I am seeing no difference between using Collection<? extends Book> and Collection<Book>. I was guessing that it only allowed for extensions of Book, but it does allow for Book as well. So scratch that. I have tried using Google, but since ? is a wildcard in google, it makes it nearly impossible to search for. I have searched stackoverflow for the answer, but all questions about this (such as List<? extends MyType> and <? extends > Java syntax) already assume knowledge of Collection<? extends T>. Here is the code that has initially intrigued my interest:

import java.util.ArrayList;
import java.util.Collection;

public class Book {
    public static void main(String[] args) {
        BookCase bookCase1 = new BookCase();
        BookCase bookCase2 = new BookCase(bookCase1);
    }
}

class BookCase extends ArrayList<Book> {
    public BookCase() {
    }

    //acts same as public BookCase(Collection<Book> c) {
    public BookCase(Collection<? extends Book> c) {
        super(c);
    }
}

<? extends T>的作用是什么?它与<T>有什么区别?

What does <? extends T> do? How does it differ from <T>?

后续问题:BookCase extends ArrayList<Book>表示BookCase扩展了Book吗?

Followup question: Does BookCase extends ArrayList<Book> mean that BookCase extends Book?

推荐答案

请考虑以下内容

class Animal { }
class Horse extends Animal { }

private static void specific(List<Animal> param) { }
private static void wildcard(List<? extends Animal> param) { }

没有扩展语法，您只能在签名中使用确切的类

Without the extends syntax you can only use the exact class in the signature

    specific(new ArrayList<Horse>()); // <== compiler error

通过通配符扩展，您可以允许Animal的任何子类

With the wildcard extends you can allow any subclasses of Animal

    wildcard(new ArrayList<Horse>());  // <== OK

通常最好使用?扩展语法，因为它使您的代码更具可重用性和面向未来.

It's generally better to use the ? extends syntax as it makes your code more reusable and future-proof.

这篇关于集合&lt ;?扩展T&gt; vs Collection T的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！