通配符背后的目的是什么?它们与泛型有什么不同? [英] What's the purpose behind wildcards and how are they different from generics?
问题描述
直到前几天,在阅读我老师的Java书籍之后,我从来没有听说过野马车,但我仍然不确定它是什么用途,为什么需要使用它。
比方说,我有一个超类 Animal
和几个子类,比如 Dog
, Cat
, Parrot
等等...现在我需要一个动物列表,我的第一个想法是这样的:
列表< Animal> listAnimals
相反,我的同事推荐如下内容:
列表< ;?延伸动物> listAnimals
为什么要使用通配符而不是简单泛型?
假设我需要一个get / set方法,我应该使用前者还是后者?它们有什么不同?
当你声明局部变量时,通配符没有什么意义,但它们是真的
想象一下你有一个方法:
int countLegs(List<?extends Animal> animals)
{
int retVal = 0;
for(Animal cur:animals)
{
retVal + = cur.countLegs();
}
return retVal;
$ b $ p
$ b 使用此签名,您可以执行此操作:
列表< Dog>狗= ...;
countLegs(狗);
列表< Cat>猫= ...;
countLegs(猫);
列表< Animal>动物园= ...;
countLegs(zoo);
但是,如果您声明 countLegs
这:
int countLegs(List< Animal>动物)
然后在前面的例子中,只有 countLegs(zoo)
会被编译,因为只有那个调用具有正确的类型。
I'd never heard about wildcars until a few days ago and after reading my teacher's Java book, I'm still not sure about what's it for and why would I need to use it.
Let's say I have a super class Animal
and few sub classes like Dog
, Cat
, Parrot
, etc... Now I need to have a list of animals, my first thought would be something like:
List<Animal> listAnimals
Instead, my colleagues are recommending something like:
List<? extends Animal> listAnimals
Why should I use wildcards instead of simple generics?
Let's say I need to have a get/set method, should I use the former or the later? How are they so different?
解决方案 The wildcards do not make a lot of sense when you declare local variables, however they are really important when you declare a parameter for a method.
Imagine you have a method:
int countLegs ( List< ? extends Animal > animals )
{
int retVal = 0;
for ( Animal cur : animals )
{
retVal += cur.countLegs( );
}
return retVal;
}
With this signature you can do this:
List<Dog> dogs = ...;
countLegs( dogs );
List<Cat> cats = ...;
countLegs( cats );
List<Animal> zoo = ...;
countLegs( zoo );
If, however, you declare countLegs
like this:
int countLegs ( List< Animal > animals )
Then in the previous example only countLegs( zoo )
would have compiled, because only that call has a correct type.
这篇关于通配符背后的目的是什么?它们与泛型有什么不同?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!