[英]Explanation of generic <T extends Comparable<? super T>> in collection.sort/ comparable code?
I use comparable interface all the time to provided natural ordering for my class through collection.sort. 我一直使用类似的界面通过collection.sort为我的班级提供自然顺序。
Basically if I have a person class, I will get it to implement Comparable interface and will provide the implementation of compareTo. 基本上如果我有一个人类,我会得到它来实现Comparable接口,并将提供compareTo的实现。 However in the definition of Collections.sort in javadocs, I see this signature 但是在javadocs中Collections.sort的定义中,我看到了这个签名
public static <T extends Comparable<? super T>> void sort(List<T> list)
I don't understand this generics definition at all? 我根本不理解这个泛型定义? Shouldn't it just say 不应该只是说
<T implements Comparable<T>>
Can someone help me with this? 有人可以帮我弄这个吗?
Actually, it means that T can implement Comparable<? super T>
实际上,这意味着T 可以实现Comparable<? super T>
Comparable<? super T>
, not just Comparable<T>
. Comparable<? super T>
,而不仅仅是Comparable<T>
。
For example, it means that a Student
class can implement Comparable<Person>
, where Student
is a subclass of Person
: 例如,这意味着Student
类可以实现Comparable<Person>
,其中Student
是Person
的子类:
public class Person {}
public class Student extends Person implements Comparable<Person> {
@Override public int compareTo(Person that) {
// ...
}
}
In this case, a List can be sorted by Collections.sort()
but only based on Person
's properties, because you pass the Student
instance into compareTo()
as a Person
(unless you downcast it, of course). 在这种情况下,List可以按Collections.sort()
排序,但仅基于Person
的属性,因为您将Student
实例作为Person
传递给compareTo()
(当然,除非你将它转发)。
In practice however, you'll never see a Student
class implement Comparable<Person>
. 但实际上,您永远不会看到Student
类实现Comparable<Person>
。 That's because Person
will probably have implemented Comparable<Person>
, and Student
inherits it implementation. 那是因为Person
可能已经实现了Comparable<Person>
,而Student
继承了它的实现。 The end result is the same however: you can pass a List<Student>
to Collections.sort()
and have it sorted on Person
's properties. 最终结果是相同的:您可以将List<Student>
传递给Collections.sort()
并将其排序在Person
的属性上。
The difference between Comparable<T>
and Comparable<? super T>
Comparable<T>
和Comparable<? super T>
之间的区别Comparable<? super T>
Comparable<? super T>
is more obvious in the overloaded version of Collections.sort() that takes a Comparator<? super T>
Comparable<? super T>
在Collections.sort()的重载版本中更明显,它带有一个Comparator<? super T>
Comparator<? super T>
: Comparator<? super T>
:
class ByAgeAscending implements Comparator<Person> {
@Override public int compare(Person a, Person b) {
return a.getAge() < b.getAge();
}
}
List<Student> students = getSomeStudents();
Collections.sort(students, new ByAgeAscending());
You always use extends with generics wildcards, even if the type parameter implements an interface. 即使type参数实现了接口,也始终使用带有泛型通配符的extends 。
If you look at a class that implements Comparable , you'll see that it actually (should) implement Comparable<T>
, where T is the class itself. 如果你看一个实现Comparable的类,你会发现它实际上(应该)实现了Comparable<T>
,其中T是类本身。
It makes sense if you think about the type paramter passed to the Comparable interface and how it's used in the compareTo() method. 如果考虑传递给Comparable接口的类型参数以及它在compareTo()方法中的使用方式,这是有意义的。
As PM 77-1 has eloquently pointed out, the super keyword allows for either the class, T, or one of its parents to implement Comparable . 正如PM 77-1雄辩地指出的那样, super关键字允许类,T或其父类之一实现Comparable 。
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