如何对Java中的泛型类型列表进行排序 [英] How to sort a list of generic types in Java

问题描述

我有一组所有共享一些共同属性的类，所以我使它们都扩展了一个公共基类， BaseEntity 。所以我有，例如<​​code> Foo扩展BaseEntity 和 Bar扩展BaseEntity 。

我也希望这些 Foo 和 Bar 对象的列表可排序，所以我实现了可比。我将类定义为 Foo extends BaseEntity implements Comparable< Foo> 和 Bar扩展BaseEntity implements Comparable< Bar> ，并且对 Foo s或 Bar 的列表进行排序按预期工作 - 当然还有排序的细节在不同的亚类中是不同的。但是当我不知道我是否会有 Foo s或 Bar 秒。例如，此代码无法编译：

  public class UtilityClass< T extends BaseEntity> {
 
 ...一堆东西... 
 
列表< T>值; 
 
 public List< T> sort（）{
 Collections.sort（values）; 
返回值; 
} 
 
 ...更多方法... 
} 
  

与错误消息绑定不匹配：类型集合的泛型方法sort（List< T>）不适用于参数（List< T>）。推断的类型T不是有界参数的有效替代< T extends Comparable< ;?我认为问题在于我试图对 BaseEntity 列表进行排序， code>对象，而 BaseEntity 本身并没有实现 Comparable 。但是现在我面临一个问题：唯一明智的做法是使 BaseEntity 对象与其他 BaseEntity 对象相比，但是当我在 BaseEntity 中添加了 implements Comparable< BaseEntity> ，编译器告诉我现在有问题了，因为我的 Foo 类试图实现 Comparable< BaseEntity> 和 Comparable< Foo> ，这显然是不允许的。

我知道我可以通过删除 implements Comparable< Foo> 然后执行 Comparable< BaseEntity> ，但是我的 compareTo 方法必须做丑陋的投射，我认为这正是使用泛型应该避免的问题。

我真正想要做的是在签名中指定， BaseEntity 它的所有子类都是 Comparable ，但只限于相同的实例子类。

感激地收到了任何帮助。

解决方案

使用交点类型，如下所示：

  public class MyList< T extends BaseEntity&可比< T>> {...} 
  

指定T必须是 BaseEntity 和 可比较自身。

I have a set of classes that all share some common attributes, so I made them all extend a common base class, BaseEntity. So I have, for example Foo extends BaseEntity and Bar extends BaseEntity.

I also want lists of these Foo and Bar objects to be sortable, so I have implemented Comparable. I have the classes defined as Foo extends BaseEntity implements Comparable<Foo> and Bar extends BaseEntity implements Comparable<Bar>, and sorting of lists of Foos or Bars works as expected - and, of course, the details of the sorting are different in the different subclasses. But I can't work out how to make my sorting work when I don't know in advance whether I'll have Foos or Bars. This code, for example, fails to compile:

public class UtilityClass<T extends BaseEntity> {

  ...bunch of stuff...

  List<T> values;

  public List<T> sort() {
    Collections.sort(values);
    return values;
  }

  ...more methods...
}

with the error message Bound mismatch: The generic method sort(List<T>) of type Collections is not applicable for the arguments (List<T>). The inferred type T is not a valid substitute for the bounded parameter <T extends Comparable<? super T>>

I think the problem is that I am attempting to sort a list of BaseEntity objects, and BaseEntity itself doesn't implement Comparable. But now I face a problem: the only sensible thing to make BaseEntity objects comparable to is other BaseEntity objects, but when I add implements Comparable<BaseEntity> to BaseEntity, the compiler tells me that I've got problems now because my Foo class is trying to implement both Comparable<BaseEntity> and Comparable<Foo>, which evidently is not allowed.

I know I could sidestep this issue by dropping the implements Comparable<Foo> and just implementing Comparable<BaseEntity>, but then my compareTo methods will have to do ugly casting, and I thought that was exactly the sort of problem using generics was supposed to avoid.

What I really want to do is specify in the signature of BaseEntity that all its subclasses will be Comparable, but only to instances of the same subclass.

Any assistance gratefully received. Thanks!

解决方案

Use an intersection type, like this:

public class MyList<T extends BaseEntity & Comparable<T>> {...}

That specifies that T must be both a BaseEntity and Comparable to itself.

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