如何在 Java 中对泛型类型设置约束？
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How to set constraints on generic types in Java?
提问by citronas
I have a generic class:
我有一个通用类：
public class ListObject<T>
{
// fields
protected T _Value = null;
// ..
}
Now I want to do something like the following:
现在我想做如下事情：
ListObject<MyClass> foo = new ListObject<MyClass>();
ListObject<MyClass> foo2 = new ListObject<MyClass>();
foo.compareTo(foo2);
Question:
题：
How can I define the compareTo()method with resprect to the generic T?
如何定义compareTo()与泛型相关的方法T？
I guess I have to somehow implement a constraint on the generic T, to tell that Timplements a specific interface (maybe Comparable, if that one exists).
我想我必须以某种方式在 generic 上实现一个约束T，以告诉它T实现了一个特定的接口（也许Comparable，如果存在的话）。
Can anyone provide me with a small code sample?
谁能给我提供一个小的代码示例？
采纳答案by nanda
Read also the discussion here: Generics and sorting in Java
另请阅读此处的讨论：Java 中的泛型和排序
Short answer, the best you can get is:
简短的回答，你能得到的最好的是：
class ListObject<T extends Comparable<? super T>> {
...
}
But there is also reason to just use:
但也有理由只使用：
class ListObject<T extends Comparable> {
...
}
回答by John Feminella
Try public class ListObject<T extends U>. Only Ts which implement U(or derive from U) will be allowable substitutions.
试试public class ListObject<T extends U>。只有T实现U（或派生自U）的s才是允许的替换。
回答by Steve Emmerson
public class ListObject<T implements Comparable> {...}
回答by JaredPar
This depends on exactly what you want the compareTo method to do. Simply defining the compareTo method to take other ListObject<T>values is done by the following
这完全取决于您希望 compareTo 方法做什么。简单地定义 compareTo 方法以采用其他ListObject<T>值是通过以下方式完成的
public class ListObject<T> {
public int compareTo(ListObject<T> other) {
...
}
}
However if you want to actually call methods on that parameter you'll need to add some constraints to give more information about the Tvalue like so
但是，如果您想实际调用该参数的方法，则需要添加一些约束以提供有关该T值的更多信息，如下所示
class ListObject<T extends Comparable<T>> {
...
}
