Java 使用泛型类实现 Comparable
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Implementing Comparable with a generic class
提问by frank.liu
I want to define a class that implements the generic Comparable interface. While in my class I also defined a generic type element T. In order to implement the interface, I delegate the comparison to T. Here is my code:
我想定义一个实现通用 Comparable 接口的类。在我的课堂上，我还定义了一个泛型类型元素T。为了实现接口，我将比较委托给T. 这是我的代码：
public class Item<T extends Comparable<T>> implements Comparable<Item> {
private int s;
private T t;
public T getT() {
return t;
}
@Override
public int compareTo(Item o) {
return getT().compareTo(o.getT());
}
}
When I try to compile it, I get the following error information:
当我尝试编译它时，我收到以下错误信息：
Item.java:11: error: method compareTo in interface Comparable<T#2> cannot be applied to given types;
return getT().compareTo(o.getT());
^
required: T#1
found: Comparable
reason: actual argument Comparable cannot be converted to T#1 by method invocation conversion
where T#1,T#2 are type-variables:
T#1 extends Comparable<T#1> declared in class Item
T#2 extends Object declared in interface Comparable
1 error
Can anybody tell me why and how to fix it?
谁能告诉我为什么以及如何解决它？
采纳答案by Radiodef
Item(without any type argument) is a raw type, so:
Item（没有任何类型参数）是一个原始类型，所以：
We could pass any kind of
ItemtoItem.compareTo. For example, this would compile:new Item<String>().compareTo(new Item<Integer>())The method
o.getT()returnsComparableinstead ofT, which causes the compilation error.In the example under the 1st point, after passing
Item<Integer>toItem.compareTo, we would then erroneously pass anIntegertoString.compareTo. The compilation error prevents us from writing the code which does that.
我们可以传递任何类型的
ItemtoItem.compareTo。例如，这将编译：new Item<String>().compareTo(new Item<Integer>())该方法
o.getT()返回Comparable而不是T，这会导致编译错误。在第 1 点下的示例中，在传递
Item<Integer>to 之后Item.compareTo，我们会错误地传递IntegertoString.compareTo。编译错误阻止我们编写执行此操作的代码。
I think you just need to remove the raw types:
我认为您只需要删除原始类型：
public class Item<T extends Comparable<T>>
implements Comparable<Item<T>> {
...
@Override
public int compareTo(Item<T> o) {
return getT().compareTo(o.getT());
}
}
回答by Njol
You're using raw types in your class definition (Item<T>is generic, but you're omitting the type parameter <T>), change it to:
您在类定义中使用原始类型（Item<T>是通用的，但省略了类型参数<T>），将其更改为：
class Item<T extends Comparable<T>> implements Comparable<Item<T>>
(Note the last <T>)
（注意最后一个<T>）
The compareTomethod will then have to be changed as well:
compareTo然后，该方法也必须更改：
public int compareTo(Item<T> o) { // again, use generics
return getT().compareTo(o.getT());
}
回答by aditya
I think, this makes more sense. I have compiled and tested the following :
我认为，这更有意义。我已经编译并测试了以下内容：
class Item<E extends Comparable<E>> implements Comparable<E> {
private int s;
private E t;
public E getE() {
return t;
}
@Override
public int compareTo(E e) {
return getE().compareTo(e);
}
public int compareTo(Item<E> other)
{
return getE().compareTo(other.getE());
}
}
Notice that you now need to have two compareTo methods.
请注意，您现在需要有两个 compareTo 方法。
