如何获得Comparator.comparing以正确推断类型参数? [英] How do I get Comparator.comparing to correctly infer type parameters?
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问题描述
我有以下课程:
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import org.apache.commons.lang3.tuple.Pair;
public class Sorter {
private List<Pair<Long, NameDTO>> dtoPairs = new ArrayList<>();
public Sorter() {
Comparator<Pair<Long, NameDTO>> bySize = Comparator.comparing(Pair::getLeft);
Comparator<Pair<Long, NameDTO>> byName = Comparator.comparing(p -> p.getRight().getName());
dtoPairs.sort(bySize.reversed().thenComparing(byName));
}
}
class NameDTO {
private String name;
public String getName() {
return name;
}
}
此编译没有问题.但是,如果我尝试内联这样的变量:
This compiles with no problems. However, if I try to inline the variables like this:
dtoPairs.sort(Comparator.comparing(Pair::getLeft).reversed().thenComparing(Comparator.comparing(p -> p.getRight().getName())));
我收到以下编译错误:
Sorter.java:[18,51] incompatible types: cannot infer type-variable(s) T,U
(argument mismatch; invalid method reference
method getLeft in class org.apache.commons.lang3.tuple.Pair<L,R> cannot be applied to given types
required: no arguments
found: java.lang.Object
reason: actual and formal argument lists differ in length)
如何编写此表达式以使Java正确理解参数?
How can I write this expression to make Java understand the parameters correctly?
推荐答案
您需要告诉编译器通用参数的类型. 试试这个:
You need to tell the compiler the type of the generic parameters. Try this:
dtoPairs.sort(
Comparator.comparing(Pair<Long, NameDTO>::getLeft)
.reversed()
.thenComparing(Comparator.comparing(p -> p.getRight().getName())));
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