问题描述
我有一个处理器类,它接受在流上运行的任意谓词,以及一个用于排序的字段。我的处理器类:
import com.example.vm.domain.Transaction;
import lombok.Builder;
import java.util.Collection;
import java.util.Comparator;
import java.util.List;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;
@Builder
public class Processor {
private Collection<Predicate<Transaction>> predicates;
public List<Transaction> process(List<Transaction> transactions) {
return process(transactions,TransactionFields.CATEGORY.getExtractor());
}
public List<Transaction> process(List<Transaction> transactions,Function<Transaction,Object> order) {
if (predicates == null) {
return transactions;
}
if (order == null) {
return transactions.stream()
.parallel()
.filter(predicates.stream().reduce(p -> true,Predicate::and))
.collect(Collectors.toList());
} else {
return transactions.stream()
.parallel()
.filter(predicates.stream().reduce(p -> true,Predicate::and))
.sorted(Comparator.comparing(Transaction::getCategory))
.collect(Collectors.toList());
}
}
}
这很好用,但我希望 Comparator.comparing(Transaction::getCategory)
成为一个参数,以便
.sorted(Comparator.comparing(Transaction::getCategory))
变成这样
.sorted(Comparator.comparing(order))
重载调用:
process(transactions,TransactionFields.CATEGORY.getExtractor());
来自枚举以返回提取器字段:
import com.example.vm.domain.Transaction;
import java.util.function.Function;
public enum TransactionFields {
CATEGORY("category",t -> t.getCategory());
private String name;
private Function<Transaction,Object> extractor;
private TransactionFields(String name,Object> extractor) {
this.name = name;
this.extractor = extractor;
}
public Function<Transaction,Object> getExtractor() {
return extractor;
}
}
我无法让它工作并且转换到比较器也失败。解决方案不一定是比较器,只要它排序即可。有大佬知道怎么解决吗?
解决方法
Function
的 Comparator#comparing
参数的通用约束是 <T,U extends Comparable<? super U>>
,这意味着该函数必须返回扩展 Comparable<…>
的内容。将您的函数从 Function<Transaction,Object>
更改为 Function<Transaction,Comparable<…>>
,您应该可以执行 .sorted(Comparator.comparing(order))
。
唯一的要求是您的 Transaction#getCategory
方法返回的是 Comparable
(字符串、数字、实现 Comparable<YourType>
的自定义类型)。