我正在尝试创建一个相当简单的Collector,将Stream<Map.Entry>
转换为Map
,但javac正在抱怨泛型。无法弄清楚它为什么不起作用。
import java.util.Map;
import java.util.Map.Entry;
import java.util.stream.Collector;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class CollectorSO {
public static <K, V> void main(String[] args) {
Collector<Entry<K, V>, ?, Map<K, V>> collector = Collectors.toMap(Entry::getKey, Entry::getValue);
Stream<Entry<String, Object>> stream = Stream.empty();
Map<String, Object> map = stream.collect(collector);
}
}
编译器输出:
Error:(11, 41) java: no suitable method found for collect(java.util.stream.Collector<java.util.Map.Entry<K,V>,capture#1 of ?,java.util.Map<K,V>>)
method java.util.stream.Stream.<R>collect(java.util.function.Supplier<R>,java.util.function.BiConsumer<R,? super java.util.Map.Entry<java.lang.String,java.lang.Object>>,java.util.function.BiConsumer<R,R>) is not applicable
(cannot infer type-variable(s) R
(actual and formal argument lists differ in length))
method java.util.stream.Stream.<R,A>collect(java.util.stream.Collector<? super java.util.Map.Entry<java.lang.String,java.lang.Object>,A,R>) is not applicable
(cannot infer type-variable(s) R,A
(argument mismatch; java.util.stream.Collector<java.util.Map.Entry<K,V>,capture#1 of ?,java.util.Map<K,V>> cannot be converted to java.util.stream.Collector<? super java.util.Map.Entry<java.lang.String,java.lang.Object>,A,R>))
This表面上看起来很相似,但即使使用JDK 1.8.0_60,我也遇到了问题。
答案 0 :(得分:5)
这是因为您为main方法声明的类型变量。它们不会在该上下文中捕获,因此您无法使用Entry<String, Object>
收集器收集Entry<K, V>, ?, Map<K, V>
流。您正在寻找的可能是这样的:
public class CollectorSO
{
public static void main(String[] args)
{
Stream<Entry<String, Object>> stream = Stream.empty();
Map<String, Object> map = stream.collect(entryToMapCollector());
}
private static <K, V> Collector<Entry<K, V>, ?, Map<K, V>> entryToMapCollector()
{
return Collectors.toMap(Entry::getKey, Entry::getValue);
}
}
祝你好运!