!529 新增单参数CollectorUtil.groupingBy重载

Merge pull request !529 from 阿超/v5-dev

hutool-core/src/main/java/cn/hutool/core/stream/CollectorUtil.java

@@ -8,12 +8,15 @@ import java.util.EnumSet;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
+import java.util.List;
 import java.util.StringJoiner;
 import java.util.function.BiConsumer;
 import java.util.function.BinaryOperator;
 import java.util.function.Function;
 import java.util.function.Supplier;
+import java.util.function.UnaryOperator;
 import java.util.stream.Collector;
+import java.util.stream.Collectors;
 
 /**
  * 可变的汇聚操作{@link Collector} 相关工具封装
@@ -111,8 +114,7 @@ public class CollectorUtil {
 		if (downstream.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)) {
 			return new SimpleCollector<>(mangledFactory, accumulator, merger, CH_ID);
 		} else {
-			@SuppressWarnings("unchecked")
-			Function<A, A> downstreamFinisher = (Function<A, A>) downstream.finisher();
+			UnaryOperator<A> downstreamFinisher = (UnaryOperator<A>) downstream.finisher();
 			Function<Map<K, A>, M> finisher = intermediate -> {
 				intermediate.replaceAll((k, v) -> downstreamFinisher.apply(v));
 				@SuppressWarnings("unchecked")
@@ -140,6 +142,18 @@ public class CollectorUtil {
 		return groupingBy(classifier, HashMap::new, downstream);
 	}
 
+	/**
+	 * 提供对null值友好的groupingBy操作的{@link Collector}实现
+	 *
+	 * @param classifier 分组依据
+	 * @param <T>        实体类型
+	 * @param <K>        实体中的分组依据对应类型，也是Map中key的类型
+	 * @return {@link Collector}
+	 */
+	public static <T, K> Collector<T, ?, Map<K, List<T>>>
+	groupingBy(Function<? super T, ? extends K> classifier) {
+		return groupingBy(classifier, Collectors.toList());
+	}
 
 	/**
 	 * 对null友好的 toMap 操作的 {@link Collector}实现，默认使用HashMap