提交Collectors.toMap的对null友好实现，避免NPE

hutool-core/src/main/java/cn/hutool/core/collection/CollStreamUtil.java

@@ -199,7 +199,7 @@ public class CollStreamUtil {
 		if (CollUtil.isEmpty(collection) || key1 == null || key2 == null) {
 			return Collections.emptyMap();
 		}
-		return groupBy(collection, key1, Collectors.toMap(key2, Function.identity(), (l, r) -> l), isParallel);
+		return groupBy(collection, key1, CollectorUtil.toMap(key2, Function.identity(), (l, r) -> l), isParallel);
 	}
 
 	/**

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

@@ -7,6 +7,7 @@ import java.util.Collections;
 import java.util.EnumSet;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Set;
 import java.util.StringJoiner;
 import java.util.function.BiConsumer;
 import java.util.function.BinaryOperator;
@@ -22,6 +23,16 @@ import java.util.stream.Collector;
  */
 public class CollectorUtil {
 
+	/**
+	 * 说明已包含IDENTITY_FINISH特征 为 Characteristics.IDENTITY_FINISH 的缩写
+	 */
+	public static final Set<Collector.Characteristics> CH_ID
+			= Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH));
+	/**
+	 * 说明不包含IDENTITY_FINISH特征
+	 */
+	public static final Set<Collector.Characteristics> CH_NOID = Collections.emptySet();
+
 	/**
 	 * 提供任意对象的Join操作的{@link Collector}实现，对象默认调用toString方法
 	 *
@@ -93,17 +104,12 @@ public class CollectorUtil {
 			A container = m.computeIfAbsent(key, k -> downstreamSupplier.get());
 			downstreamAccumulator.accept(container, t);
 		};
-		BinaryOperator<Map<K, A>> merger = (m1, m2) -> {
+		BinaryOperator<Map<K, A>> merger = mapMerger(downstream.combiner());
-			for (Map.Entry<K, A> e : m2.entrySet()) {
-				m1.merge(e.getKey(), e.getValue(), downstream.combiner());
-			}
-			return m1;
-		};
 		@SuppressWarnings("unchecked")
 		Supplier<Map<K, A>> mangledFactory = (Supplier<Map<K, A>>) mapFactory;
 
 		if (downstream.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)) {
-			return new SimpleCollector<>(mangledFactory, accumulator, merger, Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH)));
+			return new SimpleCollector<>(mangledFactory, accumulator, merger, CH_ID);
 		} else {
 			@SuppressWarnings("unchecked")
 			Function<A, A> downstreamFinisher = (Function<A, A>) downstream.finisher();
@@ -113,7 +119,7 @@ public class CollectorUtil {
 				M castResult = (M) intermediate;
 				return castResult;
 			};
-			return new SimpleCollector<>(mangledFactory, accumulator, merger, finisher, Collections.emptySet());
+			return new SimpleCollector<>(mangledFactory, accumulator, merger, finisher, CH_NOID);
 		}
 	}
 
@@ -134,4 +140,65 @@ public class CollectorUtil {
 		return groupingBy(classifier, HashMap::new, downstream);
 	}
 
+
+	/**
+	 * 对null友好的 toMap 操作的 {@link Collector}实现，默认使用HashMap
+	 *
+	 * @param keyMapper     指定map中的key
+	 * @param valueMapper   指定map中的value
+	 * @param mergeFunction 合并前对value进行的操作
+	 * @param <T>           实体类型
+	 * @param <K>           map中key的类型
+	 * @param <U>           map中value的类型
+	 * @return 对null友好的 toMap 操作的 {@link Collector}实现
+	 */
+	public static <T, K, U>
+	Collector<T, ?, Map<K, U>> toMap(Function<? super T, ? extends K> keyMapper,
+									 Function<? super T, ? extends U> valueMapper,
+									 BinaryOperator<U> mergeFunction) {
+		return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);
+	}
+
+	/**
+	 * 对null友好的 toMap 操作的 {@link Collector}实现
+	 *
+	 * @param keyMapper     指定map中的key
+	 * @param valueMapper   指定map中的value
+	 * @param mergeFunction 合并前对value进行的操作
+	 * @param mapSupplier   最终需要的map类型
+	 * @param <T>           实体类型
+	 * @param <K>           map中key的类型
+	 * @param <U>           map中value的类型
+	 * @param <M>           map的类型
+	 * @return 对null友好的 toMap 操作的 {@link Collector}实现
+	 */
+	public static <T, K, U, M extends Map<K, U>>
+	Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
+							 Function<? super T, ? extends U> valueMapper,
+							 BinaryOperator<U> mergeFunction,
+							 Supplier<M> mapSupplier) {
+		BiConsumer<M, T> accumulator
+				= (map, element) -> map.put(Opt.ofNullable(element).map(keyMapper).get(), Opt.ofNullable(element).map(valueMapper).get());
+		return new SimpleCollector<>(mapSupplier, accumulator, mapMerger(mergeFunction), CH_ID);
+	}
+
+	/**
+	 * 用户合并map的BinaryOperator，传入合并前需要对value进行的操作
+	 *
+	 * @param mergeFunction 合并前需要对value进行的操作
+	 * @param <K>           key的类型
+	 * @param <V>           value的类型
+	 * @param <M>           map
+	 * @return 用户合并map的BinaryOperator
+	 */
+	public static <K, V, M extends Map<K, V>> BinaryOperator<M> mapMerger(BinaryOperator<V> mergeFunction) {
+		return (m1, m2) -> {
+			for (Map.Entry<K, V> e : m2.entrySet()) {
+				m1.merge(e.getKey(), e.getValue(), mergeFunction);
+			}
+			return m1;
+		};
+	}
+
+
 }

hutool-core/src/test/java/cn/hutool/core/collection/CollStreamUtilTest.java

@@ -150,6 +150,13 @@ public class CollStreamUtilTest {
 		compare.put(2L, map2);
 		Assert.assertEquals(compare, map);
 
+		// 对null友好
+		Map<Long, Map<Long, Student>> termIdClassIdStudentMap = CollStreamUtil.group2Map(Arrays.asList(null, new Student(2, 2, 1, "王五")), Student::getTermId, Student::getClassId);
+		Map<Long, Map<Long, Student>> termIdClassIdStudentCompareMap = new HashMap<Long, Map<Long, Student>>() {{
+			put(null, MapUtil.of(null, null));
+			put(2L, MapUtil.of(2L, new Student(2, 2, 1, "王五")));
+		}};
+		Assert.assertEquals(termIdClassIdStudentCompareMap, termIdClassIdStudentMap);
 	}
 
 	@Test