抽取所有CollStream中group相关函数，提取一个groupThen，采纳上一个PR回复的建议，使得代码更加灵活

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

@@ -1,5 +1,6 @@
 package cn.hutool.core.collection;
 
+import cn.hutool.core.map.MapUtil;
 import lombok.AllArgsConstructor;
 import lombok.Data;
 import lombok.ToString;
@@ -7,6 +8,9 @@ import org.junit.Assert;
 import org.junit.Test;
 
 import java.util.*;
+import java.util.function.Function;
+import java.util.stream.Collector;
+import java.util.stream.Collectors;
 
 /**
  * CollectionStream测试方法
@@ -168,6 +172,39 @@ public class CollStreamUtilTest {
 		Assert.assertEquals(compare, map);
 	}
 
+	@Test
+	public void testGroupThen() {
+		// groupThen作为之前所有group函数的公共部分抽取出来，并更接近于jdk原生，灵活性更强
+
+		// 参数null测试
+		Map<Long, List<Student>> map = CollStreamUtil.groupThen(null, Student::getTermId, Collectors.toList());
+		Assert.assertEquals(map, Collections.EMPTY_MAP);
+
+		// 参数空数组测试
+		List<Student> list = new ArrayList<>();
+		map = CollStreamUtil.groupThen(list, Student::getTermId, Collectors.toList());
+		Assert.assertEquals(map, Collections.EMPTY_MAP);
+
+		// 放入元素
+		list.add(new Student(1, 1, 1, "张三"));
+		list.add(new Student(1, 2, 1, "李四"));
+		list.add(new Student(2, 2, 1, "王五"));
+		// 先根据termId分组，再通过classId比较，找出最大值所属的那个Student,返回的Optional
+		Map<Long, Optional<Student>> longOptionalMap = CollStreamUtil.groupThen(list, Student::getTermId, Collectors.maxBy(Comparator.comparing(Student::getClassId)));
+		Assert.assertEquals("李四", longOptionalMap.get(1L).get().getName());
+
+		// 先根据termId分组，再转换为Map<studentId,name>
+		Map<Long, HashMap<Long, String>> groupThen = CollStreamUtil.groupThen(list, Student::getTermId, Collector.of(HashMap::new, (m, v) -> m.put(v.getStudentId(), v.getName()), (l, r) -> l));
+		Assert.assertEquals(
+				MapUtil.builder()
+						.put(1L, MapUtil.builder().put(1L, "李四").build())
+						.put(2L, MapUtil.builder().put(1L, "王五").build())
+						.build(),
+				groupThen);
+
+		// 总之，如果你是想要group分组后还要进行别的操作，用它就对了！
+	}
+
 
 	@Test
 	public void testTranslate2List() {