diff --git "a/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/1.\346\223\215\344\275\234\345\210\206\347\273\204\347\232\204\345\205\203\347\264\240/config.json" "b/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/1.\346\223\215\344\275\234\345\210\206\347\273\204\347\232\204\345\205\203\347\264\240/config.json"
deleted file mode 100644
index 2a3402038df3f94e1383b4a032368400cecdd2a5..0000000000000000000000000000000000000000
--- "a/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/1.\346\223\215\344\275\234\345\210\206\347\273\204\347\232\204\345\205\203\347\264\240/config.json"
+++ /dev/null
@@ -1,7 +0,0 @@
-{
-  "node_id": "java-4cc01469b31d42ac80854c4024c1b15a",
-  "keywords": [],
-  "children": [],
-  "export": [],
-  "title": "操作分组的元素"
-}
\ No newline at end of file
diff --git "a/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/2.\345\244\232\347\272\247\345\210\206\347\273\204/config.json" "b/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/2.\345\244\232\347\272\247\345\210\206\347\273\204/config.json"
deleted file mode 100644
index 587e05eebc42a319cd0d48a8dd92858c53bbba03..0000000000000000000000000000000000000000
--- "a/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/2.\345\244\232\347\272\247\345\210\206\347\273\204/config.json"
+++ /dev/null
@@ -1,7 +0,0 @@
-{
-  "node_id": "java-873636226f0e44ad9269f2a13fe2adb9",
-  "keywords": [],
-  "children": [],
-  "export": [],
-  "title": "多级分组"
-}
\ No newline at end of file
diff --git "a/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/3.\346\214\211\345\255\220\347\273\204\346\224\266\351\233\206\346\225\260\346\215\256/config.json" "b/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/3.\346\214\211\345\255\220\347\273\204\346\224\266\351\233\206\346\225\260\346\215\256/config.json"
deleted file mode 100644
index 9633c11f5cd54d9896dc6a4ae78589ab3fd4dd33..0000000000000000000000000000000000000000
--- "a/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/3.\346\214\211\345\255\220\347\273\204\346\224\266\351\233\206\346\225\260\346\215\256/config.json"
+++ /dev/null
@@ -1,7 +0,0 @@
-{
-  "node_id": "java-1223956aa7ae47a9a2e517dc23c3f371",
-  "keywords": [],
-  "children": [],
-  "export": [],
-  "title": "按子组收集数据"
-}
\ No newline at end of file
diff --git "a/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/group_by.json" "b/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/group_by.json"
new file mode 100644
index 0000000000000000000000000000000000000000..e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
diff --git "a/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/group_by.md" "b/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/group_by.md"
new file mode 100644
index 0000000000000000000000000000000000000000..3f3970a5194044b6663698899ff4247a42f24761
--- /dev/null
+++ "b/data/3.Java\351\253\230\351\230\266/5.\345\210\206\347\273\204/group_by.md"
@@ -0,0 +1,51 @@
+# 分组
+
+已知类型
+
+```java
+public class Employee{
+    int id;
+    String name;
+    String department;
+}
+```
+
+将 `List<Employee> employee` 中的员工信息按部门分组，其中不正确的是：
+
+## 答案
+
+```java
+// List<Employee> employee = ...;
+Map<String, Object> result = employee.stream().collect(toMap(Employee::getDepartment, Function.identity()));
+```
+
+## 选项
+
+### 标准方法
+
+```java
+// List<Employee> employee = ...;
+Map<String, List<Employee>> result = employee.stream().collect(groupingBy(Employee::getDepartment));
+```
+
+### 朴素方法
+
+```java
+// List<Employee> employee = ...;
+Map<String, List<Employee>> result = new HashMap<>();
+for(var emp: employee){
+    var list = result.getOrDefault(emp.department, new ArrayList<>());
+    list.add(emp);
+}
+```
+
+### 朴素方法
+
+```java
+Map<String, List<Employee>> result = new HashMap<>();
+for(int index=0; index < employee.size(); index++){
+    var emp = employee.get(index);
+    var list = result.getOrDefault(emp.department, new ArrayList<>());
+    list.add(emp);
+}
+```