diff --git a/src/main/java/com/github/hcsp/collection/Main.java b/src/main/java/com/github/hcsp/collection/Main.java
index ae0f842..e452a91 100644
--- a/src/main/java/com/github/hcsp/collection/Main.java
+++ b/src/main/java/com/github/hcsp/collection/Main.java
@@ -1,12 +1,32 @@
 package com.github.hcsp.collection;
 
-import java.util.Arrays;
 import java.util.List;
 import java.util.Set;
+import java.util.Map;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Objects;
+import java.util.Arrays;
 
 public class Main {
     // 请编写一个方法，获得a和b集合中的公共元素。
-    public static Set<Person> commonElementsIn(List<Person> a, List<Person> b) {}
+    public static Set<Person> commonElementsIn(List<Person> a, List<Person> b) {
+        Set<Person> result = new HashSet<>();
+        Map<Person, Integer> hashMap = new HashMap<>(a.size() + b.size());
+        for (Person p : a) {
+            hashMap.put(p, hashMap.getOrDefault(p, 0) + 1);
+        }
+        for (Person p : b) {
+            hashMap.put(p, hashMap.getOrDefault(p, 0) + 1);
+        }
+        hashMap.forEach((key, value) -> {
+            if (value >= 2) {
+                result.add(key);
+            }
+        });
+        return result;
+    }
+
 
     // Person类，如果两个Person对象的name相等，则认为这两个对象相等。
     public static class Person {
@@ -23,6 +43,17 @@ public String getName() {
         public void setName(String name) {
             this.name = name;
         }
+
+        @Override
+        public boolean equals(Object obj) {
+            return Objects.equals(name, ((Person) obj).getName());
+        }
+
+        @Override
+        public int hashCode() {
+            return name.hashCode();
+        }
+
     }
 
     public static void main(String[] args) {