Hash set task 3

src/com/school/faang/hashset/задача_3/ContactList.java

@@ -0,0 +1,35 @@
+package com.school.faang.hashset.задача_3;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class ContactList {
+
+    private final Set<String> contacts;
+
+    public ContactList() {
+        contacts = new HashSet<>();
+    }
+
+    public ContactList(Set<String> contacts) {
+        this.contacts = new HashSet<>(contacts);
+    }
+
+    public void addContact(String contact) {
+        contacts.add(contact);
+    }
+
+    public void removeContact(String contact) {
+        contacts.remove(contact);
+    }
+
+    public boolean hasContact(String contact) {
+        return contacts.contains(contact);
+    }
+
+    public static Set<String> findMutualContacts(ContactList contactList1, ContactList contactList2) {
+        Set<String> result = new HashSet<>(contactList1.contacts);
+        result.retainAll(contactList2.contacts);
+        return result;
+    }
+}

src/com/school/faang/hashset/задача_3/Solution.java

@@ -1,7 +1,19 @@
 package com.school.faang.hashset.задача_3;
 
+import java.util.Collections;
+import java.util.Set;
+
 public class Solution {
     public static void main(String[] args) {
-
+        User firstUser = new User("first");
+        User secondUser = new User("second");
+        User thirdUser = new User("third");
+        firstUser.addContact(secondUser);
+        thirdUser.addContact(secondUser);
+        thirdUser.addContact(firstUser);
+        firstUser.addContact(thirdUser);
+        Set<String> mutualContactsForFirstAndThird = User.findMutualContacts(firstUser, thirdUser);
+        System.out.println(mutualContactsForFirstAndThird.size());
+        mutualContactsForFirstAndThird.forEach(System.out::println);
     }
 }

src/com/school/faang/hashset/задача_3/User.java

@@ -0,0 +1,53 @@
+package com.school.faang.hashset.задача_3;
+
+import java.util.Set;
+
+public class User {
+
+    private final String username;
+
+    private final ContactList contactList;
+
+    public User(String username) {
+        this.username = username;
+        contactList = new ContactList();
+    }
+
+    public User(String username, Set<String> contacts) {
+        this.contactList = new ContactList(contacts);
+        this.username = username;
+    }
+
+    public void addContact(String username) {
+        contactList.addContact(username);
+    }
+
+    public void addContact(User user) {
+        addContact(user.toString());
+    }
+
+    public void removeContact(String username) {
+        contactList.removeContact(username);
+    }
+
+    public void removeContact(User user) {
+        removeContact(user.toString());
+    }
+
+    public boolean hasContact(String username) {
+        return contactList.hasContact(username);
+    }
+
+    public boolean hasContact(User user) {
+        return hasContact(user.toString());
+    }
+
+    public static Set<String> findMutualContacts(User user1, User user2) {
+        return ContactList.findMutualContacts(user1.contactList, user2.contactList);
+    }
+
+    @Override
+    public String toString() {
+        return username;
+    }
+}