diff --git a/oop/code/oop/src/main/java/com/scaler/lld/App.java b/oop/code/oop/src/main/java/com/scaler/lld/App.java
index 58c2e48..5008657 100644
--- a/oop/code/oop/src/main/java/com/scaler/lld/App.java
+++ b/oop/code/oop/src/main/java/com/scaler/lld/App.java
@@ -1,27 +1,58 @@
 package com.scaler.lld;
 
-import com.scaler.lld.basics.OopBankAccount;
+import java.util.List;
+
+import com.scaler.lld.scaler.Student;
+import com.scaler.lld.scaler.User;
 
 /**
  * Hello world!
  *
  */
-public class App 
-{
-    public static void main( String[] args )
-    {
-        // Create accounts for Alice and Bob
-        OopBankAccount accountAlice = new OopBankAccount(1, 1000, "Alice");
-        OopBankAccount accountBob = new OopBankAccount(2, 2000, "Bob");
+public class App {
+    public static void main(String[] args) {
+        User student = new Student("student", "student@scaler.in", "batch", 100);
+        student.printInfo(); // inheritance
+    }
 
-        System.out.println("\nAlice's balance: " + accountAlice.getBalance());
-        System.out.println("Bob's balance: " + accountBob.getBalance());
+    public static void resetEmail(List<User> users) {
+        for (User user : users) {
+            user.changeEmail("");
 
-        System.out.println("\nTransferring $100 from Alice to Bob");
-        // Transfer $100 from Alice to Bob
-        accountAlice.transfer(accountBob, 100);
+            if (user instanceof Student) {
+                Student student = (Student) user;
+
+                System.out.println("Name :" + student.getName() + " " + student.getPsp());
+
+                student.setPsp(0);
+                System.out.println("Name :" + student.getName() + " " + student.getPsp());
+
+            }            
+
+        }
 
-        System.out.println("\nAlice's balance: " + accountAlice.getBalance());
-        System.out.println("Bob's balance: " + accountBob.getBalance());
     }
 }
+
+// Reusable code
+// In order to reset email, I just had to call the parent's changeEmail fn
+// Instead of defining a method for each class
+// instanceof
+// A a = (A) b;
+// 6:05
+// 10:35
+// Three types of DP
+// 1. Creational - OOP
+// Factory - Simple Factory - No OOP
+// 2. Structural - 
+// 3. Behavioural -  
+
+// 1. Subtyping
+//  - Compile and run time
+
+// Method overloading - Compile time
+// 2. Generic Polymorphism
+// 3. Adhoc polymorphism - Duck Typing
+
+// PATCH /edit-student (JSON patchBody) 
+// { field: "name", op: "set", value: "Tanmay"}
\ No newline at end of file
diff --git a/oop/code/oop/src/main/java/com/scaler/lld/scaler/Mentor.java b/oop/code/oop/src/main/java/com/scaler/lld/scaler/Mentor.java
new file mode 100644
index 0000000..94e351f
--- /dev/null
+++ b/oop/code/oop/src/main/java/com/scaler/lld/scaler/Mentor.java
@@ -0,0 +1,25 @@
+package com.scaler.lld.subtyping;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import com.scaler.lld.scaler.Student;
+import com.scaler.lld.scaler.User;
+
+import lombok.Getter;
+import lombok.Setter;
+
+// Step 1 - Extend parent class
+@Getter
+@Setter
+public class Mentor extends User {
+    private List<Student> mentees = new ArrayList<>();
+    private String company;
+
+    public Mentor(String name, String email, List<Student> mentees, String company) {
+        super(name, email);
+        this.mentees = mentees;
+        this.company = company;
+    }
+
+}
diff --git a/oop/code/oop/src/main/java/com/scaler/lld/scaler/Student.java b/oop/code/oop/src/main/java/com/scaler/lld/scaler/Student.java
index b41f93f..b70a0b0 100644
--- a/oop/code/oop/src/main/java/com/scaler/lld/scaler/Student.java
+++ b/oop/code/oop/src/main/java/com/scaler/lld/scaler/Student.java
@@ -1,12 +1,10 @@
 package com.scaler.lld.scaler;
 
 import lombok.Getter;
-import lombok.NoArgsConstructor;
 import lombok.Setter;
 
 @Getter
 @Setter
-@NoArgsConstructor
 public class Student extends User {
 
     private String batchName;
@@ -25,9 +23,16 @@ public class Student extends User {
         this.psp = psp;
     }
 
-    public void changeBatch(String batchName) {
+    public Student() {
+    }
+
+    void changeBatch(String batchName) {
         this.batchName = batchName;
     }
 
+    @Override
+    public void printInfo() {
+        System.out.println("\nStudent: " + getName() + " " + getBatchName());
+    }
 
 }
\ No newline at end of file
diff --git a/oop/code/oop/src/main/java/com/scaler/lld/scaler/User.java b/oop/code/oop/src/main/java/com/scaler/lld/scaler/User.java
index ce6e403..c81c45b 100644
--- a/oop/code/oop/src/main/java/com/scaler/lld/scaler/User.java
+++ b/oop/code/oop/src/main/java/com/scaler/lld/scaler/User.java
@@ -7,8 +7,8 @@ import lombok.Setter;
 
 @Getter
 @Setter
-@NoArgsConstructor
 @AllArgsConstructor
+@NoArgsConstructor
 public class User {
     private String name;
     private String email;
@@ -16,4 +16,26 @@ public class User {
     public void changeEmail(String email) {
         this.email = email;
     }
+
+    // Method overloading
+    // Method signature: 
+    // 1. Method name
+    // 2. # of args
+    // 3. Data type of args
+
+    public void printInfo() {
+    }
+
+    public void printInfo(String title) {
+        System.out.println(" \n User: " + title + " " + this.getName());
+    }
 }
+
+// Interfaces
+// Class - Blueprint
+// Interface - Blue print of behaviour
+// Database db = new MySqlDB();
+// Interfaces - define methods with an impl.
+// Abstract - mixture of interface and class
+// implemented methods + not-implemented
+// Abstract classes - state