feat: Add comprehensive examples from multithreading and hashing lectures

src/main/java/lecture/hash/User.java

@@ -0,0 +1,33 @@
+package lecture.hash;
+
+public class User {
+    private int id;
+    private String name;
+
+    public User(int id, String name) {
+        this.id = id;
+        this.name = name;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        User user = (User) o;
+        return id == user.id;
+    }
+
+    @Override
+    public int hashCode() {
+        return Integer.hashCode(id);
+    }
+
+    public static void main(String[] args) {
+        User u1 = new User(101, "Ali");
+        User u2 = new User(101, "Ali");
+
+        System.out.println("Are they logically equal? " + u1.equals(u2));
+
+        System.out.println("Do they have the same Hash? " + (u1.hashCode() == u2.hashCode()));
+    }
+}