Solutions for all the 8 questions

solutions/q1/NotificationServiceSolution.java

@@ -0,0 +1,16 @@
+interface NotificationService {
+    void send(String message);
+
+    default void sendWithRetry(String message, int retries) {
+        int attempts = 0;
+        while (attempts < retries) {
+            try {
+                send(message);
+                return;
+            } catch (Exception e) {
+                attempts++;
+            }
+        }
+        throw new RuntimeException("Failed after retries");
+    }
+}

solutions/q2/StringValidatorDemo.java

@@ -0,0 +1,17 @@
+@FunctionalInterface
+interface StringValidator {
+    boolean validate(String value);
+}
+
+public class StringValidatorDemo {
+
+    static boolean validate(String value, StringValidator validator) {
+        return validator.validate(value);
+    }
+
+    public static void main(String[] args) {
+        System.out.println(validate("Hello", s -> !s.isEmpty()));
+        System.out.println(validate("Welcome", s -> s.length() > 5));
+        System.out.println(validate("Java", s -> Character.isUpperCase(s.charAt(0))));
+    }
+}

solutions/q3/UserStreamExample.java

@@ -0,0 +1,31 @@
+import java.util.List;
+
+class User {
+    String name;
+    int age;
+
+    User(String name, int age) {
+        this.name = name;
+        this.age = age;
+    }
+}
+
+public class UserStreamExample {
+    public static void main(String[] args) {
+
+        List<User> users = List.of(
+            new User("Yashas", 21),
+            new User("Rohan", 20),
+            new User("Rahul", 24)
+        );
+
+        List<String> result =
+            users.stream()
+                 .filter(u -> u.age >= 18)
+                 .map(u -> u.name.toUpperCase())
+                 .sorted()
+                 .toList();
+
+        result.forEach(System.out::println);
+    }
+}

solutions/q4/PredicateDemo.java

@@ -0,0 +1,17 @@
+import java.util.List;
+import java.util.function.Predicate;
+
+public class PredicateDemo {
+    public static void main(String[] args) {
+
+        List<Integer> numbers = List.of(-2, 3, 4, 6, -8);
+
+        Predicate<Integer> isPositive = n -> n > 0;
+        Predicate<Integer> isEven = n -> n % 2 == 0;
+
+        numbers.stream()
+               .filter(isPositive.and(isEven))
+               .forEach(System.out::println);
+    }
+}
+

solutions/q5/OptionalDemo.java

@@ -0,0 +1,28 @@
+import java.util.Optional;
+
+class User {
+    String name;
+    boolean active;
+
+    User(String name, boolean active) {
+        this.name = name;
+        this.active = active;
+    }
+}
+
+public class OptionalDemo {
+
+    static Optional<User> findUser(String id) {
+        return Optional.of(new User("Yashas", true));
+    }
+
+    public static void main(String[] args) {
+
+        findUser("101")
+            .filter(u -> u.active)
+            .ifPresentOrElse(
+                u -> System.out.println(u.name),
+                () -> System.out.println("User not found")
+            );
+    }
+}

solutions/q6/Employee.java

@@ -0,0 +1,17 @@
+public record Employee(int id, double salary) {
+
+    public Employee {
+        if (salary <= 0) {
+            throw new IllegalArgumentException("Salary must be positive");
+        }
+    }
+
+    public boolean isHighEarner() {
+        return salary > 100000;
+    }
+
+    public static void main(String[] args) {
+        Employee e = new Employee(1, 150000);
+        System.out.println(e.isHighEarner());
+    }
+}

solutions/q7/PaymentProcessor.java

@@ -0,0 +1,20 @@
+sealed interface Payment permits CardPayment, UpiPayment {}
+
+record CardPayment(String cardNumber) implements Payment {}
+record UpiPayment(String upiId) implements Payment {}
+
+public class PaymentProcessor {
+
+    static void process(Payment p) {
+        if (p instanceof CardPayment cp) {
+            System.out.println("Processing card: " + cp.cardNumber());
+        } else if (p instanceof UpiPayment up) {
+            System.out.println("Processing UPI: " + up.upiId());
+        }
+    }
+
+    public static void main(String[] args) {
+        process(new CardPayment("1234-5678"));
+        process(new UpiPayment("user@upi"));
+    }
+}

solutions/q8/TextBlockDemo.java

@@ -0,0 +1,15 @@
+public class TextBlockDemo {
+    public static void main(String[] args) {
+
+        String userId = "101";
+
+        String query = """
+            SELECT *
+            FROM users
+            WHERE user_id = '%s'
+            AND active = true;
+            """.formatted(userId);
+
+        System.out.println(query);
+    }
+}