Java 17 and Java 21 Interview Questions: New Features 2026

Prerequisites for Java 17 and Java 21
Deep Dive into Java 17 and Java 21 Concepts
Step-by-Step Guide to Java 17 and Java 21 Development
Full Example Project Using Java 17 and Java 21 Features
Common Mistakes to Avoid in Java 17 and Java 21 Development
Mistake 1: Incorrect Use of Records
Mistake 2: Incorrect Use of Sealed Classes
Production-Ready Tips for Java 17 and Java 21 Applications
Testing Strategies for Java 17 and Java 21 Applications
Key Takeaways for Java 17 and Java 21 Interviews
Common Java 17 and Java 21 Interview Questions
Future Directions for Java 17 and Java 21 Development

Prerequisites for Java 17 and Java 21

To get started with Java 17 and Java 21 development, you should have a solid understanding of **object-oriented programming** concepts and **Java syntax**. Familiarity with **Java 8** features such as **lambda expressions** and **method references** is also essential. Additionally, knowledge of **Java 11** features like **var** and **HTTP Client** is recommended.

Prior to installing Java 17 or Java 21, ensure you have a compatible operating system, such as **Windows 10** or **Ubuntu 20.04**, and a code editor or **IDE** like **Eclipse** or **IntelliJ IDEA**. You should also have a basic understanding of **Java build tools** like **Maven** or **Gradle**. For more information on setting up your development environment, visit our Java Development Environment Setup guide.

To verify your Java installation, you can use the following Java code:

public class JavaVersionChecker {
 public static void main(String[] args) {
 // Get the Java version using the System.getProperty method
 String javaVersion = System.getProperty("java.version");
 // Print the Java version to the console
 System.out.println("Java Version: " + javaVersion);
 }
}

The expected output will be:

Java Version: 17.0.2

Java Version: 21.0.1

depending on your installed Java version. This code uses the **System.getProperty** method to retrieve the Java version and prints it to the console. For further reading on Java 17 and Java 21 features, visit our Java 17 Features and Java 21 Features guides.

Deep Dive into Java 17 and Java 21 Concepts

Java 17 and Java 21 introduce several new features that enhance the language’s expressiveness and performance. One of the key features is sealed classes, which allow developers to restrict the types that can extend or implement a class or interface. This is achieved using the sealed keyword, which can be applied to a class or interface to specify the permitted subclasses or implementations. For example, a sealed class can be defined as public sealed class Shape permits Circle, Rectangle.

The pattern matching feature is another significant addition to Java 17 and Java 21. This feature enables developers to use the instanceof operator to check the type of an object and perform actions based on that type. The switch statement has also been enhanced to support pattern matching, allowing for more concise and expressive code. To learn more about the pattern matching feature, visit our article on Java Pattern Matching for a detailed explanation.

Java 21 also introduces virtual threads, which are designed to reduce the overhead of thread creation and management. Virtual threads are lightweight and can be used to improve the performance of concurrent applications. The Thread class has been updated to support virtual threads, and developers can use the Thread.startVirtualThread() method to create a new virtual thread.

The combination of sealed classes, pattern matching, and virtual threads provides developers with a powerful set of tools for building efficient and expressive applications. By mastering these features, developers can write more concise and effective code, and take advantage of the performance benefits offered by Java 17 and Java 21. For further reading on concurrency in Java, see our article on Java Concurrency for a comprehensive overview.

Step-by-Step Guide to Java 17 and Java 21 Development

To get started with Java 17 and Java 21 development, you need to have a good understanding of **Java Fundamentals** and **Object-Oriented Programming** concepts. The Java Basics tutorial provides a comprehensive overview of these concepts. Java 17 and Java 21 introduce several new features, including **Sealed Classes** and **Pattern Matching for switch**.

The **Sealed Classes** feature allows you to restrict which classes can extend or implement a sealed class. This is useful for creating more robust and maintainable code. For example, you can use sealed classes to create a hierarchy of classes that represent different types of vehicles.

To demonstrate the use of sealed classes, consider the following example:

public sealed class Vehicle permits Car, Truck, Motorcycle {
 // common attributes and methods
}

public final class Car extends Vehicle {
 // car-specific attributes and methods
}

public final class Truck extends Vehicle {
 // truck-specific attributes and methods
}

public final class Motorcycle extends Vehicle {
 // motorcycle-specific attributes and methods
}

In this example, the `Vehicle` class is sealed, and only the `Car`, `Truck`, and `Motorcycle` classes are allowed to extend it.

For further reading on **Pattern Matching for switch**, see the Java Switch Pattern Matching tutorial. This feature allows you to use pattern matching in switch statements, making your code more concise and expressive.

Here’s an example of using pattern matching for switch:

public class PatternMatchingExample {
 public static void main(String[] args) {
 Object obj = "Hello";
 // using pattern matching for switch
 switch (obj) {
 case String s -> System.out.println("String: " + s); // why: we're checking if obj is a String
 case Integer i -> System.out.println("Integer: " + i); // why: we're checking if obj is an Integer
 default -> System.out.println("Unknown type");
 }
 }
}

The expected output is:

String: Hello

This example demonstrates how to use pattern matching for switch to handle different types of objects in a concise and expressive way. For more information on **Java 17 and Java 21 Features**, see the Java 17 and Java 21 Features tutorial.

Full Example Project Using Java 17 and Java 21 Features

The **Java 17** and **Java 21** releases introduced several new features that can improve the performance and readability of Java applications. One of the key features is the use of **sealed classes**, which allows developers to restrict the classes that can extend or implement a particular class or interface. To demonstrate the use of these features, we will create a simple example project that uses **sealed classes** and **pattern matching**.

The example project will be a simple banking system that allows users to create accounts and perform transactions. We will define a **sealed class** hierarchy for the different types of accounts, and use **pattern matching** to handle the different types of transactions. For more information on the **sealed classes** feature, see our article on Java 17 Sealed Classes.

The following code example demonstrates the use of **sealed classes** and **pattern matching** in the banking system:

public sealed class Account permits CheckingAccount, SavingsAccount {
 // common attributes and methods for all accounts
 private final String accountNumber;
 private double balance;

 public Account(String accountNumber, double balance) {
 this.accountNumber = accountNumber;
 this.balance = balance;
 }

 // method to perform a transaction
 public void performTransaction(Transaction transaction) {
 // use pattern matching to handle different types of transactions
 switch (transaction) {
 case Deposit deposit -> balance += deposit.getAmount();
 case Withdrawal withdrawal -> balance -= withdrawal.getAmount();
 // add more cases for other types of transactions
 }
 }
}

public final class CheckingAccount extends Account {
 // attributes and methods specific to checking accounts
 private final double overdraftLimit;

 public CheckingAccount(String accountNumber, double balance, double overdraftLimit) {
 super(accountNumber, balance);
 this.overdraftLimit = overdraftLimit;
 }
}

public final class SavingsAccount extends Account {
 // attributes and methods specific to savings accounts
 private final double interestRate;

 public SavingsAccount(String accountNumber, double balance, double interestRate) {
 super(accountNumber, balance);
 this.interestRate = interestRate;
 }
}

public sealed class Transaction permits Deposit, Withdrawal {
 // common attributes and methods for all transactions
 private final String transactionId;

 public Transaction(String transactionId) {
 this.transactionId = transactionId;
 }
}

public final class Deposit extends Transaction {
 // attributes and methods specific to deposits
 private final double amount;

 public Deposit(String transactionId, double amount) {
 super(transactionId);
 this.amount = amount;

 // why: we need to store the amount of the deposit
 }

 public double getAmount() {
 return amount;
 }
}

public final class Withdrawal extends Transaction {
 // attributes and methods specific to withdrawals
 private final double amount;

 public Withdrawal(String transactionId, double amount) {
 super(transactionId);
 this.amount = amount;

 // why: we need to store the amount of the withdrawal
 }

 public double getAmount() {
 return amount;
 }
}

The expected output of this code will depend on the specific transactions that are performed. For example, if we create a checking account and perform a deposit transaction, the output will be:

Account balance: 1000.0

For further reading on **pattern matching**, see our article on Java 21 Pattern Matching.

Common Mistakes to Avoid in Java 17 and Java 21 Development

Java 17 and Java 21 introduce several new features that can improve the efficiency and readability of your code. However, these features can also lead to common pitfalls if not used correctly. One of the key areas to focus on is the use of **records** and **sealed classes**. For more information on these features, you can visit our Java 17 Features page.

Mistake 1: Incorrect Use of Records

When using **records**, it’s essential to remember that they are immutable by default. Attempting to modify a record’s component will result in a compiler error.

public record Person(String name, int age) {
 // WRONG
 public void setAge(int age) {
 this.age = age; // compiler error: cannot assign a value to final variable age
 }
}

The error message will be: “cannot assign a value to final variable age”. To fix this, you can create a new record with the updated component:

public record Person(String name, int age) {
 public Person withAge(int newAge) {
 // create a new record with the updated age
 return new Person(this.name, newAge);
 }
}

This will allow you to create a new record with the updated age without modifying the original record.

Mistake 2: Incorrect Use of Sealed Classes

When using **sealed classes**, it’s essential to remember that they can only be extended by a fixed set of subclasses. Attempting to extend a sealed class with a new subclass will result in a compiler error.

public sealed class Shape permits Circle, Rectangle {
 // ...
}
// WRONG
public final class Triangle extends Shape { // compiler error: sealed class Shape cannot be extended by class Triangle
}

The error message will be: “sealed class Shape cannot be extended by class Triangle”. To fix this, you need to add the new subclass to the permits clause:

public sealed class Shape permits Circle, Rectangle, Triangle {
 // ...
}
public final class Triangle extends Shape {
 // ...
}

This will allow you to extend the sealed class with the new subclass. For more information on **sealed classes**, you can visit our Java 17 Sealed Classes page.

To test the corrected code, you can use the following example:

public class Main {
 public static void main(String[] args) {
 Person person = new Person("John", 30);
 Person updatedPerson = person.withAge(31);
 System.out.println(updatedPerson); // prints: Person[name=John, age=31]
 }
}

The expected output is:

Person[name=John, age=31]

By avoiding these common mistakes, you can ensure that your Java 17 and Java 21 code is efficient, readable, and free of errors. For further reading, you can visit our Java 21 Features page.

Production-Ready Tips for Java 17 and Java 21 Applications

When deploying Java 17 and Java 21 applications in production, following best practices is crucial for ensuring reliability and performance. Containerization using tools like Docker can help simplify deployment and management. The java.lang.module package provides support for modular programming, which can improve application maintainability.

Production tip: Use modular programming to organize your code into logical modules, making it easier to maintain and update your application.

To ensure smooth operation, it’s essential to monitor application performance and identify potential bottlenecks. The java.lang.management package provides classes for monitoring and managing Java applications. For more information on Java performance tuning, see our dedicated guide.

Production tip: Implement logging and monitoring mechanisms to track application performance and quickly identify issues before they become critical.

Security is another critical aspect of production-ready applications. Java 17 and Java 21 provide various security features, including TLS and cryptographic APIs. The java.security package offers classes for working with digital signatures, message digests, and other security-related tasks.

Production tip: Use secure communication protocols and follow best practices for data encryption to protect sensitive data and prevent unauthorized access.

By following these production-ready tips and staying up-to-date with the latest Java features and best practices, developers can ensure their Java 17 and Java 21 applications are reliable, performant, and secure. For further reading on Java 17 features and Java 21 features, see our in-depth guides.

Testing Strategies for Java 17 and Java 21 Applications

When developing **Java 17** and **Java 21** applications, ensuring the quality of the code is crucial. This can be achieved by utilizing various **testing frameworks** and strategies. One popular framework is **JUnit**, which provides a rich set of tools for writing and running tests. For more information on **JUnit**, refer to our article on JUnit Tutorial for Beginners.

To get started with testing, you need to understand the different types of tests, including **unit tests**, **integration tests**, and **system tests**. **Unit tests** focus on individual components or methods, while **integration tests** verify how these components interact with each other. **System tests**, on the other hand, test the entire application from end to end.

A key aspect of testing is writing **assertions**, which verify that the expected behavior matches the actual behavior. This can be done using the assert keyword in Java. For example, the following code demonstrates a simple **unit test** using **JUnit**:

package com.example.testing;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

public class CalculatorTest {
 @Test
 public void testAddition() {
 // Create a Calculator object
 Calculator calculator = new Calculator();
 // Call the add method and store the result
 int result = calculator.add(2, 3);
 // Assert that the result is 5
 assertEquals(5, result); // This assertion will pass if the result is indeed 5
 }
}

The expected output of this test would be:

Test passed: testAddition

This indicates that the **assertion** was successful, and the add method behaves as expected. For further reading on **testing strategies**, visit our article on Testing Strategies for Java Applications. Additionally, you can learn more about **Java 17** and **Java 21** features in our article on New Features in Java 17 and Java 21.

Key Takeaways for Java 17 and Java 21 Interviews

When preparing for Java 17 and Java 21 interviews, focus on sealable interfaces and pattern matching for switch statements. These features, introduced in Java 17, are crucial for any aspiring Java developer. The java.lang.String class has also been enhanced with new methods, such as String.stripIndent() and String.translateEscapes(), which can be used to improve code readability.

For Java 21 interviews, understanding of virtual threads and structured concurrency is essential. The java.lang.Thread class has been updated to support virtual threads, which can significantly improve the performance of concurrent applications. Additionally, the java.util.concurrent package has been enhanced with new classes, such as java.util.concurrent.StructuredTaskScope, which provides a way to manage concurrent tasks in a structured way.

To prepare for these interviews, it is recommended to review the Java 17 features and practice using them in real-world scenarios. This will help developers gain hands-on experience with sealed classes and pattern matching for switch statements. Furthermore, understanding the Java Memory Model and how it relates to concurrent programming is also crucial for success in these interviews.

In terms of specific topics to focus on, records and text blocks are also important features to understand. The java.lang.Record class provides a concise way to create immutable data classes, while java.lang.TextBlock provides a way to create multiline string literals. By mastering these features and understanding how to apply them in real-world scenarios, developers can significantly improve their chances of success in Java 17 and Java 21 interviews. For more information on concurrent programming, see our article on Java Concurrency.

Common Java 17 and Java 21 Interview Questions

Java 17 and Java 21 have introduced several new features that are commonly asked about in interviews. One of the key features of Java 17 is the sealed classes feature, which allows developers to restrict which classes can extend or implement a class or interface. This feature is often asked about in conjunction with the sealed keyword and the permits clause. For more information on sealed classes, see our article on Java Sealed Classes and Interfaces.

Another area of focus for Java 17 and Java 21 interview questions is the text blocks feature, which allows developers to create multiline string literals. This feature is often asked about in conjunction with the String class and the StringBuilder class. Interviewers may ask about the benefits and drawbacks of using text blocks versus other methods of creating multiline strings.

Java 21 has also introduced several new features, including the virtual threads feature, which allows developers to create lightweight threads that can improve the performance of concurrent applications. This feature is often asked about in conjunction with the Thread class and the ExecutorService interface. Developers may be asked to explain how virtual threads differ from traditional threads and how they can be used to improve the performance of concurrent applications.

Interviewers may also ask about the pattern matching for switch feature, which allows developers to use pattern matching in switch statements. This feature is often asked about in conjunction with the switch statement and the case keyword. Developers may be asked to explain how pattern matching for switch can be used to simplify code and improve readability. For further reading on Java 17 and Java 21 features, see our article on Java 17 Features and Updates and Java 21 Features and Updates.

Future Directions for Java 17 and Java 21 Development

Java 17 and Java 21 have introduced several new features, including sealed classes and pattern matching, which are expected to shape the future of Java development. The java.lang.Math class has also been updated with new methods for vectorized operations. As developers, it’s essential to stay up-to-date with these changes and understand how to apply them in real-world applications. For more information on sealed classes, refer to our article on Java 17 Sealed Classes and Their Applications.

The upcoming features in Java 21, such as virtual threads and structured concurrency, are expected to significantly improve the performance and scalability of Java applications. The java.lang.Thread class is being updated to support virtual threads, which will enable developers to write more efficient and concurrent code. To take full advantage of these features, developers should familiarize themselves with the java.util.concurrent package and its various classes, such as ExecutorService and ForkJoinPool.

Another area of focus for future Java development is cloud-native applications. As more businesses move their applications to the cloud, Java developers need to be aware of the challenges and opportunities that come with cloud-based development. This includes understanding how to use containerization and orchestration tools like Docker and Kubernetes. For further reading on cloud-native Java development, see our article on Building Cloud-Native Java Applications.

The Java community is also exploring new ways to improve the language’s type system and error handling mechanisms. The java.lang.Record class, introduced in Java 14, provides a more concise way to create immutable data classes. Additionally, the java.util.Option class offers a safer way to handle null values and avoid NullPointerExceptions. By staying current with these developments, Java developers can write more robust, maintainable, and efficient code.