Java 17 Key Features: Pattern Matching, Text Blocks, and Sealed Classes

Java has consistently proven its enduring vitality across generations of technology. Particularly, Java 17, as a Long-Term Support (LTS) release, brings a wealth of enhancements and innovative features directly applicable to real-world development. In this article, we will explore the practical applications of Java 17’s most impactful features—Pattern Matching, Text Blocks, and Sealed Classes—with clear examples and usage strategies to help you stay ahead.

Table of Contents

1. The Evolution of Java and the Significance of Java 17
2. Pattern Matching: Cleaner and Safer Type Checks
3. Text Blocks: Simplifying Multiline Strings
4. Sealed Classes: Controlled Inheritance Made Easy
5. Other Notable Features
6. Conclusion: Why Java 17 Matters for Modern Development

1. The Evolution of Java and the Significance of Java 17

Once criticized as an aging language, Java has consistently reinvented itself to remain one of the most dominant and relevant programming languages today. With the release of Java 17, designated as a Long-Term Support (LTS) version, it solidifies its position as a foundational technology for enterprises and developers alike.

Java 17 introduces transformative features that go beyond mere syntax enhancements. Among them, Pattern Matching simplifies and clarifies type checks, Text Blocks dramatically improve the readability of multiline strings, and Sealed Classes provide a robust framework for controlled inheritance.

In this article, we will dive deep into these core improvements, offering practical examples and usage scenarios. Whether you are modernizing legacy systems or building new applications, mastering Java 17’s features will be key to writing cleaner, safer, and more maintainable code. Let’s begin our journey into the new era of Java development.

2. Pattern Matching: Cleaner and Safer Type Checks

One of the standout enhancements introduced in Java 17 is Pattern Matching. This feature significantly simplifies the traditional and often verbose approach to type checking and casting, particularly when using the instanceof operator. By combining type testing and variable binding in a single concise expression, pattern matching improves both code readability and reliability.

2.1 Traditional Approach vs. Pattern Matching

Although pattern matching for instanceof was initially previewed in earlier versions, Java 17 stabilizes it for production use. Let’s compare the traditional method with the new, more elegant pattern matching style.

Traditional Approach:

Object obj = "Hello, World!";
if (obj instanceof String) {
    String str = (String) obj;
    System.out.println(str.length());
}

Pattern Matching Approach:

Object obj = "Hello, World!";
if (obj instanceof String str) {
    System.out.println(str.length());
}

As seen above, pattern matching allows you to both check the type and bind the object to a variable in a single step, reducing redundancy and minimizing the risk of casting errors.

2.2 Practical Applications of Pattern Matching

Pattern matching is especially beneficial in scenarios such as:

• Handling complex conditional logic based on multiple types
• Writing flexible utility methods that process various object types
• Combining type checking with null checks in a clean way

Here’s a practical example that shows how pattern matching can be applied in a polymorphic processing method:

public void process(Object obj) {
    if (obj instanceof String str) {
        System.out.println("String length: " + str.length());
    } else if (obj instanceof Integer num) {
        System.out.println("Integer value: " + num);
    } else {
        System.out.println("Unknown type");
    }
}

Using pattern matching, developers can now write safer and more expressive code, avoiding redundant type casting and ensuring better maintainability. Furthermore, in future versions, pattern matching will extend to switch expressions and record patterns, making it an even more essential feature for modern Java development.

3. Text Blocks: Simplifying Multiline Strings

Handling multiline strings in Java was historically cumbersome and error-prone. Developers had to manually concatenate lines, manage indentation, and escape special characters, often resulting in unreadable and messy code. Java 17 fully stabilizes Text Blocks, a feature designed to address these issues by offering a cleaner, more intuitive way to manage multiline string literals.

3.1 Challenges with Traditional String Handling

Before Text Blocks, writing multiline strings required tedious syntax, making both development and maintenance challenging. Here’s an example of the traditional approach:

Traditional Approach:

String html = "<html>\n" +
               "  <body>\n" +
               "    <h1>Hello, World!</h1>\n" +
               "  </body>\n" +
               "</html>";

This method introduces excessive noise into the code and increases the likelihood of errors, especially with complex structures like HTML, XML, or JSON.

3.2 Text Blocks: Syntax and Examples

Text Blocks utilize three double-quote characters (""") to define multiline strings naturally, preserving indentation and line breaks without the need for explicit newline characters (\n).

Text Block Example:

String html = """
    <html>
      <body>
        <h1>Hello, World!</h1>
      </body>
    </html>
    """;

This approach not only enhances readability but also makes the codebase easier to maintain and debug, aligning with modern development best practices.

3.3 Practical Tips for Using Text Blocks

When working with Text Blocks, keep the following tips in mind:

• Leading whitespace is trimmed based on the line with the least indentation.
• Escape sequences are reduced but still available when needed (e.g., \n, \t).
• No automatic line break is added at the end of a Text Block; it must be included explicitly if required.

Text Blocks are particularly useful for embedding structured data formats directly into your code, such as JSON, HTML, or SQL queries.

SQL Query Example with Text Block:

String query = """
    SELECT id, name, email
    FROM users
    WHERE active = 1
    ORDER BY name
    """;

By adopting Text Blocks, developers can significantly improve the readability and maintainability of codebases that involve complex, multiline string literals.

4. Sealed Classes: Controlled Inheritance Made Easy

Another major innovation in Java 17 is the introduction of Sealed Classes. Sealed Classes offer a way to strictly control which classes can inherit from a particular superclass or implement a given interface. This feature enhances the maintainability, security, and clarity of complex systems by explicitly defining inheritance hierarchies at the language level.

4.1 What Are Sealed Classes?

Traditionally, Java developers could prevent class inheritance using the final keyword, but there was no built-in way to restrict inheritance to a specific set of subclasses. Sealed Classes solve this by allowing a superclass to explicitly declare which classes are permitted to extend or implement it, using the sealed and permits keywords.

Example of a Sealed Class:

public sealed class Shape permits Circle, Rectangle, Square {
}

public final class Circle extends Shape {
}

public final class Rectangle extends Shape {
}

public final class Square extends Shape {
}

In this example, only Circle, Rectangle, and Square are allowed to extend Shape. Any attempt to extend Shape from an unauthorized class will result in a compilation error, enforcing a strict design contract.

4.2 Rules for Sealed Subclasses

Subclasses of a sealed class must explicitly declare one of the following:

• final: To prevent further inheritance
• sealed: To continue restricting inheritance to a defined set of classes
• non-sealed: To remove restrictions and allow open inheritance

Example of a Non-Sealed Subclass:

public non-sealed class Triangle extends Shape {
}

By marking Triangle as non-sealed, it becomes freely extensible by other classes, providing a balance between rigid control and flexible extension where necessary.

4.3 Practical Use Cases for Sealed Classes

Sealed Classes are particularly valuable in the following scenarios:

• Designing domain models with a well-defined set of states or variations
• Restricting API extension to trusted or vetted implementations
• Ensuring exhaustive handling in switch statements for better safety and clarity

Switch Statement Example with Sealed Classes:

Shape shape = new Circle();
switch (shape) {
    case Circle c -> System.out.println("Circle with radius: " + c.getRadius());
    case Rectangle r -> System.out.println("Rectangle with width: " + r.getWidth());
    case Square s -> System.out.println("Square with side: " + s.getSide());
}

Because the compiler knows the complete set of permitted subclasses, it can ensure that all cases are handled at compile time, significantly reducing runtime errors and improving code robustness.

5. Other Notable Features

Beyond the headline features, Java 17 introduces a series of important enhancements and removals that further modernize the language and improve developer experience. While they may not be as high-profile, these updates contribute significantly to writing cleaner, safer, and more efficient Java code.

5.1 Enhanced Switch Expressions

Originally introduced in Java 14 and stabilized in later versions, Switch Expressions continue to mature in Java 17. They offer a more concise, safer alternative to traditional switch statements, supporting both single-line and block expressions with better exhaustiveness checking.

Example of a Switch Expression:

String day = "MONDAY";
String typeOfDay = switch (day) {
    case "MONDAY", "FRIDAY", "SUNDAY" -> "Weekend Vibe";
    case "TUESDAY", "WEDNESDAY", "THURSDAY" -> "Workday";
    default -> throw new IllegalArgumentException("Invalid day: " + day);
};

Switch Expressions help reduce boilerplate code, prevent fall-through errors, and improve overall program safety, especially when handling complex decision logic.

5.2 The Evolution of Records

Record Classes, first formally introduced in Java 16, receive continued enhancements in Java 17. Records provide a compact syntax for declaring immutable data carriers without boilerplate code, ideal for modeling simple data-centric classes.

Example of a Record Declaration:

public record Person(String name, int age) {
}

Records automatically generate essential methods such as constructors, accessors, equals(), hashCode(), and toString(), promoting immutability and reducing manual coding errors.

5.3 Strongly Encapsulated JDK Internals

Java 17 enforces strong encapsulation of internal JDK APIs, making them inaccessible by default. Unlike previous versions where illegal reflective access could be tolerated temporarily using the --illegal-access flag, Java 17 completely restricts it without exceptions.

This change improves the security and stability of Java applications but requires developers to adapt legacy code that depended on internal JDK APIs. Migration strategies include replacing internal API usages with standard supported APIs or using officially endorsed alternatives.

5.4 Deprecations and Removals

Java 17 continues the cleanup of legacy components by deprecating and removing outdated technologies. Understanding these changes is crucial for successful migration and long-term code maintenance.

Item	Description
Applet API	Officially deprecated; planned for complete removal
Security Manager	Marked as deprecated and targeted for removal
RMI Activation System	Deprecated and removed from the JDK

When upgrading to Java 17, it is essential to audit your codebase for dependencies on deprecated or removed APIs to avoid compatibility issues and ensure a smooth transition.

6. Conclusion: Why Java 17 Matters for Modern Development

Java 17 represents far more than just another version update—it marks a pivotal advancement for the Java platform, reinforcing its relevance and adaptability in the ever-evolving world of software development. Through features like Pattern Matching, Text Blocks, and Sealed Classes, Java 17 empowers developers to write more concise, expressive, and robust code, while simultaneously enhancing maintainability and reducing common pitfalls.

From a practical perspective, Java 17 delivers:

• Significantly improved code readability and simplicity
• Enhanced security and stability through strong encapsulation of internal APIs
• Greater design control in domain modeling and API development

However, embracing Java 17 also requires awareness of deprecations, removals, and the stricter encapsulation model. Legacy systems and older libraries may need careful refactoring to achieve full compatibility with the new environment.

Looking ahead, Java’s roadmap continues with versions 18, 19, 20, and beyond, promising further innovations. By mastering Java 17 now, developers can position themselves to fully leverage the evolving capabilities of the Java ecosystem for years to come.

Java 17 is not just a bridge to the future—it’s an invitation to build better, smarter, and more resilient software today. Step confidently into the next generation of Java development.