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Unlock Java 17: Transform Your Code with Records, Sealed Classes, and Pattern Matching

This article explores JDK 17’s most impactful language enhancements—records, sealed classes, pattern matching, text blocks, var, and new APIs—showing how they reduce boilerplate, improve readability, and boost performance for Java developers transitioning from older versions.

Architect's Guide
Architect's Guide
Architect's Guide
Unlock Java 17: Transform Your Code with Records, Sealed Classes, and Pattern Matching

Are you still writing Java 8 code? It’s time to upgrade to JDK 17, a long‑term‑support release that introduces a suite of concise syntax features, making Java programming shorter and more efficient.

1. From JDK 8 to JDK 17

Why JDK 17 is a milestone

JDK 17 is not just another update; it consolidates all innovations introduced since JDK 9 and serves as the next LTS version after JDK 8 and JDK 11, marking a major step in Java’s modernization.

Significance of LTS

As an LTS release, JDK 17 receives at least eight years of support, allowing enterprises to migrate confidently and enjoy new features without frequent upgrades.

2. Record classes

Pain points of traditional JavaBeans

public class Person {
    private final String name;
    private final int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // getters, equals, hashCode, toString …
}

This boilerplate hides the class’s intent and is error‑prone.

Basic syntax and usage

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

The compiler automatically generates the constructor, getters, equals(), hashCode(), and toString(), reducing dozens of lines to a single declaration.

Records and immutability

Records are immutable by design, supporting functional‑style programming. To modify a field you create a new instance:

Person alice = new Person("Alice", 25);
// modify age
Person olderAlice = new Person(alice.name(), alice.age() + 1);

When to use and when not to

Records are ideal for DTOs, value objects, or immutable containers, but they cannot extend other classes, declare additional instance fields, or be abstract.

3. Sealed classes

Core concept

A sealed class sits between final and unrestricted inheritance, allowing you to specify which subclasses may extend it.

public sealed class Shape permits Circle, Rectangle, Triangle {
    // shared members
}

permits keyword

The permits clause lists allowed subclasses, which must be declared final, sealed, or non‑sealed:

public final class Circle extends Shape { }
public sealed class Rectangle extends Shape permits Square { }
public non‑sealed class Triangle extends Shape { }

Sealed interfaces

public sealed interface Vehicle permits Car, Truck, Motorcycle {
    void move();
}

Practical case

public sealed interface PaymentMethod permits CreditCard, DebitCard, BankTransfer, DigitalWallet {
    boolean processPayment(double amount);
}
public final class CreditCard implements PaymentMethod {
    @Override
    public boolean processPayment(double amount) {
        // credit‑card logic
        return true;
    }
}

The compiler can verify that a switch covers all permitted cases.

4. Pattern matching

Type pattern matching

// old way
if (obj instanceof String) {
    String s = (String) obj;
    if (s.length() > 5) {
        // use s
    }
}

JDK 17 allows binding a variable directly in the instanceof test:

// new way
if (obj instanceof String s && s.length() > 5) {
    // use s
}

Enhanced switch expression

Object obj = getSomeObject();
String result = switch (obj) {
    case Integer i -> "Integer: " + i;
    case String s  -> "String: " + s;
    case Person p  -> "Person: " + p.name();
    default        -> "Unknown type";
};

Pattern‑matched switches improve readability and can be optimized by the compiler.

5. Text blocks

Problems with traditional concatenation

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

Multi‑line strings are hard to maintain.

Text block syntax

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

Three double quotes delimit the block, preserving line breaks and quotes without escaping.

Formatting tricks

String query = """
        SELECT id, name, email \
        FROM users \
        WHERE status = 'ACTIVE' \
        ORDER BY name""";

JSON, SQL, and HTML examples

// JSON example
String jsonConfig = """
        {
            "appName": "神仙应用",
            "version": "1.0.0",
            "features": ["记录类","密封类","模式匹配"]
        }""";

// SQL example
String sql = """
        SELECT p.name, p.age, a.city
        FROM persons p
        JOIN addresses a ON p.id = a.person_id
        WHERE a.country = '中国' AND p.age > 18""";

6. var and enhanced switch

Type inference

// without var
Map<String, List<Person>> groupedPeople = new HashMap<>();

// with var
var groupedPeople = new HashMap<String, List<Person>>();

Switch as expression with yield

int dayOfWeek = 3;
String day = switch (dayOfWeek) {
    case 1 -> "Monday";
    case 2 -> "Tuesday";
    case 3 -> "Wednesday";
    case 4 -> "Thursday";
    case 5 -> "Friday";
    case 6, 7 -> "Weekend";
    default -> "Invalid";
};
String result = switch (status) {
    case "PENDING" -> {
        log.info("Processing pending");
        yield "In progress";
    }
    case "APPROVED" -> {
        log.info("Processing approved");
        yield "Completed";
    }
    default -> "Unknown";
};

Arrow syntax and multi‑branch handling

Season season = switch (month) {
    case 3, 4, 5 -> Season.SPRING;
    case 6, 7, 8 -> Season.SUMMER;
    case 9,10,11 -> Season.AUTUMN;
    case 12,1,2 -> Season.WINTER;
    default -> throw new IllegalArgumentException("Invalid month");
};

7. Other useful features

Private interface methods

public interface Logger {
    default void logInfo(String msg) { log(msg, "INFO"); }
    default void logError(String msg) { log(msg, "ERROR"); }

    private void log(String msg, String level) {
        System.out.println("[" + level + "] " + msg);
    }
}

Improved Stream API

// toList() without collectors
List<String> names = people.stream()
    .map(Person::name)
    .filter(name -> name.startsWith("张"))
    .toList();

// mapMulti example
List<String> words = sentences.stream()
    .mapMulti((sentence, consumer) -> {
        for (String w : sentence.split(" ")) {
            consumer.accept(w);
        }
    })
    .toList();

Enhanced NullPointerException

// JDK 17 output
Exception in thread "main" java.lang.NullPointerException:
    Cannot invoke "Person.getName()" because "person" is null

New garbage collectors

// Enable ZGC
-XX:+UseZGC

Foreign memory access API

try (MemorySegment segment = MemorySegment.allocateNative(100)) {
    MemoryAccess.setInt(segment, 0, 42);
    int value = MemoryAccess.getInt(segment, 0);
    System.out.println(value); // 42
}

These “magical” JDK 17 features dramatically simplify code, improve developer productivity, and boost runtime performance.

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JavaBackend Developmentpattern-matchingSealed ClassesrecordsJDK 17Text Blocks
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