Unlock Java 17’s Magic Syntax: Records, Sealed Classes, Pattern Matching & More
This article explores Java 17’s long‑term support release, highlighting new language features such as records, sealed classes, pattern matching, text blocks, var, enhanced switch expressions, and other practical improvements that simplify code, boost readability, and increase performance.
From JDK 8 to JDK 17
JDK 17 is a long‑term support release that incorporates all innovations since JDK 9, offering many new language features that simplify Java code.
Record classes
Traditional JavaBeans require boilerplate code. A record automatically generates constructor, getters, equals, hashCode and toString, reducing code dramatically. public record Person(String name, int age) {} Records are immutable data carriers, ideal for DTOs. To modify a field you create a new instance.
Person alice = new Person("Alice", 25);
Person olderAlice = new Person(alice.name(), alice.age() + 1);Use records when you need a simple, immutable value object; avoid them if you need inheritance or mutable fields.
Sealed classes
Sealed classes let you restrict which classes may extend a superclass, providing a middle ground between final and open inheritance.
public sealed class Shape permits Circle, Rectangle, Triangle {}The permits clause lists allowed subclasses. Subclasses 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 work the same way and are useful for closed domain models.
public sealed interface Vehicle permits Car, Truck, Motorcycle { void move(); }Pattern matching
Pattern matching for instanceof binds a variable directly, removing the need for a separate cast.
if (obj instanceof String s && s.length() > 5) { /* use s */ }Switch expressions also support pattern matching, allowing concise handling of multiple types.
String result = switch (obj) {
case Integer i -> "Integer: " + i;
case String s -> "String: " + s;
case Person p -> "Person: " + p.name();
default -> "Unknown type";
};Text blocks
Text blocks simplify multi‑line strings without concatenation or escaping.
String html = """
<html>
<body>
<h1>Hello, World!</h1>
</body>
</html>
""";var and enhanced switch
Local variable type inference with var reduces verbosity, especially when combined with the new switch expression and arrow syntax.
var groupedPeople = new HashMap<String, List<Person>>();
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 date";
};Other useful features
Java 9 introduced private interface methods, which help share implementation details among default 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); }
}Stream API now has toList() and mapMulti() for more concise pipelines.
List<String> names = people.stream()
.map(Person::name)
.filter(n -> n.startsWith("张"))
.toList();NullPointerException messages include the exact null variable, improving debugging.
JDK 17 adds new garbage collectors such as ZGC ( -XX:+UseZGC) and the foreign‑memory access API for safe off‑heap memory manipulation.
try (MemorySegment segment = MemorySegment.allocateNative(100)) {
MemoryAccess.setInt(segment, 0, 42);
int value = MemoryAccess.getInt(segment, 0);
System.out.println(value);
}These “magic syntax” features make Java code shorter, more readable, and often faster, helping developers modernize legacy applications.
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