Saturday, September 18, 2021

Java 17: Pattern Matching for Switch

In Java 17 (released only a few days ago), Pattern Matching for switch has been introduced as a preview language feature, which allows case labels with patterns rather than just constants. Here is an example showing how you can match on type patterns:

public static String typedPatternMatching(Object o) {
  return switch(o) {
    case null      -> "I am null";
    case String s  -> "I am a String. My value is " + s;
    case Integer i -> "I am an int. My value is " + i;
    default        -> "I am of an unknown type. My value is " + o.toString();

// Output:
> typedPatternMatching("HELLO")
"I am a String. My value is HELLO"

> typedPatternMatching(123)
"I am an int. My value is 123"

> typedPatternMatching(null)
"I am null"

> typedPatternMatching(0.5)
"I am of an unknown type. My value is 0.5"

You can also use a guarded pattern in order to refine a pattern so that it is only matched on certain conditions, for example:

public static String guardedPattern(Collection<String> coll) {
  return switch(coll) {
    case List list && (list.size() > 10) -> 
        "I am a big List. My size is " + list.size();
    case List list -> 
        "I am a small List. My size is " + list.size();
    default -> 
        "Unsupported collection: " + coll.getClass();

If you have a Sealed Class (made a permanent language feature in Java 17), the compiler can verify if the switch statement is complete so no default label is needed. For example:

sealed interface Vehicle permits Car, Truck, Motorcycle {}
final class Car implements Vehicle {}
final class Truck implements Vehicle {}
final class Motorcycle implements Vehicle {}

public static String sealedClass(Vehicle v) {
  return switch(v) {
    case Car c -> "I am a car";
    case Truck t -> "I am a truck";
    case Motorcycle m -> "I am a motorcycle";