j21-pattern-switch - Blog of Vincent VAUBAN

🗼 Pattern Matching for switch Expressions– With Parisian Flair

A switch statement in Java directs control flow based on the value of a selector expression. In earlier versions of Java, the selector expression had to be of certain types — such as int, short, byte, char, String, or an enum constant. Notably, types like long, float, double, and boolean were not and are still not permitted in a switch.

🚀 What's New?With the introduction of pattern matching for switch (as detailed in JEP 441), Java now allows:🟣 Selector expressions of any reference type or int🟣 case labels that use patterns, including type patternsThis enhances flexibility and makes code more concise, especially when working with type hierarchies.⚠️ Still not allowed as selector expressions: long, float, double, boolean

🏛️ Example with Paris MonumentsLet’s model some famous Parisian landmarks:

interface Monument { }

record EiffelTower(int heightInMeters) implements Monument { }
record LouvreMuseum(int roomCount) implements Monument { }
record NotreDame(boolean isOpenToVisitors) implements Monument { }

🧓 Before Java 21: Verbose instanceofTraditionally, if you wanted to perform actions based on the type of Monument, you'd use instanceof checks:

public static String describe(Monument monument) {
    if (monument instanceof EiffelTower e) {
        return "Eiffel Tower stands at " + e.heightInMeters() + " meters tall.";
    } else if (monument instanceof LouvreMuseum l) {
        return "Louvre has " + l.roomCount() + " rooms.";
    } else if (monument instanceof NotreDame n) {
        return "Notre-Dame is currently " + (n.isOpenToVisitors() ? "open" : "closed") + " to visitors.";
    } else {
        throw new IllegalArgumentException("Unknown monument");
    }
}

✨ Java 21+ Pattern Matching with switch ExpressionLet’s refactor the same logic using a pattern matching switch expression:

public static String describe(Monument monument) {
    return switch (monument) {
        case EiffelTower e -> "Eiffel Tower stands at " + e.heightInMeters() + " meters tall.";
        case LouvreMuseum l -> "Louvre has " + l.roomCount() + " rooms.";
        case NotreDame n -> "Notre-Dame is currently " + (n.isOpenToVisitors() ? "open" : "closed") + " to visitors.";
        default -> throw new IllegalArgumentException("Unknown monument");
    };
}

🧭 Same with a switch StatementYou can also use pattern matching in a classic switch statement format:

public static String describe(Monument monument) {
    switch (monument) {
        case EiffelTower e:
            return "Eiffel Tower stands at " + e.heightInMeters() + " meters tall.";
        case LouvreMuseum l:
            return "Louvre has " + l.roomCount() + " rooms.";
        case NotreDame n:
            return "Notre-Dame is currently " + (n.isOpenToVisitors() ? "open" : "closed") + " to visitors.";
        default:
            throw new IllegalArgumentException("Unknown monument");
    }
}

📌 Summary

Java is evolving toward a more expressive and safer language, and this pattern matching feature is a major step—ideal even for expressing the grandeur of monuments like the Eiffel Tower or the Louvre.