GeezerVison is showing javaevolved.dev — Guarded patterns with when | java.evolved

Home / Language / Guarded patterns with when

Language Intermediate

Guarded patterns with when

Add conditions to pattern cases using when guards.

Code Comparison

✕ Java 8

if (shape instanceof Circle) {
    Circle c = (Circle) shape;
    if (c.radius() > 10) {
        return "large circle";
    } else {
        return "small circle";
    }
} else {
    return "not a circle";
}

✓ Java 21+

return switch (shape) {
    case Circle c
        when c.radius() > 10
            -> "large circle";
    case Circle c
            -> "small circle";
    default -> "not a circle";
};

Share 𝕏

See a problem with this code? Let us know.

Why the modern way wins

🎯

Precise matching

Combine type + condition in a single case label.

📐

Flat structure

No nested if/else inside switch cases.

📖

Readable intent

The when clause reads like natural language.

Old Approach

Nested if

Modern Approach

when Clause

Since JDK

Difficulty

Intermediate

JDK Support

Guarded patterns with when

Available

Widely available since JDK 21 LTS (Sept 2023)

How it works

Guarded patterns let you refine a type match with an additional boolean condition. This keeps all the branching logic in the switch instead of nesting if statements inside cases.

Record patterns (destructuring)

Java 8

if (obj instanceof Point) {
    Point p = (Point) obj;
    int x = p.getX();
    int y = p.getY();
    System.out.println(x + y);
}

Java 21+

if (obj instanceof Point(int x, int y)) {
    IO.println(x + y);
}

Hover to see modern ➜

JDK 21+ →

Language Beginner

Type inference with var

Java 8

Map<String, List<Integer>> map =
    new HashMap<String, List<Integer>>();
for (Map.Entry<String, List<Integer>> e
    : map.entrySet()) {
    // verbose type noise
}

Java 10+

var map = new HashMap<String, List<Integer>>();
for (var entry : map.entrySet()) {
    // clean and readable
}

Hover to see modern ➜

JDK 10+ →

Language Beginner

Compact source files

Java 8

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println(
            "Hello, World!");
    }
}

Java 25

void main() {
    IO.println("Hello, World!");
}

Hover to see modern ➜

JDK 25+ →