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🏷 Pattern Matching 7 patterns

Topic: Pattern Matching

All Java patterns related to Pattern Matching — java.evolved

Pattern matching for instanceof

Old if (obj instanceof String) { String s = (String) obj; int length = s.length(); // do something with 'length' }

Modern if (obj instanceof String s) { int length = s.length(); // do something with 'length' }

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JDK 16+ learn more →

Record patterns (destructuring)

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

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

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JDK 21+ learn more →

Unnamed variables with _

Old try { parse(input); } catch (Exception ignored) { log("parse failed"); } map.forEach((key, value) -> { process(value); // key unused });

Modern try { parse(input); } catch (Exception _) { log("parse failed"); } map.forEach((_, value) -> { process(value); });

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JDK 22+ learn more →

Pattern matching in switch

Old String format(Object obj) { if (obj instanceof Integer i) return "int: " + i; else if (obj instanceof Double d) return "double: " + d; else if (obj instanceof String s) return "str: " + s; return "unknown"; }

Modern String format(Object obj) { return switch (obj) { case Integer i -> "int: " + i; case Double d -> "double: " + d; case String s -> "str: " + s; default -> "unknown"; }; }

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JDK 21+ learn more →

Guarded patterns with when

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

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

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JDK 21+ learn more →

Primitive types in patterns

Old String classify(int code) { if (code >= 200 && code < 300) return "success"; else if (code >= 400 && code < 500) return "client error"; else return "other"; }

Modern String classify(int code) { return switch (code) { case int c when c >= 200 && c < 300 -> "success"; case int c when c >= 400 && c < 500 -> "client error"; default -> "other"; }; }

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JDK 25+ learn more →

Exhaustive switch without default

Old // Must add default even though // all cases are covered double area(Shape s) { if (s instanceof Circle c) return Math.PI * c.r() * c.r(); else if (s instanceof Rect r) return r.w() * r.h(); else throw new IAE(); }

Modern // sealed Shape permits Circle, Rect double area(Shape s) { return switch (s) { case Circle c -> Math.PI * c.r() * c.r(); case Rect r -> r.w() * r.h(); }; // no default needed! }

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JDK 21+ learn more →