Pattern Matching for switch

Switch on Types (Java 21)

switch can match type patterns, add conditions with when (guards), and handle case null. Cases are checked top to bottom, so put more specific cases first.

Example

public class Main {
    static String classify(Object o) {
        return switch (o) {
            case null -> "null value";
            case Integer i when i < 0 -> "negative int " + i;
            case Integer i -> "int " + i;
            case String s when s.isBlank() -> "blank string";
            case String s -> "string '" + s + "'";
            case int[] arr -> "int array of size " + arr.length;
            default -> "other: " + o.getClass().getSimpleName();
        };
    }

    public static void main(String[] args) {
        Object[] inputs = { -5, 7, "  ", "UPI", new int[]{1, 2, 3}, 2.5, null };
        for (Object in : inputs) System.out.println(classify(in));
    }
}

Output

negative int -5
int 7
blank string
string 'UPI'
int array of size 3
other: Double
null value

Rules

A switch over Object needs default; a switch over a sealed type does not if all subtypes are covered.
A case that can never match because an earlier case covers it (e.g. case Integer i before case Integer i when i > 0) is a compile error ("dominated").
Without case null, a null selector throws NullPointerException, as before.