Unnamed variables and patterns

Definition

An underscore _ (Java 22) marks a variable you must declare but will never use, such as a loop variable, a caught exception or a part of a pattern. It tells the reader, and the compiler, that the value is ignored on purpose.

Example code

Underscore for values you do not need

import java.util.List;

record Point(int x, int y) { }

class Demo {
    public static void main(String[] args) {
        int count = 0;
        for (var _ : List.of("a", "b", "c")) {          // the element itself is not needed
            count++;
        }
        System.out.println(count);

        try {
            Integer.parseInt("abc");
        } catch (NumberFormatException _) {              // the exception object is not needed
            System.out.println("not a number");
        }

        Object obj = new Point(3, 9);
        if (obj instanceof Point(int x, _)) {            // only x matters
            System.out.println("x = " + x);
        }
    }
}

// Output:
// 3
// not a number
// x = 3