Java Add Two Counts Safely

Understand the idea

Use an integer type for whole counts, validate the domain and select checked arithmetic if silently wrapping would be wrong. The inputs below represent two independently validated groups. A reusable method should expose its accepted range and its overflow behaviour instead of returning an apparently plausible negative total.

Run it locally

Use a local JDK 21 or newer. Save the complete example as Main.java in a separate lab folder, then compile and run it with the commands below. Java examples do not run in Bhylu’s browser playground. The -ea option enables assertion checks used by some lessons.

JDK and terminal setup

javac --release 21 -encoding UTF-8 -d out Main.java
java -ea -cp out Main

A worked example

public class Main {
    static int addCounts(int first, int second) {
        if (first < 0 || second < 0) throw new IllegalArgumentException("negative count");
        return Math.addExact(first, second);
    }

    public static void main(String[] args) {
        System.out.println(addCounts(7, 6));
    }
}

Expected output

13

Both counts pass the non-negative check, and addExact returns thirteen. Integer.MAX_VALUE plus one would throw rather than wrap. Test the zero identity, ordinary values and a boundary that cannot fit, since those cases exercise different parts of the contract.