Java Test a Contract Instead of a Happy Path Only

Understand the idea

Java’s assert statements are normally disabled unless assertions are enabled for the relevant code. Use ordinary checks and exceptions for required validation. For a repeatable example, an explicit check helper can throw AssertionError on a wrong result and run without an external testing framework. Larger projects often use a test framework, but the quality of the chosen cases matters more than the amount of test syntax.

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 remaining(int capacity, int booked) {
        if (capacity < 0 || booked < 0 || booked > capacity) throw new IllegalArgumentException("invalid counts");
        return capacity - booked;
    }
    static void check(boolean condition) { if (!condition) throw new AssertionError("wrong remaining seats"); }

    public static void main(String[] args) {
        check(remaining(8, 0) == 8);
        check(remaining(8, 8) == 0);
        try {
            remaining(8, 9);
            throw new AssertionError("overbooking was accepted");
        } catch (IllegalArgumentException expected) {
            System.out.println("Boundary checks passed");
        }
    }
}

Expected output

Boundary checks passed

The tests cover an empty room, a full room and an invalid overbooking. The final catch treats only the expected rejection as a passing case. A different exception or a silently accepted overbooking still fails the program, which prevents the test from masking an unrelated defect.