SampleBadMethodsTest.cls

@isTest
private with sharing class SampleBadMethodsTest {
    @isTest
    private static void iAddUpPositiveIntegers_positive() { // Happy path
        // GIVEN
        Integer a = 10;
        Integer b = 20;

        // WHEN
        Test.startTest();
        Integer result = SampleBadMethods.iAddUpPositiveIntegers(a, b);
        Test.stopTest();

        // THEN
        System.assertEquals(30, result);
    }

    @isTest
    private static void iAddUpPositiveIntegers_aIsNegative() {
        // GIVEN
        Integer a = -10;
        Integer b = 20;

        // WHEN
        try {
            Test.startTest();
            Integer result = SampleBadMethods.iAddUpPositiveIntegers(a, b);
            Test.stopTest();

            System.assert(false, 'Exception expected');
        } catch(Exception e) {
            // THEN
            System.assert(e.getMessage().contains('I only take positive integers!'));
        }
    }

    @isTest
    private static void iAddUpPositiveIntegers_bIsNegative() {
        // GIVEN
        Integer a = 10;
        Integer b = -20;

        // WHEN
        try {
            Test.startTest();
            Integer result = SampleBadMethods.iAddUpPositiveIntegers(a, b);
            Test.stopTest();

            System.assert(false, 'Exception expected');
        } catch(Exception e) {
            // THEN
            System.assert(e.getMessage().contains('I only take positive integers!'));
        }
    }

    @isTest
    private static void iAddUpPositiveIntegers_aIsNull() {
        // GIVEN
        Integer a = null;
        Integer b = 20;

        // WHEN
        try {
            Test.startTest();
            Integer result = SampleBadMethods.iAddUpPositiveIntegers(a, b);
            Test.stopTest();

            System.assert(false, 'Exception expected');
        } catch(Exception e) {
            // THEN
            System.assert(e.getMessage().contains('I only take positive integers!'));
        }
    }

    @isTest
    private static void iAddUpPositiveIntegers_bIsNull() {
        // GIVEN
        Integer a = 20;
        Integer b = null;

        // WHEN
        try {
            Test.startTest();
            Integer result = SampleBadMethods.iAddUpPositiveIntegers(a, b);
            Test.stopTest();

            System.assert(false, 'Exception expected');
        } catch(Exception e) {
            // THEN
            System.assert(e.getMessage().contains('I only take positive integers!'));
        }
    }
}