How do you test a vending machine
One time I was asked this question over the phone. In a site interview I was asked how to test a calculator. In another site interview I was also asked how to test an elevator system. All these none software related questions are similar in the sense that they aim to detect your ability to think and solve problems that you never thought about before. They also intend to discover your testing approach because there is no single correct answer. What determines the best answer is your way of thinking. Generally speaking here is how I answer questions of this kind. I suggest the most important test case categories for example functional testing, usability testing, security testing and so on. In each category I provide few test cases. You can fill out the whole page with individual test cases but the way you approach your solution is the most important factor. Actually suggesting test types helps you come out with more test cases. For example in functional testing you can mention the mapping between numbers and items, prices and change, etc. In usability testing you can mention how easy to learn and use the machine, finding keys and screen navigation, etc. In security testing you can mention cases when the user inserts invalid coins although one might argue that this could be functional test but this is besides the point because you should try your best to demonstrate your way of thinking not how many test cases you get unless the interviewer is stressing on that.
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