What Is an Edge Case?

Written by Coursera Staff • Updated on

Learn what an edge case is, the significance of edge cases in software development and testing, and how to deal with them when they arise.

[Featured image] A software engineer tests her code for edge cases.

In software development and testing, an edge case is a problem or situation that falls outside normal procedures and on the boundary of your operating framework.  Essentially, when you test for an edge case you’re checking how your software behaves under unexpected or rare conditions. Just as software testers should test the regular, expected scenarios (commonly known as “happy paths”), it’s also important to test edge cases to ensure the software handles all possible situations gracefully. This helps to ensure your software handles unexpected events reliably and your confidence in your software’s functionality.

Why are edge cases important in software development and testing?

Edge cases are necessary because they involve testing inputs or scenarios that fall beyond the usual operational conditions or just at the boundaries. For instance, if you're developing a login system, an edge case could be an extremely long email address or a password consisting of only special characters. While these scenarios may not be typical, they are possible, and your software should be equipped to handle them.

These cases play a critical role in software testing, which is a process where you check if a software system behaves as expected in different scenarios. By considering a variety of test cases, you make your software more robust and resilient. Dealing with edge cases also enhances user experience, as users are less likely to encounter unexpected errors or crashes, even if they use your software in ways you didn't initially predict.

How to identify edge cases

Uncovering edge cases often requires thinking creatively. It's not just about testing whether the software works in expected situations, but whether it can handle the unexpected.

For example, if you're designing a website to sell books, you might test the purchase process with one or two books in the cart. But what if someone tries to order 10,000 books at once? Or what if they try to check out with an empty cart? These are edge cases, and while they may not happen frequently, your website should be able to handle them without crashing or encountering issues.

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