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 their code for an edge case.

Key takeaways

  • An edge case is an issue that occurs outside standard procedures and at the boundary of your operating framework when developing and testing software.

  • An edge case in programming is when you’re developing a login system and need to account for an extremely long email address or a password consisting only of special characters.

  • Edge case testing plays a critical role in software testing because it enhances user experience, as users are less likely to encounter unexpected errors or crashes, even when they use your software in unexpected ways.

Examples of edge cases typically fall into two categories: stress loads and user interactions.

Discover more about edge cases, why they are important, and how to identify them below.

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What is an edge case?

In software development and testing, an edge case is a problem or situation that falls outside normal procedures and is 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 builds your confidence in your software’s functionality.

Why is edge case testing important in software development and testing?

Testing edge cases is 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.

Read more: User Experience Research: Methods, Best Practices, and Career Potential

What is the difference between an outlier and an edge case?

An outlier is a data point that significantly deviates from the data set, while an edge case is a scenario or input that occurs at the extreme boundaries of operating conditions. Outliers typically arise from measurement errors or rare events. Edge cases, on the other hand, are valid but rare situations that software testers use to ensure the system design is robust against extreme use cases.

Edge case research: 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.

What are edge cases in software testing?

In software testing, edge cases generally fall into two categories: stress loads and user interactions. Stress loads refer to high network usage caused by unusual weather conditions, disasters, or other situations in which several unusual circumstances lead to high network volume. User interactions, on the other hand, involve unusual user behaviors. This may result from limited understanding of the technology, cultural differences, or language barriers, among other reasons.

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