A Mock is basically a fake version of something your code depends on, like a database or an API, that you use in unit testing. Instead of calling the real thing, which might be slow or unpredictable, a mock just returns whatever you tell it to. This helps keep tests simple, fast, and consistent because you control exactly how the mock behaves.
Mocks are super useful because they let you test one piece of code without worrying about the rest of the system. They make tests run faster since you're not waiting on real databases or external services. Plus, they help you check if your code handles different situations properly, like errors or missing data. In JUnit, a tool like Mockito makes it easy to create and use mocks, so you can focus on testing just what matters.
Let’s say you’re building a shopping app, and there’s a method that calculates the total price of a customer’s order. This method needs to fetch product prices from a database. But in a unit test, you don’t want to actually connect to the database because it’s slow and could change over time. Instead, you use a mock to fake the database and return fixed prices.
For example, if a customer buys two items, the mock can be set up to return $10 for one and $20 for the other, so you know the total should be $30. This way, you can test if your calculation method works correctly without depending on a real database. Plus, you can easily test different cases, like what happens if an item is missing or if the database throws an error. This makes your tests faster, more reliable, and easier to control.
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