When to use JUnit instead of Java's assert? [duplicate]

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• assert vs. JUnit Assertions [closed] 7 answers

When testing the over all functionality of a class, when should Java's built in assert be used instead of JUnit and vice-versa? It's been my observation that JUnit adds unnescary complexity if you're not using anything but it's assertEquals but I'm fairly new to testing in general. Also I heard having the assert in the source code adds to readability and understanding.

Is that a problem that I only test the over all functionality and not the individual functions (because if over all it works it implies the individual pieces do too)?