Module 5 · A real program, end to end Lesson 14 of 15
Unit tests with JUnit
A unit test calls one method and checks the result. For a fresher it is proof the code works, and for a team it is the safety net that lets anyone change code without fear.
The concept
Test the edges, not the middle - an empty list, a single element, a value exactly on a threshold - because that is where bugs live.
A test that never fails is not a test; change the code on purpose, watch it go red, then put it back. That red proves the test does something.
class StudentServiceTest {
@Test
void averageOfMarks() {
var service = new StudentService(new FakeRepo(List.of(70, 80)));
assertEquals(75.0, service.average(), 0.001);
}
@Test
void averageOfNothingIsZero() {
var service = new StudentService(new FakeRepo(List.of()));
assertEquals(0.0, service.average(), 0.001);
}
}Worked example
assertEquals(75.0, service.average(List.of(70, 80)), 0.001) runs with one command and fails loudly the moment the rule breaks.
Your practice task
Write three JUnit tests for your service: a normal case, an empty input and a boundary value.
Do this before moving on. Reading a solution teaches the shape of it; typing it teaches you.
Key takeaway
Test the edges, not the middle - an empty list, a single element, a value exactly on a threshold - because that is where bugs live.
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