Assertions
- Kotlin course
- Intermediate
- Lesson 108/116
- 5 min
Learn assertions with assertEquals and assertTrue.
Assertions: assertEquals, assertTrue
Assertions are functions that compare the expected value with the actual value in a test. assertEquals(expected, actual) verifies two values are equal, assertTrue(condition) verifies a condition is true. If the verification fails an exception is thrown and the test fails.
assertEquals(5, c.add(2, 3))
assertTrue(c.add(2, 3) > 0)Here the first line verifies that add(2, 3) is 5, the second verifies the result is positive. Assertions produce clearer error messages than check; they show the expected and actual values. JUnit's Assertions object provides these functions; the kotlin.test package also offers compatible versions.
Assertion error messages
When an assertion fails JUnit produces a detailed error message. assertEquals includes the expected and actual values in the message; so you understand what went wrong. assertTrue states that the condition was false. To add extra context a third string argument can be given.
assertEquals(5, result, "add(2,3) should be 5")
assertTrue(list.isEmpty(), "list should be empty")Here each assertion carries a message; it explains what went wrong when the test fails. Good messages let you quickly grasp why a test failed. When many assertions pile up in one test, splitting the test is a good habit.
Practice Verify a test with assertEquals
Define class Calculator; inside it add fun add(a: Int, b: Int): Int = a + b. Then define a helper fun assertEquals(expected: Int, actual: Int) whose body throws AssertionError when the values are not equal. In main create an instance with val c = Calculator(); verify with assertEquals(5, c.add(2, 3)); print println("all passed").
Starter code
class Calculator {
fun add(a: Int, b: Int): Int = a + b
}
fun assertEquals(expected: Int, actual: Int) {
if (expected != actual) throw AssertionError("expected $expected but was $actual")
}
// main: get add(2, 3) result, verify with assertEquals expecting 5
// print "all passed"
fun main() {
// write here
}Sample solution
fun main() {
val c = Calculator()
assertEquals(5, c.add(2, 3))
println("all passed")
}Expected output
all passed
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does assertEquals(4, calc.add(2, 2)) verify?
- That add returns 2
- That add returns 0
- That add returns 4 Correct answer
- That add returns 22
Explanation: assertEquals(4, ...) expects the actual value to be 4.
2 Verify equality
____(5, c.add(2, 3))assertEquals(5, c.add(2, 3))Explanation: assertEquals(5, ...) expects the value to be 5.
3 Match the assertion types
| Term | Meaning |
|---|---|
assertEquals(a, b) |
Equality |
assertTrue(cond) |
Truth |
assertNull(x) |
Nullness |
assertFalse(cond) |
Falsity |