Kotlin test assertions - Step-by-Step Execution

Concept Flow - Kotlin test assertions

Write test function

↓

Call function to test

↓

Use assertion to check result

↓

Assertion passes?

No→Test fails, show error

Yes↓

Test passes, continue or finish

This flow shows how a Kotlin test calls a function, checks the result with assertions, and passes or fails the test.

Execution Sample

Kotlin

fun add(a: Int, b: Int) = a + b

fun testAdd() {
    val result = add(2, 3)
    assert(result == 5) { "Expected 5 but got $result" }
}

This code tests the add function by asserting the result equals 5.

Execution Table

Step	Action	Evaluation	Result
1	Call add(2, 3)	2 + 3	5
2	Store result	result = 5	5
3	Assert result == 5	5 == 5	True, assertion passes
4	Test ends	No errors	Test passes

💡 Assertion passes because result equals expected value 5

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
result	undefined	5	5	5

Key Moments - 2 Insights

Why does the test fail if the assertion condition is false?

Because the assertion throws an error stopping the test, as shown in the execution_table step 3 where a false condition would cause failure.

What does the message inside assert { } do?

It shows a clear error message if the assertion fails, helping to understand what went wrong, as seen in the code sample.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'result' after step 2?

A 5

B 3

C undefined

D 0

💡 Hint

Check the 'Store result' row in the execution_table where result is set to 5.

At which step does the assertion check happen?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Assert result == 5' action in the execution_table.

If the add function returned 4 instead of 5, what would happen at step 3?

A Result changes to 5

B Assertion passes

C Assertion fails and test stops

D Test continues without error

💡 Hint

Refer to key_moments about assertion failure stopping the test.

Concept Snapshot

Kotlin test assertions check if a condition is true.
Use assert(condition) { "message" } inside test functions.
If condition is false, test fails with the message.
Assertions help verify code works as expected.
They stop the test immediately on failure.

Full Transcript

This visual trace shows how Kotlin test assertions work. First, a test function calls the function to test. The result is stored in a variable. Then an assertion checks if the result matches the expected value. If true, the test passes and continues. If false, the assertion throws an error and the test fails immediately. The message inside the assertion helps explain the failure. Variables like 'result' change as the test runs. Understanding these steps helps beginners see how tests verify code correctness.