When writing JUnit tests for a class Foo
, the common practice is to create a FooTest
class, which will contain various test methods.
Suppose we want to write tests for the IntPair
class below.
public class IntPair {
int first;
int second;
public IntPair(int first, int second) {
this.first = first;
this.second = second;
}
public int intDivision() throws Exception {
if (second == 0){
throw new Exception("Divisor is zero");
}
return first/second;
}
@Override
public String toString() {
return first + "," + second;
}
}
Here's a IntPairTest
class to match (using JUnit 5).
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.fail;
public class IntPairTest {
@Test
public void testStringConversion() {
assertEquals("4,7", new IntPair(4, 7).toString());
}
@Test
public void intDivision_nonZeroDivisor_success() throws Exception {
assertEquals(2, new IntPair(4, 2).intDivision());
assertEquals(0, new IntPair(1, 2).intDivision());
assertEquals(0, new IntPair(0, 5).intDivision());
}
@Test
public void intDivision_zeroDivisor_exceptionThrown() {
try {
assertEquals(0, new IntPair(1, 0).intDivision());
fail(); // the test should not reach this line
} catch (Exception e) {
assertEquals("Divisor is zero", e.getMessage());
}
}
}
Notes:
- Each test method is marked with a
@Test
annotation. - Tests use
Assert.assertEquals(expected, actual)
methods to compare the expected output with the actual output. If they do not match, the test will fail. JUnit comes with other similar methods such asAssert.assertNull
andAssert.assertTrue
. - Java code normally use camelCase for method names e.g.,
testStringConversion
but when writing test methods, sometimes another convention is used:whatIsBeingTested_descriptionOfTestInputs_expectedOutcome
e.g.,intDivision_zeroDivisor_exceptionThrown
- There are several ways to verify the code throws the correct exception. The third test method in the example above shows one of the simpler methods. If the exception is thrown, it will be caught and further verified inside the
catch
block. But if it is not thrown as expected, the test will reachAssert.fail()
line and will fail as a result. - The easiest way to run JUnit tests is to do it via the IDE. For example, in Intellij you can right-click the folder containing test classes and choose 'Run all tests...'
Adding JUnit 5 to your IntelliJ Project -- by Kevintroko@YouTube
Resources
- JUnit Official User Guide
- JUnit 5 Tutorial – Common Annotations With Examples - a short tutorial
- How to test private methods in Java? [ short answer ] [ long answer ]