JUnit

When writing JUnit tests for a class Foo, the common practice is to create a FooTest class, which will contain various test methods.

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;
    }
}

import org.junit.Test;
import org.junit.Assert;

public class IntPairTest {

    @Test
    public void testStringConversion() {
        Assert.assertEquals("4,7", new IntPair(4, 7).toString());
    }

    @Test
    public void intDivision_nonZeroDivisor_success() throws Exception {
        Assert.assertEquals(2, new IntPair(4, 2).intDivision());
        Assert.assertEquals(0, new IntPair(1, 2).intDivision());
        Assert.assertEquals(0, new IntPair(0, 5).intDivision());
    }

    @Test
    public void intDivision_zeroDivisor_exceptionThrown() {
        try {
            Assert.assertEquals(0, new IntPair(1, 0).intDivision());
            Assert.fail(); // the test should not reach this line
        } catch (Exception e) {
            Assert.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 as Assert.assertNull and Assert.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 reach Assert.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...'
• Optionally, you can use static imports to avoid having to specify Assert. everywhere.

  import static org.junit.Assert.assertEquals;
  //...
  @Test
  public void testStringConversion() {
      assertEquals("4,7", new IntPair(4, 7).toString());
  }
  

Some intermediate JUnit techniques that may be useful:

• It is possible for a JUnit test case to verify if the SUT throws the right exception.
• JUnit Rules are a way to add additional behavior to a test. e.g. to make a test case use a temporary folder for storing files needed for (or generated by) the test.
• It is possible to write methods thar are automatically run before/after a test method/class. These are useful to do pre/post cleanups for example.
• Testing private methods is possible, although not always necessray