6.3. Exercises: Unit Testing

Work on these exercises in the IntelliJ java-web-dev-exercises project. You will update your CarTest.java file by adding more test cases.

6.3.1. testGasTankAfterDriving()

Add a test for the third TODO, “gasTankLevel is accurate after driving within tank range”.

  1. Your test must use the Car method drive()

  2. With a value of 50 miles passed into drive(), we expect test_car to have a gasTankLevel of 9.

    assertEquals(9, test_car.getGasTankLevel(), .001);

Check your solution

6.3.2. testGasTankAfterExceedingTankRange()

Add a test for the fourth TODO, “gasTankLevel is accurate after attempting to drive past tank range”.

  1. You’re on your own for this one. You’ll need to simulate the Car travelling farther than it’s gasTankLevel allows.

Check your solution

6.3.3. testGasOverfillException()

The test for our last TODO is a little different. We are going to perform an action on our car object, and we are expecting the object to throw an error. In this case, we are going to attempt to add gas to our car that exceeds the gas tank size.

  1. First, we’ll add some text to our @Test annotation to tell JUnit to expect an exception.

    //TODO: can't have more gas than tank size, expect an exception
    @Test(expected = IllegalArgumentException.class)
    public void testGasOverfillException() {

    This lets JUnit know that this test should pass if an IllegalArgumentException is thrown at any point during this test.

  2. Update the Car class to include an addGas() method.

    public void addGas(double gas) {
      this.setGasTankLevel(gas + this.getGasTankLevel());
  3. Back in CarTest, implement the new addGas() method and a fail() scenario.

    fail("Shouldn't get here, car cannot have more gas in tank than the size of the tank");

    The fail() message will be displayed if the test fails. We have to import fail into this class to use it.

  4. Run the test. It should fail! In the output is an unexpected exception. This test was expecting an IllegalArgumentException, but it got an AssertionError exception. This caused the test to fail. Further down in the output log, we can see that our fail() statement printed out the statement about not being able to add more gas than is possible.

  5. We need to refactor Car to throw an exception when too much gas is added to the tank. Find the setGasTankLevel method and modify it:

    public void setGasTankLevel(double gasTankLevel) {
       if (gasTankLevel > this.getGasTankSize()) {
          throw new IllegalArgumentException("Can't exceed tank size");
      this.gasTankLevel = gasTankLevel;
  6. Now, run the test - it should pass!

Check your solution