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Student: ex10


Part of a series of tutorial articles about a Student class.

Code examples referred to on this page can be found here:


In this example we show how to use exceptions to add error checking to inputs.

The only two data values we have so far are name and perm. Since names can take a variety of forms, we’ll focus our attention on perm.

A perm number should be a positive integer between 1 and 9999999 (i.e. any seven digit number other than 0). Furthermore, a perm can take either a six digit form, or a seven digit form; if it has a seven digit form, the last digit is a check digit (we think, produced by the Luhn Algorithm).

So we may want to prevent a Student object from being produced with an invalid perm.

We’ll start by defining the desired behavior with some test cases. Each test case name is something that describes the behavior we expect. In the cases where the perm number passed in is not acceptable, we will expect the code to throw an IllegalArgumentException.

Note: If you are not familiar with exceptions, you may find it helpful to do some reading in the textbook(s):

  • HFJ2 Chapter 11 covers Exceptions
  • pp. 75-77 in Chapter 2 of JN7 also cover Exceptions

Here’s how we write a test for an expected exception. We’ll need to unpack this a bit, since it introduces a few elements of Java syntax that aren’t in our Head First Java textbook (though they are in the JN7 textbook) and that you may not have seen before, namely:

  • the IllegalArgumentException.class expression
  • the lambda expression: () -> { }

First, here’s the whole test. Then we’ll unpack it a little at a time:

    public void test_constructor_zeroPerm() {
        assertThrows( IllegalArgumentException.class, () -> {
                Student s = new Student("Test",0);

So let’s start by looking just at just the assertThrows method.

It takes two parameters.

  • The first is IllegalArgumentException.class. This is an instance of the java.lang.Class<T> class documented here. That is, the expression IllegalArgumentException.class refers to a Java Object that respresents the class IllegalArgumentException, and has methods such as getMethods(), getFields(), etc.
  • The second is the entire expression:

    () -> {
       Student s = new Student("Test",0);

    This is a so-called Lambda Expression, and is a way of representing a Java function(method) that can be called with no parameters (hence the empty ()), and the body of the method is the part in {}.

    A lambda expression allows us, among other things, to pass a block of code as a parameter to a function. In this case the type of the parameter is org.junit.jupiter.api.Executable documented here that that isn’t really important to understand at this point. Later in the course, we’ll come back and talk much more about lambda expressions, and their relationship to Java Interfaces, and specifically a kind of interface called a “functional interface”.

For now, it’s enough to know this:

  • When you want to test whether some code throws an exception in a JUnit test, use assertThrows
  • Pass the name of the expected exception followed by .class as the first parameter
  • For the second parameter, write () -> { codeGoesHere(); } where codeGoesHere(); can be as much Java code as you need to generate the exception.

Here are the rest of the tests that we introduce in this version:

    public void test_constructor_negPerm() {
        assertThrows( IllegalArgumentException.class, () -> {
            Student s = new Student("Test",-1);

    public void test_constructor_tooBigPerm() {
        assertThrows( IllegalArgumentException.class, () -> {
            Student s = new Student("Test",10000000);

    public void test_constructor_perm_1_ok() {
        Student s = new Student("Test",1);

    public void test_constructor_perm_9999999_ok() {
        Student s = new Student("Test",9999999);

We add these tests, and we get some test failures, as we expected:

Failed tests:   
  edu.ucsb.cs156.student.StudentTest.test_constructor_zeroPerm(): Expected java.lang.IllegalArgumentException to be thrown, but nothing was thrown.
  edu.ucsb.cs156.student.StudentTest.test_constructor_negPerm(): Expected java.lang.IllegalArgumentException to be thrown, but nothing was thrown.
  edu.ucsb.cs156.student.StudentTest.test_constructor_tooBigPerm(): Expected java.lang.IllegalArgumentException to be thrown, but nothing was thrown.

Tests run: 9, Failures: 3, Errors: 0, Skipped: 0

So now we set about making the tests pass.

Here is the new version of our constructor:

   public Student(String name, int perm) {

        if (perm < 1 || perm > 9999999) {
            throw new IllegalArgumentException("Unacceptable value for perm: " + perm);
        } = name;
        this.perm = perm;

We see with this than mvn test produces no errors:

Results :

Tests run: 9, Failures: 0, Errors: 0, Skipped: 0

Further, we check the mutation test coverage:

ucsb-cs156/student-tutorial % mvn clean test org.pitest:pitest-maven:mutationCoverage
>> Line Coverage: 13/13 (100%)
>> Generated 7 mutations Killed 7 (100%)
>> Mutations with no coverage 0. Test strength 100%
>> Ran 11 tests (1.57 tests per mutation)

So, that’s a win. But how about that check digit?

We’ll tackle that in ex11.