Algorithms and OOD (CSC 207 2013F) : Readings

Unit Testing and JUnit

Summary: We consider “unit testing”, an approach to software development and to the verification of computer programs. We also introduce JUnit, a unit testing framework for Java.


Most computer programmers strive to write clear, efficient, and correct code. It is (usually) easy to determine whether code is clear. With some practice and knowledge of the correct approaches, one can determine how efficient code is (or should be). However, it is often difficult to determine whether or not code is correct.

The gold standard of correctness is a formal proof that the procedure or program is correct. However, in order to prove a program or procedure correct, one must develop a rich mathematical toolkit and devote significant effort to writing the proof. Such effort is worth it for life-critical applications, but for many programs, it is often more than can be reasonably expected.

There is also a disadvantage of formal proof: Code often changes and the proof must therefore also change. Why does code change? At times, the requirements of the code change (e.g., a procedure that was to do three related things is now expected to do four related things). At other times, with experience, programmers realize that they can improve the code by making a few changes. If we require that all code be proven correct, and if changing code means rewriting the proof, then we discourage programmers from changing their code. (Automated proof systems can help with all of this, but they are beyond the scope of this course.)

Hence, we need other ways to have some confidence that our code is correct. A typical mechanism is a test suite, a collection of tests that are unlikely to all succeed if the code being tested is erroneous. One nice aspect of a test suite is that when you make changes, you can simply re-run the test suite and see if all the tests succeed. To many, test suites encourage programmers to experiment with improving their code, since good suites will tell them immediately whether or not the changes they have made are successful.

What should a test suite look like? First, it should be as comprehensive as possible. That is, it should poke into the nooks and crannies of the code, looking at not only special cases, but also a variety of general cases. Second, it should be as automatic as possible. Evidence suggests that programmers are less likely to take the time to test “by hand” or to compare actual output to expected output. (Computers are also really good at comparing things, so why would you ask a human to do so?) An ideal test suite is easy to run (usually just one button), and gives either a quick affirmation that all tests passed or a summary of what tests failed. (Some only tell you the first test that failed, which is useful, too. You shouldn't use code that doesn't pass all of its tests.)

Testing is also a core component of many agile software development methodologies. Agile programmers have found that writing tests early helps programmers think about what they want their code to achieve and having convenient and comprehensive tests gives programmers confidence to try new approaches.

Most agile testing starts with a concept called “Unit Testing”. In unit testing, you focus on testing the small units of a program - individual procedures, objects, and perhaps small sets of cooperating objects. While you will eventually need to test the combination of smaller units, making sure that your basic units work correctly is an important place to start.

Most tests resemble the things you would do by hand - When I give this procedure this input, do I get this output? You'd probably run experiments like that while developing your code, so why not document them as code? It doesn't take long (in most cases, less time than it would to run the program and compare answers by hand). You can also take advantage of the computer to run more tests and to automate tests. For example, if you were writing a square root procedure, instead of just checking that the square root of 4 is 2 and the square root of 100 is 10, you can confirm that the square root of the square of i is i for every i from, say, 1 to 100.

Unit Testing and JUnit

One of the first unit testing frameworks was SUnit, designed for the smalltalk language. It was soon ported to Java, as JUnit. It remains one of the most popular unit testing frameworks for Java, and so it will be the framework we use in this course.

JUnit provides most of what you want in a testing framework:

  • It's easy to run a series of tests once you've specified them.
  • It's easy to group test suites into bigger test suites.
  • You can test a variety of behaviors, not just “are these two things equal”, but also “does this code throw an exception” and such.
  • You can easily scaffold your tests, setting up some objects and such before each test.

Using JUnit in Eclipse

We also use JUnit because it is integrated into Eclipse.

To create a new JUnit test for a class in Eclipse, right click the class that we want to test then choose New > JUnit Test Case. A “New JUnit Test Case” window should appear. Once there, click the Next button. You can now select the methods that you want to test. After that, click the Finish button. You now have a test class, typically in the same package as your original class.

Within the test class that you just made, there will be methods that are prefixed with the word test and the annotation @Test. These are what you will use to test each method you have chosen. At the moment they all should have a simple body, something like,

        fail("Not yet implemented");

We will replace this line with our test or tests. To start with, we'll use the assertEquals method, which takes three parameters: a string that describes the test, the expected value, and the expression to test. For example, if I've written a method called average in the Math class, I might do a simple test with

        assertEquals("averaging 1 and 3", 2, Calculator.average(1,3));

While assertEquals will be our primary tools, JUnit provides a variety of other kinds of assertion tests. You can read documentation at

You can put as many assertions in a test as you want, but JUnit will only tell you if they all succeeded - the first one that fails will trigger an error.

You may also find it useful to split your test into multiple methods. As long as you annotate each with @Test (and, in some versions of JUnit, make sure the procedure name starts with test), JUnit will run them. But even if you have multiple methods, JUnit will generally still stop with the first one.

For example, suppose we want to test this class.

package taojava.examples.junit;

 * A simple set of mathematical methods for testing with JUnit.
public class MyMath {

     * Compute the average of two integers, rounded down.
     * @param left
     *   One of the two integers
     * @param right
     *   The other integer
    public static int average(int left, int right) {
      return (left + right) / 2;
    } // average(int,int)

     * Determine if a number, n, is even.
    public static boolean isEven(int n) {
      return ((n % 2) == 0);
    } // isEven
} // class MyMath

A preliminary set of tests might look something like the following.

package taojava.examples.junit;

import static org.junit.Assert.*;

import org.junit.Before;
import org.junit.Test;

public class MyMathTest {
    public void testAverage() {
	assertEquals("zero", 0, MyMath.average(0, 0));
	for (int i = 0; i < 100; i++) {
	    assertEquals(i + " vs " + -i, 0, MyMath.average(i, -i));
	} // for
    } // testAverage

    public void testIsEven() {
	for (int i = -100; i < 100; i += 2) {
	    assertEquals(i + " is even", true, MyMath.isEven(i));
	    assertEquals((i + 1) + " is odd", false, MyMath.isEven(i + 1));
	} // for
    } // testIsEven
} // MyMathTest

As you may have noted, in order to use the static procedures from MyMath, we have to precede them with the class name. Java expects you to be very careful on naming the methods you use.

Note that all of the tests must be preceded by @Test. If you remove that text, JUNit will no longer run it. (Try it and see.)

JUnit may not run the tests in the order you expect. So make sure not to rely on values you set up in one test in another test. For example, if you are working on lists and add an element to the list in testA, you may not find that the element is there in testB. Similarly, at first glance it might seem that these two tests could not both succeed:

    int x = 3;

    public void testOne() {
      assertEquals(3, x);
      x += 1;
      assertEquals(4, x);
    } // testOne
    public void testTwo() {
      assertEquals(3, x);
      x += 1;
      assertEquals(4, x);
    } // testTwo

However, JUnit essentially resets the environment between every tests. And that's a good thing. You should be able to write tests that are not affected by what happens in other tests. (Yes, that's probably another form of encapsulation.)

So, what should we do if we want to do the same setup for all of our tests, setup that requires additional code. One possibility is to create a method and then just call it from each test. But JUnit provides another option. In particular, JUnit provides the @Before annotation which you can add to any procedure you want called before every test. Traditionally, that procedure is called setUp, and you should probably use that name for clarity.

Warning! On occasion, Eclipse seems to forget about the @Before annotation. In those cases, creating a new JUnit Test Case and clicking the box to include setUp usually works.

Wrapping Up

Important Terms

  • Unit Test
  • Assertion
  • setUp
  • @Test
  • @Before

Review Questions

  1. Why might you write test cases before implementing code?
  2. Why should we test with a testing framework, rather than a bunch of print statements?
  3. How do we set up a JUnit test in Eclipse?

Exploratory Questions

  1. What are standard agile approaches to unit testing?
  2. If you have multiple test classes, how do you group them together into a test suite?
  3. What other annotations does JUnit provide? What purpose do those annotations serve?
  4. What unit test would you write for binary search? How about for a sorting algorithm? (Just consider the tests that you would write; you don't actually have to write them.)

Copyright (c) 2013 Samuel A. Rebelsky.

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