Assumed Knowledge:
Learning Outcomes:
  • Understand the roll of Unit Testing in the programming process
  • Understand when tests are sufficient
  • Be able to write simple tests using JUnit.

Unit testing allows us to check - by running other code - that each method of our code, individually, works correctly. Of course, it doesn’t guarantee that the the methods interact correctly or not.

Unit testing is based on the fundamental assumption that for a given input, there is an expected output. Creating a list of such input-output mappings, we can determine if the method, in fact, works correctly.

Conside a method supposed to return the highest of three numbers. An input-output mapping for such a method is listed below.

Input Output
5, 7, 9 9
1, 10, 100 100
0, 0, 0 0

Is the above set of mappings sufficient?

solution

No, in all cases, the third value is the answer. A function that simply returns the third value will pass this insufficient test

A more comprehensive set of mappings is provided below.

Input Output
5, 7, 9 9
100, 1, 10 100
0, 8, 0 8
-5, -100, -1 -1
-9, -9, 0 0
6, 6, 6 6

Write a set of input-output mappings for a function that when passed an integer, returns true if all the digits in the integer are even (0/2/4/6/8), and false otherwise.

solution
Input Output
284666604 true
284661604 false
148 false
227 false
-2486 true
-9486 false
0 true

Write a set of input-output mappings for a function that when passed an integer array, returns true if the array is in ascending order (for each item of thearray, the item is less than or equal to the next item (if any)), and false otherwise.

solution
Input Output
{5, 8, 12, 100} true
{6, 6, 6} true
{20} true
{} true
null not well-defined
{5, 8, 12, 11} false
{6, 4, 4, 8} false

Write a set of input-output mappings for a function that when passed an integer array, returns true if each item of the array occurs exactly once, and false otherwise. Return false if the array is null.

solution
Input Output
{5, 8, 12, 100} true
{6, 6, 6} false
{20} true
{} true
null false
{2, 7, 1, 9, 3, -5, -5} false
{2, -5, 7, 1, 9, 3, -5} false

JUnit test cases

JUnit is a unit testing framework for Java. It operates using assertions to determine if a particular test passes or not.

A list of important assertions are given below:

  • assertTrue(boolean expression): passes if the boolean expression passed to the assertion is true, fails otherwise.

    1
2

      assertTrue(5 > 3): pass
  assertTrue(6 == 12/3): fail

  • assertFalse(boolean expression): passes if the boolean expression passed to the assertion is false, fails otherwise.

    1
2

      assertFalse(2 >= 3): pass
  assertFalse(6 == 12/2): fail

  • assertEquals(expected integer value, integer expression): passes if the integer expression passed to the assertion equals the expected integer value, fails otherwise.

    1
2

      assertEquals(5, 10/2): pass
  assertEquals(5, 2*3): fail

  • assertEquals(expected floating-point value, floating-point expression, tolerance): passes if the floating-point expression passed to the assertion is within tolerance distance of the expected floating-point value, fails otherwise.

    1
2

      assertEquals(1.2, 1.21, 0.02): pass
  assertEquals(1.2, 1.23, 0.02): fail

  • assertNull(reference): passes if the reference (object/array) passed to the assertion is null, fails otherwise.

    1
2

      assertNull(null): pass
  assertNull(new int[]{1,7,2,9}): fail

  • assertNotNull(reference): passes if the reference (object/array) passed to the assertion is not null, fails otherwise.

    1
2

      assertNotNull(new int[]{1,7,2,9}): pass
  assertNotNull(null): fail

  • assertArrayEquals(expected integer array, actual integer array): passes if the actual integer array passed to the assertion equals the expected integer array, fails otherwise.

    Assuming three arrays created as following:

    1
2
3

      int[] a = {1,7,2,9};
  int[] b = {1,7,2};
  int[] c = {1,7,2,9};

    1
2

      assertArrayEquals(a,c): pass
  assertArrayEquals(a,b): fail

How to write a JUnit test

First you need a method that you need to test.

Let’s take a look at the following code that contains a single method sumEven.

The JUnit test is a separate class created to test this method. The following video describes the steps to create a JUnit test. Generally a test is written before implementing a method.