Ref:

What is JUnit?

JUnit is a simple, open source framework to write and run repeatable tests. It is an instance of the xUnit architecture for unit testing frameworks. JUnit features include:

  • Assertions for testing expected results
  • Test fixtures for sharing common test data
  • Test runners for running tests

When should tests be written?

Tests should be written before the code. Test-first programming is practiced by only writing new code when an automated test is failing.

Good tests tell you how to best design the system for its intended use. They effectively communicate in an executable format how to use the software. They also prevent tendencies to over-build the system based on speculation. When all the tests pass, you know you're done!

Whenever a customer test fails or a bug is reported, first write the necessary unit test(s) to expose the bug(s), then fix them. This makes it almost impossible for that particular bug to resurface later.

Test-driven development is a lot more fun than writing tests after the code seems to be working. Give it a try!

Basic test template

import org.junit.*;import static org.junit.Assert.*;

public class SampleTest {

    private java.util.List emptyList;

    /**     * Sets up the test fixture.      * (Called before every test case method.)        */    @Before    public void setUp() {        emptyList = new java.util.ArrayList();    }

    /**     * Tears down the test fixture.      * (Called after every test case method.)     */       @After    public void tearDown() {        emptyList = null;    }

    @Test    public void testSomeBehavior() {        assertEquals("Empty list should have 0 elements", 0, emptyList.size());       }

    @Test(expected=IndexOutOfBoundsException.class)    public void testForException() {        Object o = emptyList.get(0);    }}

Handling Exception

* How test that passes when an expected exception is thrown?

Add the optional expected attribute to the @Test annotation. The following is an example test that passes when the expected IndexOutOfBoundsException is raised: 

    @Test(expected=IndexOutOfBoundsException.class)    public void testIndexOutOfBoundsException() {        ArrayList emptyList = new ArrayList();	Object o = emptyList.get(0);    }

* How test that fails when an unexpected exception is thrown?

Declare the exception in the throws clause of the test method and don't catch the exception within the test method. Uncaught exceptions will cause the test to fail with an error.

The following is an example test that fails when the IndexOutOfBoundsException is raised: 

    @Test    public void testIndexOutOfBoundsExceptionNotRaised()         throws IndexOutOfBoundsException {

        ArrayList emptyList = new ArrayList();        Object o = emptyList.get(0);       }

Assertion Statement Reference

This is a list of the different types of assertion statements that are used to test your code. Any Java data type or object can be used in the statement. These assertions are taken from the JUnit API.

  • assertEquals(expected, actual)
  • assertEquals(message, expected, actual)
  • assertEquals(expected, actual, delta) – used on doubles or floats, where delta is the difference in precision
  • assertEquals(message, expected, actual, delta) – used on doubles or floats, where delta is the difference in precision
  • assertFalse(condition)
  • assertFalse(message, condition)
  • assertNotNull(object)
  • assertNotNull(message, object)
  • assertNotSame(expected, actual)
  • assertNotSame(message, expected, actual)
  • assertNull(object)
  • assertNull(message, object)
  • assertSame(expected, actual)
  • assertSame(message, expected, actual)
  • assertTrue(condition)
  • assertTrue(message, condition)
  • fail()
  • fail(message)
  • failNotEquals(message, expected, actual)
  • failNotSame(message, expected, actual)
  • failSame(message)

Ref:

Pre and Post condition

  • A pre-condition is an expression of type Boolean that should always hold before the execution of the body of the method,conditions for each function that must be true for it to behave correctly
  • A post-condition is an expression of type Boolean that should always hold after the execution of the body of the method,conditional expressions that must be true when a method returns
  • The strategy is to construct a JUnit test case for each accessible method in the class under test, where each test case verifies both the preconditions and postconditions of the method. Example 
    public class RangeTest extends TestCase {    private Range mRange;  public RangeTest(String name) {      super(name);    }

  protected void setUp() throws Exception {    super.setUp();       mRange = new Range(1, 5);   }

  public void testConstructor() {     // pre: aLower < aUpper    try {        new Range(6, 4);      fail("[6,4] should be precluded");    }        catch (AssertionError e) {    }    // pre: aLower == aUpper    new Range(-3, -3);    // post: aLower == getLower()    Assert.assertEquals("[1,5].getLower() should be 1", 1,       mRange.getLower());       // post: aUpper == getUpper()    Assert.assertEquals("[1,5].getUpper() should be 5", 5,       mRange.getUpper());  }

  public void testGetLower() {    // pre: object constructed    Assert.assertNotNull("[1,5] could not be constructed", mRange);       // post: getLower() == aLower provided to c'tor     Assert.assertEquals("[1,5].getLower() should be 1", 1,      mRange.getLower());  }

  public void testGetUpper() {     // pre: object constructed       Assert.assertNotNull("[1,5] could not be constructed",      mRange);    // post: getUpper() == aUpper provided to c'tor     Assert.assertEquals("[1,5].getUpper() should be 5", 5,         mRange.getUpper());  }

  public void testIntersects() {    // pre: anOther !=  null     try {      mRange.intersects(null);      fail("intersects(null) should be precluded");       }    catch (AssertionError e) {    }       // post: true if two ranges have an integer in common      Assert.assertTrue(!mRange.intersects(new Range(-3, -1)));      Assert.assertTrue(!mRange.intersects(new Range(6, 7)));         Assert.assertTrue(!mRange.intersects(new Range(-2, 1)));      Assert.assertTrue(!mRange.intersects(new Range(5, 7)));      Assert.assertTrue(mRange.intersects(new Range(0, 2)));      Assert.assertTrue(mRange.intersects(new Range(3, 4)));         Assert.assertTrue(mRange.intersects(new Range(4, 8)));}}
  • If these test cases pass, then we can be confident that all methods in Range are:
  • Checking for violation of their preconditions (and objecting by throwing a Runtime exception of some sort)
  • Adhering to their postconditions.

Ref: http://www.hacknot.info/hacknot/action/showEntry?eid=17