• Technique for developing software, guided by tests
• Kent Beck
• Martin Fowler
• Forces us to think about interface first
• Makes refactoring and bug fixes convenient (no hidden regression)

• JUnit is a Java library to automate unit testing
• @Before, @BeforeEach use to initialize test object before each test
• @BeforeClass called when the test class is initialized, use for shared data between test cases
• @After, @AfterEach clean up resources after each test case
• @AfterClass after the entire test class
• @Test marks a single test

public class ThreeDigitsTests {
   // reference to object under test
  private ThreeDigits td;
  
  @Before
  public void setUp() { td = new ThreeDigits(); }
  
  @After
  public void tearDown() { td = null; }
  
  @Test
  public void testSetDigits() {
    // implement the test
  }

}

• assertEquals checks that a result is equal to an expected result
• assertArrayEquals checks content equality of arrays
• assertThrows checks that an expected exception is thrown
• assertTrue, assertFalse, assertNull
• assertThat checks a predicate

public class ThreeDigitsTests {
  private ThreeDigits td;
  
  // ... test setup and tear-down
  
  @Test
  public void testSetDigits() {
    td.setDigits(0, 1, 2);
    assertArrayEquals(new int[] {0, 1, 2}, td.getDigits());
  }

  @Test
  public void testSetTooManyDigits() {
    Throwable e = assertThrows(
      IllegalArgumentException.class,
      () -> td.setDigits(0, 1, 2, 3)
    );
    assertEquals("Expected 3 digits, but got 4", e.getMessage());
  }
}