A calculator class example Junit 4 test case

CalculatorTest

public class CalculatorTest {

  private Calculator classUnderTest;

  @Before
  public void setUp() throws Exception {
    this.classUnderTest = new Calculator();
  }

  @After
  public void tearDown() throws Exception {
    classUnderTest = null;
  }

  @Test
  public void testAdd_positiveNumbers_shouldReturnResult() {
    assertEquals("add", 7, classUnderTest.add(3, 4));
  }

  @Test(expected = IllegalArgumentException.class)
  public void testAdd_negativeNumbers_shouldThrowException() {
    classUnderTest.add(-3, -4);
  }

  @Test
  public void testSubstract() {
    assertEquals("substract", 2, classUnderTest.substract(5, 3));
  }

  @Test
  public void testMultiply() {
    assertEquals("multiply", 56, classUnderTest.multiply(7, 8));
  }

}