JUnit: new instance before invoking each @Test method. What are the benefits?

Question

Currently, I am reading "JUnit in action" book. In this book I found text below:

JUnit creates a new instance of the test class before invoking each @Test method. This helps provide independence between test methods and avoids unintentional side effects in the test code. Because each test method runs on a new test class instance, we can’t reuse instance variable values across test methods.

Now I do not see much point in this approach:

For example:

public class CalculatorTest {
    @Test
    public void testAdd_1() {
        Calculator calculator = new Calculator();
        double result = calculator.add(1, 1);
        assertEquals(2, result, 0);
    }

    @Test
    public void testAdd_2() {
        Calculator calculator = new Calculator();
        double result = calculator.add(2, 2);
        assertEquals(4, result, 0);
    }
}

For test class CalculatorTest there are no any benefits.

Ok, lets go pay attention on another example:

public class OneTest {

    static byte count;

    public OneTest() {
        count++;
    }

    @Test
    public void test1() {
        System.out.println(count);
    }

    @Test
    public void test2() {
        System.out.println(count);
    }
}

For test class OneTest I found a way to use the same variable count for the many test methods...

so, How to see the real benefits of the approach described in the book?

Answer 1

How to see the real benefits of the approach described in the book?

The purpose of separate instance is not for any benefit but to maintain the contract that each test should be independently executed without any effect of the execution of a previous test. There is just no other way to ensure this contract other than using a different instance for each test.

For example, the Spring transaction management makes sure to rollback all changes made to the database by a test, by default, to maintain the same contract.

So, using static variables in a test is generally discouraged as it will defeat the whole purpose of one-instance-per-test to have a clean slate for each test.