How to test void method in JUnit [duplicate]

Question

How to test void method using JUnit.

void add(int a, int b) {
 int c= a + b;
}

How to test above method by using Junit in Java.

Answer 1

If a method has side effects, you can check them in your JUnit test.

In your example, the method has no side effects, so there is nothing to test.

If you still want to test it, you can use reflection to do so, but I don't think it is good practice.

You can check JUnit FAQ related section:

How do I test a method that doesn't return anything?

Often if a method doesn't return a value, it will have some side effect. Actually, if it doesn't return a value AND doesn't have a side effect, it isn't doing anything.

There may be a way to verify that the side effect actually occurred as expected. For example, consider the add() method in the Collection classes. There are ways of verifying that the side effect happened (i.e. the object was added). You can check the size and assert that it is what is expected:

@Test
public void testCollectionAdd() {
    Collection collection = new ArrayList();
    assertEquals(0, collection.size());
    collection.add("itemA");
    assertEquals(1, collection.size());
    collection.add("itemB");
    assertEquals(2, collection.size());
}

Another approach is to make use of MockObjects.

A related issue is to design for testing. For example, if you have a method that is meant to output to a file, don't pass in a filename, or even a FileWriter. Instead, pass in a Writer. That way you can pass in a StringWriter to capture the output for testing purposes. Then you can add a method (e.g. writeToFileNamed(String filename)) to encapsulate the FileWriter creation.