How to mock a void method with no arguments? - junit

Question

For Example:
Class A{
string s = null;
public void method(){
s="Sample String";
}
}
I have a void method with similar scenario. How can I test such void method?

Answer 1

With void methods you should test the interaction with its dependent objects within the void method. I think a void method with no argument is rarely useful to test (but if you have a valid use case, please add it to your question). I provided you a simple example for a method with an argument but void as a return type:
public class A {
private DatabaseService db;
private PaymentService payment;
// constructor
public void doFoo() {
if(n < 2) {
db.updateDatabase();
} else {
payment.payBill();
}
}
}
And the unit test for this can look like the following
#RunWith(MockitoJUnitRunner.class)
public class ATest {
#Mock
DatabaseService db;
#Mock
PaymentService payment;
#Test
public void testDoFooWithNGreaterTwo() {
A cut = new A(db, payment); // cut -> class under test
cut.doFoo(3);
verify(payment).payBill(); // verify that payment was called
}
#Test
public void testDoFooWithNLessThanTwo() {
A cut = new A(db, payment); // cut -> class under test
cut.doFoo(1);
verify(db).updateDatabase(); // verify that db was called
}
}