Trying to use Junit to verify console output matches a string

Question

I'm trying to use Junit to verify that something output by a method matches a string using assertequals, but I can't get it to work. Because it says that the test case is undefined for (String, Void) My code for the method is below

public void speciesInformation() {
     switch (species){ 

     case"Penguin":System.out.println("Penguin Trivia fact");
     break;

     default: System.out.println("There is no current information for that species");
     break;
     }       
}

and the junit code I'm attempting to use is(other tests omitted):

public class AnimalsTest extends junit.framework.TestCase{
public void testSpeciesInfo() {   
    assertEquals(" Penguin Trivia fact/n", penguin.speciesInformation());
}
}

All my other tests work because they're based on a return, but this one is supposed to be based on what's printed by the public void method and I can't tell how to do it or find information on how.

score 1 · Answer 1 · answered Feb 23 '19 at 03:38

1

You can use ByteArrayOutputStream for this, set new PrintStream of byteArrayOutputStream in System.setOut, then you can assert what was logged in console easily.

See this for example

answered Feb 23 '19 at 03:38

Purnima Joshi

13
4

score 1 · Answer 2 · answered Feb 23 '19 at 03:42

In general, it's not always easy to assert some things, especially if they are static methods.

In this case, you can set another Stream for the System.out, what is explained in another answer.

However, if we talk about testing in general, you should ask yourself: "What do I really need to test?" Actually, you probably want to test that correct string was printed, right? So:

Was at least something printed?
Was the string that was printed the expected string?

You can check the second one using the tool you've already mentioned - asserting correctness of a method, returning a string. To do this, you can refactor your method a bit:

public void speciesInformation() {
  System.out.println(getSpeciesInforationText(species));
}

public void getSpeciesInforationText(String species) {
     switch (species){ 

     case"Penguin":
         return "Penguin Trivia fact");

     default: 
         return "There is no current information for that species");
     } 
}

This way you can easily test the getSpeciesInforationText correctness.

And the first method becomes very simple - you just call print method there. As a result you can either not test it all (because it's very simple) or you can use a technique called Mocking to test that you called the method. But it will require much more actions from your side. If you are interested in it, check out what Mockito is and how to use it.

Trying to use Junit to verify console output matches a string

2 Answers2