JUnit Testing - Comparing the stdout output of a void type method with a String

Question

I want to test a void type method that writes a String to the stdout. For this matter my team and I are trying to compare its output with the String / text we expect to see when we call the method.

Note: I'm a newbie both to Java and StackOverflow, so if there's a better way of doing this I'd be thankful to be notified about that.

 public static boolean test_printAdequateOption(String txt, double[]
 sol, Formatter out, String expectedOutput) {
         //Obj.
         ByteArrayOutputStream baos = new ByteArrayOutputStream();
         PrintStream ps = new PrintStream(baos);
         PrintStream sout = System.out;

         System.setOut(ps);

         printAdequateOption(txt, sol, out);

         System.out.flush();
         System.setOut(sout);



     return  baos.toString().equals(expectedOutput);
 }

The thought behind this test is to print the output of the method printAdequateOption to a PrintStream in such way that it allows me to save it as a String and thus compare it to an expected ouput parameter.

Answer 1

I would suggest refactoring your code to make it more testable. Obviously, your method printAdequateOption(...) both formats/creates the string and then calls System.out.println() (or something like that I assume).

You do not really care to test that the JDK call to print to standard output works for correct input, but you are interested in checking that the string that will be printed has the expected format. So essentially you need to extract the string creation/formatting into a separate method that will return the formatted string. The testing of such a method should be straightforward.

Answer 2

First:

1. Naming: you only use the _ char within SOME_CONSTANT; so your method would better be called testPrintAdequateOption()
2. Making the test itself a JUnit @Test; and then; instead of returning a boolean; you make a real assert within the test code; so that the test really fails on unexpected results (see below for how to do that)
3. In case you really want to keep it this way, the name is slightly misleading. You would rather call your method doesPrintAdequateOptionGiveCorrectResultsOn() or something like that: methods returning a boolean should indicate in their name that they are about, well a boolean choice!

From a pure technical point the above code is OK. Besides the one design problem ... if testing content written to stdout is the "only" way to test your code; then you should look into make real testing possible.

In other words: your test is the best you can do; but the thing that you intend to test; respectively the fact that you have to do it this way ... is the real problem here!

Edit: I somehow overlooked that you were really asking for JUnit unit tests. Thus lets give a better answer and simply show how a real JUnit test should look like here:

@Test
public boolean testPrintAdequateOption() {   
     // overwrite stdout to allow for capturing print statement
     ByteArrayOutputStream baos = new ByteArrayOutputStream();
     PrintStream ps = new PrintStream(baos);
     PrintStream sout = System.out;
     System.setOut(ps);

     // call method with specific input
     String text = ...
     double[] sol = ...
     Formatter formatter = ...
     printAdequateOption(text, sol, formatter);

     System.out.flush();
     System.setOut(sout);

     assertThat(baos.toString(), is("expected output));
  }

That is how a reasonable test would look like; to be executed using JUnit; for example within your eclipse IDE.

Please note: I am using the assertThat assert, together with the is hamcrest matcher. See here about those two.

And the funny thing is: when you write such a test, it becomes really obvious that you should simply change your logic; very much as tomkab suggests in his answer - you don't want that printAdequateOption() produces a string and just prints that to System.out. Instead, you want a method that produces that string ... that you can directly. So no such detour as you are doing in your code!