how to write junit test cases for void function which prints output to console

Question

I've two functions addKeywords and search which have void return type and search function prints results to the console. Here is the code

 void addKeywords(String video)throws IOException{


    InputStreamReader ip = new InputStreamReader(System.in);
    BufferedReader br = new BufferedReader(ip);

    Integer previousVal=0;

    if(!videoKeyword.containsKey(video) && videoNames.contains(video)){
        videoKeyword.put(video,new HashSet<String>());
    }
    else if(!videoNames.contains(video)){
        System.out.println("Video is not a part of lookup");
    }

    System.out.println("Enter keywords for video");
    String keyword =br.readLine();

    if(!keywordLength.containsKey(video))
        keywordLength.put(video, 0);

    if((keywordLength.get(video)+keyword.length())<500){
        videoKeyword.get(video).add(keyword);
        previousVal=keywordLength.get(video);
        keywordLength.put(video, previousVal+keyword.length());
    }
    else{
        System.out.println("Maximum length exceeded for video "+ video);
    }
    if(!kewordVideo.containsKey(keyword)){
        kewordVideo.put(keyword,new HashSet<String>());
    }
    kewordVideo.get(keyword).add(video);
 }

 void search(String searchKey){
    for (Entry<String, HashSet<String>> entry : videoKeyword.entrySet()) {
        for (String s : entry.getValue()) {
            if (s.startsWith(searchKey)) {
                System.out.println(searchKey+" is mapped to "+entry.getKey());
                break;
            }
        }
     }
 }

I have written junit tests

 public class MyUnitTest extends CultureMachineAssignment {
      CultureMachineAssignment testObj =new CultureMachineAssignment();
      testObj.insertDataIntoSet();
      testObj.addkeywords("video1");

     @Test
     public void testVideo() {
        assertEquals("video1", testObj.search("abcd"));
    }
}

I am getting following errors

The method assertEquals(Object, Object) in the type Assert is not applicable for the arguments (String, void)

• Syntax error on token ""video1"", delete this token
• Syntax error on token(s), misplaced construct(s)

I am not sure if this is the correct way to write junit test cases for functions with void return type and which prints output to console. Can someone please tell me correct code ?

Answer 1

You can redirect the System.out or System.err messages with System.setOut(printStream) and System.setErr(printStream) functions.
That way you will be able to get the System.out messages into a printstream object and can read it.

Answer 2

You need to pass System.out to the constructor of the class in the form of PrintStream and then write to it later. So you can replace it in a unit test with a custom stream into a String and verify it:

final ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
PrintStream ps = new PrintStream(byteArrayOutputStream);

ps.println("...");

assertEquals("expectedOutput", byteArrayOutputStream.toString())

(You should also pass System.in as a parameter to further decouple)

---

Update

As @kryger pointed out: You should return the result as a String and move printing responsibility to the caller.