How to Mock the System.in Stream for a Unit Test

Question

im really new to programming and want to ask how to mock the System.in Stream for a Unit-Test. My task is to programm the game and unit test the existing Methods. I dont know how to unit test a Method requiring an Input Stream. More Specific: I dont know how to mock it. In the following Code example is my method. The method reads in 4 accepted color-Strings with a Scanner and returns the Strings in an String-Array. Is there any Method to mock the System.in Stream

public String[] guess(){ //Read in the 4 colors a player want to guess
        int k = 1;
        Scanner valueIn = new Scanner(System.in);
        String[] guess = new String[4];
        for(int i = 0; i < 4;i++){
            k = 1;
            while(k==1) {
                System.out.println("now the " + (i + 1) + "color ");
                guess[i] = valueIn.nextLine();
                if(guess[i].equals("red")||guess[i].equals("blue")||guess[i].equals("yellow")||guess[i].equals("green")||guess[i].equals("purple")||guess[i].equals("brown")){
                    k = 0;
                }
            }
        }
        return guess;
    }

Answer 1

Use System.setIn() method.

@Test
public void test(){
    String[] expectedOutputArray = {"red", "green", "yellow", "brown"};
    String input = "red\ngreen\nyellow\nbrown";
    InputStream in = new ByteArrayInputStream(input.getBytes());
    System.setIn(in);
    String[] outputArray = guess(); //call your guess function
    Assert.assertArrayEquals(expectedOutputArray, outputArray);
}

• System.in is basically an InputStream which reads from the console (hence your input in the console).
• So prepare an InputStream and set to in property of System class, using System.setIn(in)

---

I am putting another approach for how the code can be reformatted for testing guess method, removing dependency on System.in

Please put each class, in different java file

public class GuessRefactor {
    private SystemClass systemObj;

    public String[] guess(SystemClass systemObj){
        int k = 1;
        String[] guess = new String[4];
        for(int i = 0; i < 4;i++){
            k = 1;
            while(k==1) {
                System.out.println("now the " + (i + 1) + "color ");
                guess[i] = systemObj.getInput();
                if(guess[i].equals("red")||guess[i].equals("blue")||guess[i].equals("yellow")||guess[i].equals("green")||guess[i].equals("purple")||guess[i].equals("brown")){
                    k = 0;
                }
            }
        }
        return guess;
    }

    public static void main(String[] args) {
        System.out.println(Arrays.toString(new GuessRefactor().guess(new SystemClass())));
    }
}


public class SystemClass {
    Scanner valueIn = new Scanner(System.in);

    public String getInput(){
        return valueIn.nextLine();
    }
}


@RunWith(MockitoJUnitRunner.class)
public class TestClass {
    @Mock
    SystemClass systemClass;

    @Test
    public void test(){
        GuessRefactor guessRefactor = new GuessRefactor();
        Mockito.when(systemClass.getInput()).thenReturn("red");
        String[] expectedOutput = {"red", "red", "red", "red",};
        String[] output = guessRefactor.guess(systemClass);
        Assert.assertArrayEquals(expectedOutput, output);
    }
}