eclipse junit system.in java

Question

I am trying to create a junit test on a method that calls for a user input using scanner and System.in.

The method being tested looks like this:

public void setUserAnswer(){
    Scanner input = new Scanner(System.in);
    userAnswer = input.nextInt();
}

Currently, I am using the following set up in jUnit:

StringBuilder sb = new StringBuilder();
        sb.append("");
        sb.append((problem.getOperand1()+problem.getOperand2()));
        String data = sb.toString();
        System.setIn(new ByteArrayInputStream(data.getBytes()));

operand1 and operand2 are random generated numbers which the user adds and enters the answer for.

The problem I have is the System.setIn doesn't pass from the jUnit class to the class being tested.

Help?

Answer 1

To cut the discussion, your method could be tested in following way:

public class MathProblem {

  private int userAnswer;

  public void setUserAnswer(){
    Scanner input = new Scanner(System.in);
    userAnswer = input.nextInt();
  }

  public int getUserAnswer() {
    return userAnswer;
  }
}

and now the Test:

public class MathProblemTest {

  private MathProblem mathProblem;

  @Before
  public void before() throws Exception {
    mathProblem = new MathProblem();
  }

  @Test
  public void testGetUserAnswer() throws Exception {
    StringBuilder sb = new StringBuilder();
    sb.append("41 20");
    String data = sb.toString();
    System.setIn(new ByteArrayInputStream(data.getBytes()));

    mathProblem.setUserAnswer();

    assertThat(mathProblem.getUserAnswer(), equalTo(41));
  }

}

As the others already mention: This code behaves as expected.

So if you do not see a correct behaviour the test might be broken or the problem is in another region of your code, that you have not posted. So ask your self: Where does your example differ from this example?

Answer 2

You should not be messing with System.in during a unit test.

Ask yourself what it is you're testing. Are you testing the Scanner code, or the code that uses whatever values are entered by the user?

Your code probably does 3 things:

• Ask user for input
• Perform operation
• Display result

In a unit test, you're testing the operation, not the user prompting or the display logic. Or if you do, they should be 3 different unit tests, and remember that if you're testing the user prompting, you're testing the use of the Scanner, not whether the Scanner can read System.in.

So, first split your code to:

public class MyClass {
    public static void main(String[] args) {
        Input input = promptUserForInput(new Scanner(System.in), System.out);
        Result result = performOperation(input);
        printResult(System.out, result);
    }
    // methods here
}

You don't have to create new classes for Input and Result if they are simple values. They could also be the same class, e.g. an instance of MyClass.

This way you can test if the operation works for various inputs, which is your primary concern.

@Test
public void testOperation1() {
    Input input = new Input(5, 15, true); // true means subtract
    Result result = MyClass.performOperation(input);
    assertEquals(-10, result.getValue());
}

@Test
public void testOperation2() {
    Input input = new Input(5, 15, false); // false means add
    Result result = MyClass.performOperation(input);
    assertEquals(20, result.getValue());
}

You can also test user prompting and result printing, if needed.

@Test
public void testPrompt() {
    String userInput = "5 15\nYes";
    PrintStream out = new PrintStream(new ByteArrayOutputStream());
    Input input = MyClass.promptUserForInput(new Scanner(userInput), out);
    assertEquals(5, input.getNum1());
    assertEquals(15, input.getNum2());
    assertTrue(input.isSubtractRequested());
}

@Test
public void testPrint() {
    ByteArrayOutputStream buf = new ByteArrayOutputStream();
    try (PrintStream out = new PrintStream(buf)) {
        MyClass.printResult(out, new Result(-10));
    }
    String outText = new String(buf.toByteArray());
    assertEquals("Result is -10\r\n", outText);
}