在测试 JUnit 中强制私有方法返回值

Question

I'm trying to test a public method, which calls a private method that returns a random integer to use in the public method.

I am finding it hard to write a test since my expected outpoint always varies to my result due to the random number generator. So i'm trying to make the private method return a specified value when called (in test).

Is this possible?

Method:

public String addLine(String arrayString){
    String[] numbers = arrayString.split(",");
    int startingSize = numbers.length;

    int newLineCounter = generateLineSpacing(startingSize);

    for (int index  =0;index <numbers.length; index++ ,newLineCounter--){
        if (newLineCounter == 0){
            numbers[index] = "\n," + numbers[index];
            newLineCounter = generateLineSpacing(startingSize);
        }
    }
    return StringUtils.arrayToCommaDelimitedString(numbers);
}


private int generateLineSpacing(int sizeOfArray) {
    Random random = new Random();

    //odd +1
    if (sizeOfArray % 2 == 1){
        sizeOfArray += 1;
    }

    //create new line in random position within range 1 - half array size
    return random.nextInt(sizeOfArray/2  -1 ) + 1;
}

Test:

@Test
public void shouldAddSymbolAtNewLineLocation(){
    //given
    String expected = "2,3,\n,4";
    String startingString = "2,3,4";

    //mock out random number generator to return 2

    //when
    String result = underTest.addLine(startingString);

    //then
    assertEquals(expected, result);
}

Answer 1

You cannot usually mock a private method. By declaring the method private you are making sure no one will modify its behaviour, a test should not do that either.

What you can do here is moving the random number generator into a Supplier passed as a dependency to the class, or as a method parameter, this can be mocked then.

As an altenative, this answer methions that you can use PowerMock to mock private methods, I have personally not used it but I generally think it should not be done (again because you are changing a private method).