[FIXED] How to unit test a random string generator using Junit?

Issue

The question I have is simple and straightforward

Let's say I have this function which returns a random string

fun generateRandomId(): String {
    val randomString = UUID.randomUUID().toString().replace("-", "")
    return randomString
}

And I wanna test the above function using JUnit

@Test
fun generateRandomId() {
    Assertions.assertEquals()
}

Not sure how to assert that the randomly generated String doesn't have any "-".

Solution

assertFalse(generateRandomId().contains("-"))

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Previous answer to the original question:

If the function uses another class to generate the random String, then inject that other class in the constructor of your class:

class MyClass(someOtherClass: SomeOtherClass) {


    fun generateRandomId(): String {
        val randomString = someOtherClass.generateRandom()
        return randomString
    }
}

and in your tests you mock SomeOtherClass and all you need to do is to check that whatever SomeOtherClass returns, is returned in your function.

@Test
fun testGenerateRandomId() {
   // given
   val someOtherClassMock: SomeOtherClass = mock() // 1
   val myClass = MyClass(someOtherClassMock) // 2
   val randomString = "randomString"
   whenever(someOtherClassMock.generateRandom()).thenReturn(randomString) // 3

   // when
   val result = myClass.generateRandomId()

   // then
   assertEquals(randomString, result)
}

1 and 3: mock the behaviour of SomeOtherClass with your preferred mocking framework / fake implementation

1 and 2: these lines usually go in the setUp method of your test class

---

Now if you don't use an external class to generate your random String, you will have to paste your code for further help.

Answered By - Gustavo