Java jUnit：如何断言列表中的所有对象都来自某个类？

Question

I've built the following class:

public class BibliotecaApp {

    public static void main(String[] args) {
        System.out.print("Welcome!");
    }

    public static ArrayList<Book> getBooksList(){
        return new ArrayList();
    }
}

I need to test if the method getBooksList() return an actual list of books.

I wrote the following test:

@Test
public void everyItemInTheBookListIsABookInstance() throws Exception{
    assertThat(BibliotecaApp.getBooksList(), everyItem(isA(Book.class)));
}

The test is passing - and it shouldn't. Note that in the actual method I return an instance of ArrayList (I'm practing TDD), although I defined the method signature as ArrayList<Book> - is that why the test passes? If so, how should I write a test to ensure that every item in the listem belong to a certain class ( Book in this context)?

Thanks in advance.

Answer 1

The behaviour of everyItem is documented as :

only matches when [...] items [...] are all matched

So an empty list will always match.

If that's a weird diea, then another way to think about it is that everyItem fails only if it finds an item that doesn't match the inner matcher. ¹ There are no such items in an empty list.

If you want the test to fail if there are no items at all, then you'll also ² need to assert that explicitly:

assertThat(BibliotecaApp.getBooksList(), not(empty()));

---

_{1. Even if one doesn't agree on that equivalence, one can take a look at the implementation to see that this is indeed what's happening.}

_{2. But you may as well drop the original assertion, because the compiler already enforces the type.}