In which case `ParameterizedType#getActualTypeArguments` return an array which has more than one elements

Question

I tried to get the classtype in the generic,and code below works:

@Test
public void test3() throws NoSuchFieldException {
    Class<?> clazz = (Class<?>) ((ParameterizedType)Role.class.getDeclaredField("users")
            .getGenericType()).getActualTypeArguments()[0];
    assert clazz.equals(User.class) : "error!";
}

static class Role {
    public List<User> users;
}
static class User {

}

So, I learned something about ParameterizedType .

And I want to know in which case ParameterizedType#getActualTypeArguments return an array which has more than one elements.I tried a lot of cases, but all the results are an array of size 1.

Any ideas?

Answer 1

It return an array of more than one element if you have several generic parameters like in Map.

Example:

@Test
public void exampleTest() {
    Map<Integer, String> myMap = new HashMap<>();
    ParameterizedType type = (ParameterizedType) myMap.getClass().getGenericSuperclass();
    System.out.println("Array of actual types : " + Arrays.toString(type.getActualTypeArguments()));
}

And the output is:

Array of actual types: [K, V]