assertTrue(isNotNull.apply("test"));
        assertTrue(isNotNull.apply(123));
        assertTrue(isNotNull.apply(new Object()));
        assertFalse(isNotNull.apply(null));
    }

    public void test_apply_boolean() {
        BooleanFunction<Boolean> identity = b -> b != null && b;

        assertTrue(identity.apply(true));
        assertFalse(identity.apply(false));
        assertFalse(identity.apply(null));
    }