@Test public void testsGetHeadLinked() { FunctionalList testList = new FunctionalLinkedList(); assertEquals("get Head on empty", ErrorMessage.EMPTY_STRUCTURE, testList.head().getError()); testList.add("zero"); testList.add("one"); testList.add("two"); assertEquals( "get Head when not empty, no error", ErrorMessage.NO_ERROR, testList.head().getError()); assertEquals("get Head when not empty, correct item", "zero", testList.head().getReturnValue()); }