无法访问子类方法，泛型是否不利？ 或者我想念一些东西

Question

List<Employee> empLList = new LinkedList<>();
        empLList.add(...)
Iterator<Employee> descItr = empLList.descendingIterator();

in the above code i'm unable to access descendingIterator with following error

cannot find symbol
  symbol:   method descendingIterator()
  location: variable empLList of type List<Employee>

to get descendingIterator i have to recast empLList to LinkedList

 Iterator<Employee> descItr = ((LinkedList) empLList).descendingIterator();

My question : In general is the above a disadvantage of using Generics, ie, everytime we need to cast the object back to subclasss to access subclass' methods or is Generics supposed to work like that.

OR we should not use generics in case where we depend on too many of subclass' methods

OR i am missing something

I am curious about the use of GENERICS in the example not the collections used.

Answer 1

It is not about Generics.

Look at APIs. descendingIterator is method of Deque not List

LinkedList implements Deque

My question : In general is the above a disadvantage of using Generics, ie, everytime we need to cast the object back to subclasss to access subclass' methods or is Generics supposed to work like that.

Your snippet error is nowhere related to generics concepts. Logic based on casting is very very bad and should be replaced with polymorphism by correcting design.

I am curious about the use of GENERICS in the example not the collections used.

Then ask right questions.

Change to following to make it work:

Deque<Employee> empLList = new LinkedList<>();
empLList.add(...)
Iterator<Employee> descItr = empLList.descendingIterator();

Answer 2

This has nothing to do with generics. The List interface is a contract, defining the methods all of its implementations have to provide. Some, like LinkedList , may provide additional methods (such as the afore mentioned descendingIterator() ).

Your list of employees is regarded as being any implementation of List , thus all the List methods are available for use, nothing more. If you know your implementation is LinkedList , you may cast , but this is bad practice . Better keep it as LinkedList then:

LinkedList<Employee> empLList = new LinkedList<>();
empLList.add(...)
Iterator<Employee> descItr = empLList.descendingIterator();

... or (if you accept any List , but want to use the descendingIterator() ), create a new LinkedList out of it:

List<Employee> empLList = ... // any implementation 
empLList.add(...) 
Iterator<Employee> descItr = new LinkedList(empLList).descendingIterator();

Answer 3

It is not because of Generics. It is because, descendingIterator() method is not available in List. In this line

  Iterator<Employee> descItr = empLList.descendingIterator();

You are calling the method on a list type referenece variable. As a result you are getting compile error. When it comes to the below line

  Iterator<Employee> descItr = ((LinkedList) empLList).descendingIterator();

You are casting your empList to LinkedList and method is available in LinkedList as it implements Deque(Implementation of descendingIterator() is provided in LinkedList)