为什么Java编译器在以下代码中为LinkedListDescingIterator给出“错误：找不到符号”？

Question

Why does this code:

import java.util.*;
class Playground {
    public static void main(String[ ] args) {
        List<Integer> l = new LinkedList<>();
        Iterator<Integer> i = l.descendingIterator();
    }
}

Generate this compiler error

./Playground/Playground.java:5: error: cannot find symbol
        Iterator<Integer> i = l.descendingIterator();
                               ^
  symbol:   method descendingIterator()
  location: variable l of type List<Integer>
1 error

• Here is the relevant JavaDocs API
• Running Java 7.. In case that is issue. Thought it had been around for donkeys years.
• Here is a canned example elsewhere.
• You could copy/paste code from here into this website to see,

Answer 1

Following the principal

“Coding to interfaces, not implementation.”

I suggest to use the Deque interface that provides descendingIterator() method

Deque<Integer> deque = new LinkedList<>();
Iterator<Integer> iterator = deque.descendingIterator();

instead.

Answer 2

You are trying to call descendingIterator on a List reference. The compiler doesn't know that the runtime type is a LinkedList , hence the compilation error.

If you want to access this method, you could define the reference as a LinkedList :

LinkedList<Integer> l = new LinkedList<>();

Answer 3

List is an interface and LinkedList is an implementation of List

You have the option of explicit typecasting like the following

Iterator<Integer> i = ((LinkedList<Integer>)l).descendingIterator();

or change your code to be the following:

import java.util.*;
class Playground {
    public static void main(String[ ] args) {
        LinkedList<Integer> l = new LinkedList<>();
        Iterator<Integer> i = l.descendingIterator();
    }
}