为什么 LinkedList 不实现 RandomAccess 接口来使元素的检索更快？

Question

Why only ArrayList and Vector implements RandomAccess interface and LinkedList doesn't? As I understand it, the RandomAccess implemented class can make the retrieval of elements faster in collection.

Also, if I try to make a custom class which extends java.util.LinkedList and also implements RandomAccess interface, does that make the custom class just like LinkedList but with RandomAccess?

Here is code which is implementing what I stated in the second point:

public class ExtendedLinkedList extends java.util.LinkedList implements java.util.RandomAccess {
    
    public static void main(String[] args){
        
        ExtendedLinkedList ell = new ExtendedLinkedList();
        
        System.out.println(ell instanceof java.util.RandomAccess); // True
        System.out.println(ell instanceof java.util.LinkedList); // True
    }
    
}

Answer 1

Implementing the RandomAccess interface does not make random access faster.

Indeed, it doesn't do anything at all , really.

It only exists as a marker interface: it's used to mark list implementations that already provide efficient random indexed access .

In other words: ArrayList and Vector implement RandomAccess because they provide efficient random access. LinkedList does not implement it, because it does not provide efficient random access (accessing a specific index from the List interface requires potentially iterating up to half of the list).

If you extend LinkedList then you should not implement RandomAccess unless you also change the implementation of get(int) to be efficient (at which point I'd say you probably shouldn't be extending LinkedList at all).