Should variables always be declared using interfaces in Java?

People often see the suggestion that variables should be declared with an interface rather than implementing a class For example:

List<Integer> list = new ArrayList<>();

However, I say that the algorithm used in this list does rely on O (1) random access of ArrayList (such as Fisher Yates shuffling) In this case, the key abstraction ArrayList represents for me is its array like nature, not just its list nature In other words, if someone appears and changes the list to LinkedList, it will be problematic, even if the code will compile In this case, declare whether the implementation type can be used?, For example:

ArrayList<Integer> list = new ArrayList<>();

Solution

Emphasize what Oliver Charlesworth said in his comments: whether the information is public or not is the most important difference

From the way you write the question, it seems that you are talking about a field (not a method parameter or return value) By definition, a private field is an implementation detail that you can specify at will Therefore, it is feasible to declare it as ArrayList - although strictly speaking, it is irrelevant Use this sentence to express: what do you say

Someone who can change the statement

private List<T> list = new ArrayList<T>();

to

private List<T> list = new LinkedList<T>();

You can also change the declaration

private ArrayList<T> list = new ArrayList<T>();

to

private LinkedList<T> list = new LinkedList<T>();

The only practical way to prevent this is to add this (important) implementation detail as a comment, for example:

/**
 * The list that stores ... whatever. 
 *
 * This list should have a complexity of O(1) for random access,* because ...
 */
private final List<T> list = new ArrayList<T>();

(this can also be enforced by internally passing the list to the method that expects it to be randomaccess. This was proposed by Alex R, but refers to the public method. For private methods, even this will not prevent someone from changing the method signature if there is no record of the intention and reason for requiring randomaccess)