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Declare variables and parameters with interfaces rather than concrete classes for greater flexibility

Medium
OOPClean code
What is it?​

This practice is activated when variables or parameters are declared using concrete classes instead of interfaces, which can be recognized by declarations like ConcreteClass varName instead of InterfaceType varName.

For instance, in Java, variables should have List, Set or Map implementations instead of ArrayList, HashMap or HashSet.

Why apply it?​

This rule matters because using interfaces for declarations provides flexibility and promotes loose coupling, allowing the system to be more adaptable to change and easier to extend or modify.

How to fix it?​

Refactor the code by replacing declarations of variables and parameters with the appropriate interface type rather than the concrete class.

Read more:​

https://www.oodesign.com/programming-to-an-interface.html

Examples​

Example 1:​

Positive​

Correct implementation following the practice.

package org.example;

import java.util.ArrayList;
import java.util.List;

//TIP To <b>Run</b> code, press <shortcut actionId="Run"/> or
// click the <icon src="AllIcons.Actions.Execute"/> icon in the gutter.
public class Main {

    private void ListInitialisation() {
        List<Integer> list = new ArrayList<>(100);
    }



}

Example 2:​

Positive​

Correct implementation following the practice.

package dev.best.practices;

import java.util.HashSet;
import java.util.Set;

public class SetExample {

    public void init() {
        final Set<String> names = new HashSet<>();
    }
}

Negative​

Incorrect implementation that violates the practice.

package org.example;

import java.util.ArrayList;
import java.util.List;

//TIP To <b>Run</b> code, press <shortcut actionId="Run"/> or
// click the <icon src="AllIcons.Actions.Execute"/> icon in the gutter.
public class Main {

    private void ListInitialisation() {
        ArrayList<Integer> list = new ArrayList<>(100);
    }

}