我试图了解Java中“程序到接口”的含义

Question

I am trying to understand what it means by "Program to an interface" in Java. Should we program to interface because of changing client needs. Is this the only reason

how will below program run into problem from client and developer perceptive?

public interface SuperHeros {
    public List<String> getCharacters();
}
public class DCSuperHeros implements SuperHeros {
    List<String> characters = List.of("Super Man", "Bat Man", "Wonder Woman", "Cyborg", "Flash");

    public List<String> getCharacters() {
        return characters;
    }
}
public class MarvelSuperHeros implements SuperHeros {
    List<String> characters = List.of("Spider Man", "Captain America", "Daredevil", "Wolverine", "Iron Man");

    public List<String> getCharacters() {
        return characters;
    }
}

**SuperHeros marvelHeros = new MarvelSuperHeros(); //Line 15
SuperHeros dcHeros = new DCSuperHeros(); //Line 16

what is the problem with line 15 and 16. Can somebody please explain with an use case which will run this example into a problem because of coding to implementation?**

Answer 1

Imagine you had a library that is used by both a CLI program and a server. This library uses a cache that is just a map of key-value pairs.

The interface you'd want the library to use would be Map because for the CLI program you might be able to use HashMap, but for the server, you would probably want to use ConcurrentHashMap for thread safety. Which implementation you use would be injected at runtime. You might even want to use a Redis implementation.

Another similar example would be a library that manages a data repository. You might need an implementation that uses AWS or Azure storage, and an implementation that uses MongoDB or SQL. You would want that library to present the same interface to the programs that uses it, and let the implementation that is used be selected at runtime.

Or for something less boring, imagine you are programming Super Mario Bros. 2. There are 4 characters that you can choose from: Mario, Luigi, Princess Peach, and Toad. Each one behaves slightly differently when you press the D-pad or A or B buttons, but they all have the same interface.

Regarding Line 15/16

You could have a wiki system that is used by 2 websites, one is DC and one is marvel.

A class that is used by both would take in the interface

SuperHeros marvelHeros = new MarvelSuperHeros(); //Line 15
SuperHeros dcHeros = new DCSuperHeros(); //Line 16

SuperHeroDataBase marvelDataBase = new SuperHeroDataBase(marvelHeros);
// these aren't necessarily in the same program, could be 2 totally different server programs that use the same library
SuperHeroDataBase dcDataBase = new SuperHeroDataBase(dcDataBase);

with the ctor for SuperHeroDataBase:

public SuperHeroDataBase(SuperHeros superHeros){
    this.superHeros = superHeros);
}