The importance of interfaces in Java [closed]

It’s:

public int howFast(Airplane airplane) {
    return airplane.speed();
}

public int howFast(Tiger tiger) {
    return tiger.speed();
}

public int howFast(Rocket rocket) {
    return rocket.speed();
}

public int howFast(ThingThatHasntBeenInventedYet thingx) {
    // wait... what? HOW IS THIS POSSIBLE?!
    return thingx.speed();
}

...

vs

public int howFast(Mover mover) {
    return mover.speed();
}

Now, imagine having to go back and change all those howFast functions in the future.

Interface or no, each class will have to implement or inherit the speed() method. An interface lets you use those disparate classes more easily, because they’ve each promised to behave in a certain way. You’ll call speed(), and they’ll return an int, so you don’t have to write separate methods for every possible class.

When you find you need to handle speed differently because of a breakthrough in relativistic physics, you only need to update the methods that call speed(). When your great-granddaughter writes a class for HyperIntelligentMonkeyFish, she doesn’t have to disassemble your ancient binary and make changes so that your code can monitor and control her mutant army. She needs only declare that HyperIntelligentMonkeyFish implements Mover.