DBObject object or User object confusion

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I have managed to solve the task by checking the other users their help questions here. However, i am still stuck on why the code needs to be changed to User and it can't be a reference to the User object through the DBObject interface.

interface DBObject {
    DBObject initializeIdAndName(long id, String name);
}

static class User implements DBObject {
    long id;
    String name;

    @Override
    public User initializeIdAndName(long id, String name) {
        this.id = id;
        this.name = name;
        return this;
    }

As learned on a previous section here: https://codegym.cc/groups/posts/101-why-interfaces-are-necessary-in-java It tells me i can create object references by using the interface class. It would help me a lot if someone can explain me in detail why here it cant be a reference to a User object by returning a DBObject reference to that User object (as Java will know it will be a user object, simply because we are returning this.) I do know that an interface class can't be instantiated, but then again, every object creation is merely a reference to that specific object. This part realllllyy confuses me, and if someone could explain me this or send me a link to clear this up it would be highly appreciated!

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Roy

Level 22 , Bangkok, Thailand

27 April 2019, 07:04

So i was just doing the task; com.codegym.task.task13.task1328 Which made me able to try out many things, and i found out i am overthinking it all and making it harder than it is. I considered the interface reference always had to be an object of the underlying class so that it always had its methods and variables as well. Best is (for anyone having the same issue) to simply stick with the super/subclass referencing and think of it as a remote control for TV. The remote control has some methods, like volume , change channel etc, and when creating a reference for the the TV using " Remote remote = new TV" you will only be able to use the methods available on the remote control and not all the other methods the TV has. Edit; level 14 explains this all in detail

Roy

Level 22 , Bangkok, Thailand

26 April 2019, 21:55

Hey Guys, I really appreciate your replies, though im still confused about this specific tasks method error. I do understand the concept of interfaces and their usage. Going back to this specific task and my question why the code needs to be changed to User and it can't be a reference to the User object through the DBObject interface.

@Override
public DBObject initializeIdAndName(long id, String name) {
    this.id = id;
    this.name = name;
    return this;
}

This won't let the task be completed and caused a lot of confusion, especially as CodeGym tells me after i ran it that the method should return a user object, even though it does return a user object(?!?) and all the requirements needed to finish the task.

I just tested it:

@Override
public String toString() {
    return String.format("The user's name is %s, id = %d", name, id)+ " "+this.getClass().getSimpleName();
}

Returns User in the end.

Bill Wu

Level 29 , Wellington, New Zealand

27 April 2019, 02:55

You have to specifically use User in the signature because if you put the interface there, the object returned is considered as DBObject. User implements it, and they have similarities but they are DIFFERENT.

Roy

Level 22 , Bangkok, Thailand

27 April 2019, 04:11

Hey Bill, thanks for your reply! Can i ask; "the object returned is considered as DBObject". How? I would love for you to explain this further by any link or example.

static class Matrix {
    public static DBObject NEO = new User().initializeIdAndName(1, "Neo");
    public static DBObject TRINITY = new User().initializeIdAndName(2, "Trinity");
}

User object is created. Id and Name are being initialized through the method and are returned. What exactly is this DIFFERENCE you are talking about when returning as DBObject or as User? (sorry for my many questions here, i really try to get a deep understanding of the matter of why and how it works :))

Bill Wu

Level 29 , Wellington, New Zealand

28 April 2019, 21:56

useful

Java is "type safe", which means the type of a variable should match the object. All the inherited types will match the interface type, but they are not the same type. For example: interface IT {}, class A implements IT, class B implements IT. if you declare a variable IT ob, it can be initialised by an object either from A or B, but in JVM, ob will be considered as type IT, not A or B. Hopefully that helps. You may google some stuff about interface, which are a lot. Read them and be patient, with some time it will be clearer.

Guadalupe Gagnon

Level 37 , Tampa, United States

26 April 2019, 13:49

Interfaces don't contain any useable code. All that they contain is method blueprints. Think of them more like a contract: Any class that implements the interface GUARANTEES that the methods contained in the interface will be implemented in the class. The code will not compile all the methods are at least contained in the class. This tasks is just a small example of an interface and by no means explains or deomstrates the full power of them. Bryce gave a little better example, though you will need a lot more solid examples before the concepts starts making sense. Just remember my first couple sentences 1) they contain methods with no bodies, sort of like blue prints 2) Classes that implement the interface MUST contain those methods.

Bryce Lindley

Level 9 , Olympia, United States

26 April 2019, 06:46

Think of interfaces as a means to create objects with abstract methods that you would like to add to classes. Interfaces are not meant to create objects but add methods to which you normally wouldn't use for the classes. For instance you make an RPG with hero classes. Class warrior, ranger, cleric, mage. And you have a superclass called Hero that extends these classes. The superclass Hero might have: int strength; int health; int attack; You would use an interface for those who have magic spells to create those abstract methods because having all those heroes extend magic spells wouldn't make sense. interface magic { void castSpell(); } class Mage implements magic { void castSpell() { // do mage stuff } class Cleric implements magic { void castSpell() { // do cleric stuff }