Please help me understand an interface

Frage zur Aufgabe

Gelöst

Hello, I don't really understand what HOBBY is. 1. Can we create an object of another class inside an interface? 2. So the reference HOBBY is an constant now, public static final. I can't access the hi() method: System.out.println(Dream.HOBBY.hi()); Why? 3. I cant' access the hi() method: System.out.println(new Hobby().hi()); Why? So what can I do with HOBBY? Please help me understand this :)

interface Desire {
}

interface Dream {
     public Hobby HOBBY = new Hobby();
}

public static class Hobby implements Dream, Desire {
     static int INDEX = 1;

    @Override
    public String toString() {
        INDEX++;
        return "" + INDEX;
    }

    public void hi(){
        System.out.println("hello");
    }
}

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Guadalupe Gagnon

Level 37 , Tampa, United States

27 Januar 2021, 18:54

Lösung

1: Yes 2: Because that hi() method returns void and that line is trying to print the returned value of the hi() method. 3: Same reason as number 2

Sylwia

Level 16 , Deutschland , Germany

28 Januar 2021, 17:13

Hi Guadalupe, thank you for your reply:) So pls tell me, how can HOBBY (an object of class Hobby) access his metkod hi()?

Guadalupe Gagnon

Level 37 , Tampa, United States

28 Januar 2021, 17:40

Lösung

Your problem wasn't that you weren't accessing the hi() method correctly, it was that you were trying to print out the result of that method that doesn't return any result. Because hi() is not a static method you will need to access it through an object. There is an object in the Dream interface. This would be a static variable so you can access that in a static fashion using the Dream interface. Otherwise you would need to create a Hobby object and call the method on that.

public static void main(String[] args) {
   Dream.HOBBY.hi(); //access the static object in the Dream class
   new Hobby().hi();  // create an object and access
   Hobby hob = new Hobby();  // create an object saved to a variable
   hob.hi();  // access it from the variable
}

Sylwia

Level 16 , Deutschland , Germany

28 Januar 2021, 18:17

things are more clear now... ;) What do you mean with "it was that you were trying to print out the result of that method that doesn't return any result." Why ist type void not allowed here? I thougt a class can have methods with different types? Thank you very much :)

Guadalupe Gagnon

Level 37 , Tampa, United States

28 Januar 2021, 18:18

Passing 'void' to the println() method can't be done. For example:

System.out.println(Dream.HOBBY.hi());
System.out.println(new Hobby().hi());

Sylwia

Level 16 , Deutschland , Germany

28 Januar 2021, 18:39

now I understand, that was a beginner's mistake (I'm still a beginner at level 13 :D ). Thank you for your help :)

Guadalupe Gagnon

Level 37 , Tampa, United States

28 Januar 2021, 18:51

No worries. We are all constantly learning and I made plenty of beginner mistakes myself when I was a beginner. I now am more of an intermediate programmer and make intermediate mistakes.