I've solved this task with step by step method, but I'm not sure about whole code overall in terms of working.

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Rozwiązane

Can someone explain how list returns output from getOutputByBookType() method? Or main part with adding objects to books list?

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Guadalupe Gagnon

Poziom 37 , Tampa, United States

9 kwietnia 2021, 18:31

System.out.println() calls the toString() method on any object passed to it. The toString() method in the book class returns getOutputByBookType()

Guadalupe Gagnon

Poziom 37 , Tampa, United States

9 kwietnia 2021, 18:38

rozwiązanie

small example:

class MyClass{
    private static int ID = 1;
    private int myId;

    public MyClass() {
        this.myId = ID++;
    }

    @Override
    public String toString() {
        return "Its my toString() implementation of object: " + myId;
    }
}

public class Solution {
    public static void main(String[] args) throws IOException {
        ArrayList<MyClass> test = new ArrayList<>();
        test.add(new MyClass()); test.add(new MyClass()); test.add(new MyClass());
        System.out.println(test);
    }
}

no toString() method is called in the above code, but it still outputs the implementation of toString() for each object in the ArrayList.

Igor Karlik

Poziom 19 , Wrocław, Poland

10 kwietnia 2021, 00:32

That's exactly what I couldn't understand at first, thank a lot :)

Gellert Varga

Poziom 23 , Szekesfehervar, Hungary

8 kwietnia 2021, 21:32

For example, in that case if the Book is type of AgathaChristieBook, then the returned value will come from this line: String agathaChristieOutput = author + ": " + getBook().getTitle() + " is a detective"; 1.) 2.) 3.) 1.) 'author' is an instance-variable of the current Book-instance. You can write it this way too: this.author but it's enough if you write this way: author because of here we are inside an instance method, so the mentioned variable in the code basically means the variable of the current object, even without the 'this' keyword. 2.) getBook() method: you can look at this for example inside the class AgathaChristieBook: public AgathaChristieBook getBook() { return this; } So the getBook() returns the current object! So: getBook() == 'this' keyword!! (== 'this' object.) So: getBook().getTitle() == this.getTitle() But as i mentioned it above, here you don't need use the 'this' keyword: getBook().getTitle() == this.getTitle() == getTitle() // try out the propgram with this code in your IDE-compiler. 3.) getTitle(): it's a getter method what returns to you the title variable of the class AgathaChristieBook.

Igor Karlik

Poziom 19 , Wrocław, Poland

9 kwietnia 2021, 17:40

Sorry, I should've explain more specifically, my question is how method getOutputByBookType() is called in the main method. I see only its declaration in Books class :/