Object call versus class-calls ?

Under discussion

I try to understand the difference between example 1 and example 2 below, and why it matters, what to call. I think, I understand that, Example 1 I call the class Idea and Example 2 I call the object of Idea-class. But why does it matter, and when to prefer the example 1 over example 2, and the other way around.. ? Example 1:

public class Solution {
    public static void main(String[] args) {
        printIdea(new Idea());
    }

    public static void printIdea(Idea idea) {
        System.out.println(Idea.getDescription());

    }

    public static class Idea {
        public static String description = "Anita";

        public static String getDescription() {
            return description;
        }
    }
}

vs. Example 2:

public class Solution {
    public static void main(String[] args) {
        printIdea(new Idea());
    }

    public static void printIdea(Idea idea) {
        System.out.println(idea.getDescription());

    }

    public static class Idea {
        public static String description = "Anita";

        public String getDescription() {
            return description;
        }
    }
}

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Gellert Varga

Level 23 , Szekesfehervar, Hungary

17 August 2021, 18:53

Guadalupe has written some very good examples. However, I need to correct a little now: Your first example also works fine, because getDescription() is static there. Which one is better (1. or 2.), I can't give you a clear answer yet, maybe when I become a skilled programmer someday. For now, I can only give you hints and assumptions: - If we pay respect to Java is an OOP, where all things are objects preferably, then I think version 2 is better. - If I assume that for different ideas I would probably like to see different descriptions printed out, then version 2. Because in version 1, even if you create a lot of Idea objects, the static getDescription() method will always return the same thing, no matter how many different Idea-objects you have. But in version 2, where each Idea object has its own getDescription() instance method, it would be possible to return different values in case of each object - (just not like in this example, where the instance method returns the same single static variable).

Guadalupe Gagnon

Level 37 , Tampa, United States

16 August 2021, 13:24

Example 1 is incorrect because the getDescription() method is not static. You can't call a non static member of a class from the class itself, you MUST use an object (like example 2). Because the String 'description' is static, you can call that by the class without an object. See the code and the notes below:

public class Solution {
    public static void main(String[] args) {
        // because description is static you can call it from the class;
        System.out.println(Idea.description);

        Idea x = new Idea();

        // you can also call static members from objects:
        System.out.println(x.description);
        // though you can do this, it is improper and you should
        // call static members from the class only

        // You CAN NOT call non-static members from the class
        //!!ERROR System.out.println(Idea.getDescription());

        // you can only call the non-static members with an initialized object
        System.out.println(x.getDescription());

        // non-initialized objects will result in a null pointer exception, the
        // compiler won't even let this compile until temp is set to something
        Idea temp;
        System.out.println(temp.getDescription());

    }

    public static class Idea {
        public static String description = "Anita";

        public String getDescription() {
            return description;
        }
    }
}