Thread: Assigning a Value to a Class extending a Superclass

I'm learning Java programming and today I was into a document on classes.

I've created a superclass and a subclass as the following:

package objects;
public class Objects {
 
    String carrier="Zain JO";
    String color="Black";
    int battery=1;
 
    public void ObjectsCon(int b){
        battery=b;
    }
 
    public class Android extends Objects{
 
        String company="Google";
        String model="Nexus";
        double screen=4.5;
 
        void AndroidCon(double s){
            screen=s;
        }
    }
 
    public static void main(String[] args) {
 
        Objects obj=new Objects();
        obj.ObjectsCon(2);
 
    }
}

As you see, I have assigned a value of "2" to the superclass, successfully. but my problem is How to assign a value to "s" through its method "AndroidCon"?

I tried to:

Android device=new Android();

but it didn't work. I get: on-static variable this cannot be referenced from a static context

but it didn't work. I get: on-static variable this cannot be referenced from a static context

You are attempting to access an inner class from a static context. Either define the Android class as static, or have the Android class as its own class (rather than an inner class). The following link provides a bit of reasoning as to why this is the case:
Nested Classes (The Java™ Tutorials > Learning the Java Language > Classes and Objects)

cs13, this is the third time I've sen you (poorly) spoon-feeding. Please read this before you post again: http://www.javaprogrammingforums.com...n-feeding.html

Continued spoon-feeding will cause me to request that you are banned, for the reasons outlined in that article.

I just want to point out that it was copeg's suggestion, not mine (I don't disagree with him, I just don't want to take the credit).

And it's good that you can learn from code like that, but you're in the minority. There is a non-code, non-teachable skill of problem solving that is only perfected through practice, and full-code solutions like cs13's rob people of that practice.

Also, my real problem with cs13's approach is that it's not exactly the correct answer, and he continues to post slightly-off solutions. In his example, the inner class does not rely on anything in the outer class, but by making it non-static, he's adding the overhead required to keep around the unused references to the outer class. Making the inner class static is the way to go in that case.

Well Kevin, I don't think I am that skilled to judge that but it seemed to me as a new approach of accessing nested classes. The truth is that I didn't consider the AndroidCon as a nested class but a subclass inheriting from the superclass. The issue with the Oracle online tutorial and thus the book version is not telling about the structure. Yes, they tell you how to let a subclass inherit the superclass but they don't explain the structure.

I've been googling a lot on classes structure but I have nothing. I mean questions like:
- Can I write a class outside the superclass which its end "}" comes in the end of the public static void main..?
- Can I write a class in a new .java file and make it inherit objects from the superclass which is written in another .java file within the same project?

Actually a lot of questions all about the structure. I understand the syntax but the structure is missing from Oracle online tutorial.

However, thank you so much again for the help and I am really glade to find this great forum. I just hope to be a more active and useful to the other members as I am learning.

Originally Posted by ubermoe

Well Kevin, I don't think I am that skilled to judge that but it seemed to me as a new approach of accessing nested classes. The truth is that I didn't consider the AndroidCon as a nested class but a subclass inheriting from the superclass. The issue with the Oracle online tutorial and thus the book version is not telling about the structure. Yes, they tell you how to let a subclass inherit the superclass but they don't explain the structure.

I see. In your first example, you had your subclass inside your superclass, which makes your subclass also an inner class, which slightly complicates things. If your question has nothing to do with nested classes, I would take out the nesting.

Originally Posted by ubermoe

I've been googling a lot on classes structure but I have nothing. I mean questions like:
- Can I write a class outside the superclass which its end "}" comes in the end of the public static void main..?

I'm not sure I understand the question. What happened when you tried? Are you perhaps talking about anonymous inner classes?

Originally Posted by ubermoe

- Can I write a class in a new .java file and make it inherit objects from the superclass which is written in another .java file within the same project?

Sure! But again, what happened when you tried?

Originally Posted by ubermoe

Actually a lot of questions all about the structure. I understand the syntax but the structure is missing from Oracle online tutorial.

Many of your questions are going to be answered simply by writing a small program (an SSCCE) and trying it out.

Originally Posted by ubermoe

However, thank you so much again for the help and I am really glade to find this great forum. I just hope to be a more active and useful to the other members as I am learning.

No problem. I'm glad we could help.

Usually, each class has its own file. When you create a new class, you create a new file. You're trying to shove all your classes into one file, which is giving you a headache.

You certainly can combine classes into a single file, if that actually fits your design. One way to do that is by nesting classes, where you have an inner class inside an outer class. This is useful when the inner class depends on something in the outer class, or when the inner class is only used in the outer class. The inner class can be static or non-static, based on the things I mentioned above.

Another way to do it, and what I think you're trying to do, is to simply list multiple classes inside a single file. The problem with that though, which the compiler is telling you about, is any classes other than the class for which the file is named must not be public. This is because the normal approach, with classes in different files, will not know where to find a class that is listed inside another class file!

Please split your classes up into multiple files, as I have no idea what you're trying to run.

Also, please see the tutorial on properly calling constructors: Providing Constructors for Your Classes (The Java™ Tutorials > Learning the Java Language > Classes and Objects)