Thread: How to make operational objects?

I'm a beginner, so, please be patient

I'm trying to make messages between classes coherent.
Hence, if I'm instancing a class I want to be able to declare the variable storing the instance by the class name it is installing.

Following a MVC design pattern, but omitting the view (not relevant for this purpose) I have got the next self-explanatory code:

Model class

/**
 * 
 */
package model;
 
import java.lang.*;
import java.io.*;
 
/**
 * @author jonathan
 *
 */
public class UsersM {
 
	UsersM users;
 
	/**
	 * 
	 */
	public UsersM() {
		// TODO Auto-generated constructor stub
	}
 
	public UsersM get() {
		return users;
	}
 
}

Controller class

/**
 * 
 */
package controller;
 
import model.UsersM;
 
/**
 * @author jonathan
 *
 */
public class UsersC {
 
	/**
	 * 
	 */
	public UsersC() {
		// TODO Auto-generated constructor stub
	}
 
	public UsersM get() {
		return new UsersM().get();
	}
 
}

Main class

/**
 * 
 */
package Main;
 
import controller.*;
/**
 * @author jonathan
 *
 */
public class Main {
 
	/**
	 * 
	 */
	public Main() {
		// TODO Auto-generated constructor stub
	}
 
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		UsersC users = new UsersC();
 
		System.out.println(users.get());
	}
 
}
 
//The output after run the code is null.

which does nothing because I'm not able to actually store anything in the variables, and also I'm forced to limit the get() method to the type of variable I'm retrieving the message from. I would like to know if there is a reasonable way work around this because I look for examples and documentation but I cannot find anything about this.

Originally Posted by Norm

Please explain why not.

Because once I declare a variable type Class I cannot assign values type Set, List, ArrayList to it, it throws me a mismatch type error.

Originally Posted by Norm

Can you explain what that means?
What does "open the class" mean?
What does "type is returning" mean? Java requires that methods specify what data type they are returning. True, some methods return data of type Object but that is often a poor design.

It means I have to open the class with the IDE (Eclipse in my case) and check what the method returns in order to make the variable compatible with that object. And that is not really abstraction, is it?

Originally Posted by Norm

Yes that is the way java works.
If you declare variables as type Object, then any class's object can be assigned to that variable because all classes extend Object.
However you can not assign primitives like int or char to that variable.

Not primitives, but, how about data structures like Set, List, ArrayList, HashMap? Any book to read that talks about all this? I have not clear many things.

Originally Posted by ClipOS

...I want to not have to open the class I am retrieving to check what type is returning...
...Not primitives, but, how about data structures like Set, List, ArrayList, HashMap? ...

I am not sure you got an answer already but I wrote my answer below :

Because of two statements above, I ended up that : "you want to check what is the returning object's class, whether it is Set or ArrayList or HashMap etc." am I right?

If I am right, then ; there is an instanceof() operator in Java , and you can find more on https://docs.oracle.com/en/java/java...-operator.html. In that page you can find that different classes defined for shape and which one of them returned determined by instanceof() operator. You can do similar for Set, ArrayList or any other classes I think. Also classes on the link(i.e Rectangle,Circle tc.) are all derived from(in exact word implemented from) an abstract class(i.e. interface) Shape. You can catch and follow this hint/tip and you can go further from there.

However, if you want to do something else, both my assumption and answer is not valid.