Не могу использовать переменную с одного класса в другом!

public class LaunchUrlIndicator {
 
    public static void main(String[] args) {
        Program start = new Program();
        Check checkObj= new Check();
        
        start.begin();
        checkObj.firstCheck();
}

import javax.swing.*; 
public class Program {
    Program start;
 
    String url ="0";
 
    public void begin(){
        url=JOptionPane.showInputDialog("Input the URL adress, please. Input 0 (zero) to exit");
    }
}

public class Check {
    Program start;
    Check checkObj;
 
    String url="0";
    String errorMsg = "The URL inputed incorectly. It must start whis: [url]http://,[/url] [url]https://,[/url] or directly whis Domaine Name, and consist of not less than 4 symbols";
    int lengthUrl;
    boolean check1;
    
    public void firstCheck(){
        url = start.url;
        lengthUrl= url.length();
        if(url.equals("0")) System.out.println("Programm stoped");
        else if(lengthUrl < 4) System.out.println(errorMsg);
        else check1 = true;
    }
}

public class LaunchUrlIndicator {
 
    public static void main(String[] args) {
        Program start = new Program();
        Check checkObj= new Check(start);
        
        start.begin();
        checkObj.firstCheck();
}

import javax.swing.*; 
public class Program {
 
    String url ="0";
 
    public void begin(){
        this.url=JOptionPane.showInputDialog("Input the URL adress, please. Input 0 (zero) to exit");
    }
}

public class Check {
    Program start;
    public Check(Program p){
        this.start = p;
    } 
    String errorMsg = "The URL inputed incorectly. It must start whis: [url]http://,[/url] [url]https://,[/url] or directly whis Domaine Name, and consist of not less than 4 symbols";
    int lengthUrl;
    boolean check1;
    
    public void firstCheck(){
        url = start.url;
        lengthUrl= url.length();
        if(start.url.equals("0")) System.out.println("Programm stoped");
        else if(lengthUrl < 4) System.out.println(errorMsg);
        else this.check1 = true;
    }
}