public class A {
   
    private int x=3; 
    int y=5; 
    
    
   void  printFieldsClassA(){
       System.out.println("printFieldsClassA: x = "+x+" y = "+y); 
    } 
   
}

public class B extends A{
   
     int w=1; 
     int t=1; 
   
   B(){
     
   } 
    
      void  printFieldsClassB(){
          System.out.println("printFieldsClassB: w = "+w+" t = "+t); 
      } 
    
}

public static List startRef(Class startClass,List listRes) throws ClassNotFoundException, IllegalArgumentException, IllegalAccessException{
           
           
           Field[] fi=startClass.getDeclaredFields();
           
           
           for (Field f : fi) {
               System.out.println("Field in class: "+startClass.getName()+" field: "+f); 
                  if(f.getName().equals("x")){
                      f.setAccessible(true);


                      f.setInt(Class.forName(startClass.getName()), 33);


                  }

           }
                  
                     if(startClass.getSuperclass()!=null){
                          startRef(startClass.getSuperclass(),listRes);
                     }
            
                   return listRes;
    }

Exception in thread "main" java.lang.IllegalArgumentException: Can not set int field com.test.testappmaven.A.x to java.lang.Class

f.setInt(Class.forName(startClass.getName()), 33);

A a=new A();

Class myClass= Class.forName("com.test.testappmaven.A");
        
    Field[] fi=myClass.getDeclaredFields();
           
           for (Field f : fi) { 
                        f.setAccessible(true);

                        f.setInt(a, 8);

           } 

A a=new A();
f.setInt(a, 8);