是否可以在具有私有构造函数的类中访问私有变量并从另一个类更改最终变量值

Question

How can I access private variable inside a class having private constructor. And how can I change it's value if the variable is declared final from another class.Have tried few links but not able to get as most of the solutions are having public constructor like:

Link i have tried

Code I have tried:

  package com.test.app;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;

public class Demo {

    static Demo instance = null;
    private int checkvalue = 10;
    final private int checkvalue1 = 12;

    private Demo() {
        System.out.println("Inside Private Constructor");
        System.out.println(checkvalue);
    }

    static public Demo getInstance() {
        if (instance == null)
            instance = new Demo();

        return instance;
    }

}

class Main {

    public static void main(String args[]) {
        try {
            ///this is showing me the value inside the constructor
            Class clas = Class.forName("com.test.app.Demo");
            Constructor<?> con = clas.getDeclaredConstructor();
            con.setAccessible(true);
            con.newInstance(null);


            ///how can i get the value of the private variables inside the class
            Field f = Demo.class.getDeclaredField("checkvalue");
            f.setAccessible(true);
            ////throwing me a error 
            //java.lang.IllegalArgumentException: Can not set int field com.test.app.Demo.checkvalue to java.lang.reflect.Constructor
            System.out.println("" + f.get(con));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Answer 1

如果要更改checkValue1变量，则应考虑不要将此变量作为最终变量。