Change the value of final variable

By using reflections, you can change the value of final fields. Following step-by-step procedure explains how to change the value of final fields.

Step 1: Get the class object.

Ex:

   Employee emp = new Employee();

   Class myClass = emp.getClass();

Step 2: get the declared field.

Ex: 

  Field myField = myClass.getDeclaredField("id")

Step 3: suppress Java language access checking by enabling the accessibility to true for this field.

Ex: 

  myField.setAccessible(true);

Step 4: Set the field value.

class Employee {
    private final int id, age;
    
    Employee(int id, int age){
      this.id = id;
      this.age = age;
    }
    
    Employee(){
      id = -1;
      age = -1;
    }
    
    int getId(){
        return id;
    }
    
    int getAge(){
        return age;
    }
            
}

import java.lang.reflect.Field;

public class Test {

  public static void display(Employee emp) {
    System.out.println("---------------------------");
    System.out.println("id : " + emp.getId());
    System.out.println("age : " + emp.getAge());
  }

  public static void main(String args[]) throws Exception {
    Employee emp = new Employee();
    Class myClass = emp.getClass();

    System.out.println("Setting id");
    Field myField = myClass.getDeclaredField("id");
    myField.setAccessible(true);
    myField.setInt(emp, 1);
    display(emp);

    System.out.println("Setting age");
    myField = myClass.getDeclaredField("age");
    myField.setAccessible(true);
    myField.setInt(emp, 26);
    display(emp);
  }
}

Output

Setting id
---------------------------
id : 1
age : -1
Setting age
---------------------------
id : 1
age : 26

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