We could use reflection to modify a field or call a method which is private of a class.

Configuration

Java Compilation:
Java Runtime:

JDK 11.0.12
JRE HotSpot 11.0.12

We could use java.lang.reflect to modify a field or call a method which is private of a class

This could be due to no access to source code to change them to public

The following class Person has a private field age and private method growUp:


public class Person {

    private int age;

    public Person() {
        age = 20;
    }

    public int getAge() {
        return age;
    }

    private void growUp(int year) {
        age += year;
    }
}

To call the private growUp, first getting the method object by using getDeclaredMethod:


Method growUp = personA.getClass().getDeclaredMethod("growUp", int.class);

Then setting the accessibility to true:


growUp.setAccessible(true);

Now the method can be invoked:


growUp.invoke(personA, 5); // 5 is the argument of the method

For similar concept, to access the private field, we use getDeclaredField:


Field age = personA.getClass().getDeclaredField("age");
age.setAccessible(true);
age.set(personA, 30);

See the main class:


public class MainClass {

    public static void main(String args[]) throws Exception {

        Person personA = new Person();

        System.out.println("personA's present age: " + personA.getAge());

        Method growUp = personA.getClass().getDeclaredMethod("growUp", int.class);
        growUp.setAccessible(true);
        growUp.invoke(personA, 5);

        System.out.println("personA's age after growUp for 5 years: " + personA.getAge());

        Field age = personA.getClass().getDeclaredField("age");
        age.setAccessible(true);
        age.set(personA, 30);

        System.out.println("personA's age after setting to 30 years: " + personA.getAge());
    }
}

Output:


personA's present age: 20

personA's age after growUp for 5 years: 25

personA's age after setting to 30 years: 30