Main.java

import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class Main {
    public static void main(String[] args)  throws Exception {
        MyClass obj = new MyClass("abc");

//      Compile error
//      obj.print();

        Method method = obj.getClass().getDeclaredMethod("print", new Class[] {});
        method.setAccessible(true);
        method.invoke(obj, new Object[] {});

        Field nameField = obj.getClass().getDeclaredField("str");
        nameField.setAccessible(true);
        nameField.set(obj, "def");
        method.invoke(obj, new Object[] {});
    }
}

MyClass.java

public class MyClass {
    private final String str;

    public MyClass(String str) {
        this.str = str;
    }

    private void print() {
        System.out.println(str);
    }
}