OnJava8-Examples/reflection/ModifyingPrivateFields.java

// reflection/ModifyingPrivateFields.java
// (c)2021 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
import java.lang.reflect.*;

class WithPrivateFinalField {
  private int i = 1;
  private final String s = "I'm totally safe";
  private String s2 = "Am I safe?";
  @Override public String toString() {
    return "i = " + i + ", " + s + ", " + s2;
  }
}

public class ModifyingPrivateFields {
  public static void
  main(String[] args) throws Exception {
    WithPrivateFinalField pf =
      new WithPrivateFinalField();
    System.out.println(pf);
    Field f = pf.getClass().getDeclaredField("i");
    f.setAccessible(true);
    System.out.println(
      "f.getInt(pf): " + f.getInt(pf));
    f.setInt(pf, 47);
    System.out.println(pf);
    f = pf.getClass().getDeclaredField("s");
    f.setAccessible(true);
    System.out.println("f.get(pf): " + f.get(pf));
    f.set(pf, "No, you're not!");
    System.out.println(pf);
    f = pf.getClass().getDeclaredField("s2");
    f.setAccessible(true);
    System.out.println("f.get(pf): " + f.get(pf));
    f.set(pf, "No, you're not!");
    System.out.println(pf);
  }
}
/* Output:
i = 1, I'm totally safe, Am I safe?
f.getInt(pf): 1
i = 47, I'm totally safe, Am I safe?
f.get(pf): I'm totally safe
i = 47, I'm totally safe, Am I safe?
f.get(pf): Am I safe?
i = 47, I'm totally safe, No, you're not!
*/