Modify final fields

Doh.java

public class Doh {
  public static void main(String[] args) throws Exception {
    Person p = new Person("Name");
    setName(p, "New Name");
    setDefault("Bob");
    System.out.println(p.getName());
    System.out.println(Person.DEFAULT);
  }

  /**
   * Allows to change the state of an immutable instance. Huh?!?
   */
  private static void setName(Person p, String newName) throws Exception {
    Field nameField = p.getClass().getDeclaredField("name");
    setValue(p, nameField, newName);
  }

  /**
   * Allows to change the state of final statics. Huh?!?
   */
  private static void setDefault(String newDefault) throws Exception {
    Field staticField = Person.class.getDeclaredField("DEFAULT");
    setValue(null, staticField, newDefault);
  }

  /**
   * 
   * Set the value of a field reflectively.
   */
  protected static void setValue(Object owner, Field field, Object value) throws Exception {
    makeModifiable(field);
    field.set(owner, value);
  }

  /**
   * 
   * Force the field to be modifiable and accessible.
   */
  protected static void makeModifiable(Field nameField) throws Exception {
    nameField.setAccessible(true);
    int modifiers = nameField.getModifiers();
    Field modifierField = nameField.getClass().getDeclaredField("modifiers");
    modifiers = modifiers & ~Modifier.FINAL;
    modifierField.setAccessible(true);
    modifierField.setInt(nameField, modifiers);
  }
}

/**
 * An immutable data class.
 */
class Person {
  // avoid inlining by calling newString("unnamed")
  public static final String DEFAULT = new String("unnamed");
  private final String name;

  public Person(String name) {
    this.name = name == null ? DEFAULT : name;
  }

  public String getName() {
    return name;
  }
}

ForceMakeModifiable.java

/**
 * Force the field to be modifiable and accessible.
 */
protected static void makeModifiable(Field nameField) throws Exception {
  nameField.setAccessible(true);
  int modifiers = nameField.getModifiers();
  Field modifierField = nameField.getClass().getDeclaredField("modifiers");
  modifiers = modifiers & ~Modifier.FINAL;
  modifierField.setAccessible(true);
  modifierField.setInt(nameField, modifiers);
}

MakeModifiable.java

/**
 * Force the field to accessible.
 */
protected static void makeModifiable(Field nameField) throws Exception {
  nameField.setAccessible(true);
}

Person.java

/**
 * An immutable data class.
 */
class Person {
  private final String name;
  
  public Person(String name) {
    this.name = name;
  }
  
  public String getName() {
    return name;
  }
}