Steven Yong's Weblog

In Java, a final field is generally considered a constant which can’t be modified once it is initialized. It’s partly true as most of the programmers know that reflection can break this rule. For example i have a class Person:

class Person {
  private final String name; 
  Person(String name) { this.name = name; } 
  public String toString() { System.out.println(“name: ” + name); }
}

And the code to test the “final” modification:

public static void main(String[] args) {
    Person jacky = new Person("Jacky"); // initialize the name with constructor
    Field nameField = Person.class.getDeclaredField("name");
    nameField.setAccessible(true);
    nameField.set(jacky, "Andy"); // try to modify the value here
    System.out.println(jacky);
}

The output is: name: Andy

Now what I’m able to modify the value of the final field, name. But if I initialize a value to the name during declaration:

class Person {
  private final String name = “Jacky”; 
  public String toString() { System.out.println(“name: ” + name); }
}

With this test:

public static void main(String[] args) {
    Person jacky = new Person(); // default constructor
    Field nameField = Person.class.getDeclaredField("name");
    nameField.setAccessible(true);
    nameField.set(jacky, "Andy"); // try to change the value here
    System.out.println(jacky);
} 

The output is: name: Jacky (the name field wasn’t changed)

Just want to share some funny points of Java with you.


PS: above code behaves differently with JDK 1.2, JDK 1.3, JDK 1.4 and JDK 5.