What is the significance of final instance variables?
class-variables, constants, instance-variables, java, static
Solution
So my question is, what is the point of having final instance variables if they can't have a different values per different instance.
They can, but those values can't be changed after creation. You can only assign values to final instance fields within instance variable initializers and constructors, and you must assign values to all instance fields that way.
Note that in your code here:
Foof()
{
whuffie=42; //compile time error
}
... the comment is incorrect. That assignment should be perfectly valid.
Final fields are useful for implementing immutability - which helps make it easy to reason about an object. For example, `String` is immutable, so if you validate a string and then keep a copy of the reference, you know that validation will still be correct later on.
Compare that with `java.util.Date`, where if you really want to have any faith in validation being useful, you need to create a defensive copy of the `Date` value and not provide the reference to any other code, in case it changes the underlying instant being represented.
Problem
I am having some confusion between `static final class variable` and `final instance variable`. Here is the sample code:- ``` class Foof{ final int size=3; final int whuffie; Foof() { whuffie=42; } public static void main(String [] args){ Foof obj1 = new Foof(); Foof obj2 = new Foof(); obj1.size=53; //compile time error System.out.println(obj1.size); obj2.whuffie=45; //compile time error System.out.println(obj2.whuffie); } } ``` Error:- ``` ankit@stream:/home/Data/JAVA/practice/src/test.com/test-20121031_static_demystified$ javac Foof.java Foof.java:14: error: cannot assign a value to final variable size obj1.size=53; //compile time error ^ Foof.java:16: error: cannot assign a value to final variable whuffie obj2.whuffie=45; ^ 2 errors ``` So my question is, what is the point of having final instance variables if they can't have a different values per different instance. I mean if they have same value for each instance then why don't we declare them as class variable(static final class variable) as this would serve the same purpose and the we don't need to create objects in order to access them. EDIT -1:- ``` class Foof{ final int size=3; final int whuffie; Foof() { whuffie=42; size = 23; //compile-time error. } public static void main(String [] args){ Foof obj1 = new Foof(); Foof obj2 = new Foof(); //obj1.size=53; System.out.println(obj1.size); //obj2.whuffie=45; System.out.println(obj2.whuffie); } } ``` Error:- ``` Foof.java:8: cannot assign a value to final variable size size = 23; ^ 1 error ``` According to the error, i can make out that first size is assigned a value =3 during object creation.