private static final fields

Question

I just would like to clarify this. What is the difference between -

private static int STUDENT_AGE = 18; 

and

private static final int STUDENT_AGE = 18; 

within the field?

Jon Skeet explained as "not related to a particular instance at all", ok I think I understand it. Then what does final do in this case exactly?

Below code does not work is it because student age is assigned as static final? Does it mean the default age can not be overwritten at all? then is it possible to create the constructor that specify an age other than the default?

private String myName;
private String myAddress;
private int myYearEnrolled;
private String myGender; 
private static final int STUDENT_AGE = 18; 

public Student(String name, String address, int year, String gender) {
    myName = name;
    myAddress = address;
    myYearEnrolled = year;
    myGender = gender; 
}

   public Student(int age)
{
     STUDENT_AGE = age; 
}

Answer 1

 private static final int STUDENT_AGE = 18;

It's constant declaration. You can't change the value.

private static int STUDENT_AGE = 18;

It's a static declaration but not constant. The value can be changed.

Answer 2

static means "not related to a particular instance at all" - final means you cannot change this value after initialization and this value must be initialized.

The combination of final and static gives you the ability to create constants. This is no longer recommended in a public way (totally ok for e.g. magic numbers in a private context) as it's not typesafe. Use Enum post java 1.5 or create your own typesafe enums pre java 1.5 as suggested in Joshua Blochs Effective Java and this question.

Remark: reading this about a year later, I think I need to emphasize that there is nothing wrong with public static final fields in general, just that named constants should be implemented with enums or another type safe alternative.