Should a private final field be static too?

Question

I was wondering, if I have this field in my class : private final int foo = ..., should I put it in static private static final int foo = ...? Beca

Answer 1

If every instance of your class should have the same immutable value for foo, then you should make foo final and static. If each instance of your class can have a different (but still immutable) value for foo, then the value should just be final.

However, if every instance of your class should have the same immutable value for foo, then it is a really a constant. By convention, that is typically coded as follows:

private static final int FOO = ...

Note the caps to denote a constant...

Answer 2

There is a big difference between the two non-access modifiers.

• final means your variable can be assigned a value once.
• static sets its scope to pertain to the class (rather than an instance or a local context).

There is no reason why they should go together unless by design.

A static final variable is de facto a constant.

For instance, fields declared in interfaces are implicitly public, static and final.

Amongst the usage examples for final, non static fields you can find:

• fields declared outside an anonymous class and being referenced inside it
• local fields (declared in a method's body) being referenced inside a local class (declared in the same method's body)
• etc...

Answer 3

if you initiate its value in the constructor then it should not be static like

private final int foo;

public MyClass(int m){
    foo=m;
}

because its value depends on entry value.

---

but if you initiate its value inline like

private final int foo = 100;

then its preferred to be static to have just one instance of it, since the final field will be created on each instance of class; but static will be created once.

Answer 4

It is a constant and you will not want to have one copy of the variable for each instance of the class,so make it static. Also, if you want to use them from a static method, make it static.

Static variables are those variables which are common for all the instances of a class.If one instance changes it, then value of static variable would be updated for all other instances.

If you do not want this behavior, keep it non-static.

Answer 5

A final member can still be set by the constructor: therefore, if each instance of your class can set foo in the constructor and this value should pertain only to that instance, it should only be final, NOT static.

If however, foo is only set at declaration time, it might mean that this is a common value for all instances and you would win a little memory by declaring it static also. Beware though that if foo was not a primitive but a reference member, final only means that the reference can't change, not the content of the object, therefore a final and non static member that is a reference should not automatically be static. You could want one immutable reference per instance of your class, with different object state.