在Java中分配最终字段

Question

I am working on a class that has some fields with final access modifier like: final textField and I am allowed to assign to them forsome reason. When I change them to static final I can no longer assign anything to them (it complains that they are final like it should have done in the first place). Any ideas why this is happening ?

Example for the first case:

final LabelField label_title;

label_title = new LabelField(
        "Press the button to launch the speed test",
        LabelField.FIELD_HCENTER);

Answer 1

You can assign final fields in constructor, you can't assign static final fields in a constructor. You shouldn't change static fields in a constructor in any case.

Answer 2

When a field is defined as final , it has to be initialised when the object is constructed, ie you're allowed to assign value to it inside a constructor.

A static field belongs to the class itself, ie one per class. A static final field is therefore not assignable in the constructor which is one per object.

Hope it makes sense to you!

Answer 3

The value of final members can't be changed. But it is allowed to initialize a final instance field in the constructor. This is not allowed for class members. The next snippets shows what's allowed and what's not allowed:

public class Final {

    final static Integer INT1;  // compile error
    final static Integer INT2 = new Integer(2);
    final Integer int3;
    final Integer int4 = new Integer(4);

    public Final() {
        int3 = new Integer(3);
        int3 = new Integer(3); // compile error
    }
}

Answer 4

From the JLS - final fields : -

A field can be declared final (§4.12.4). Both class and instance variables (static and non-static fields) may be declared final.

It is a compile-time error if a blank final (§4.12.4) class variable is not definitely assigned (§16.8) by a static initializer (§8.7) of the class in which it is declared.

A blank final instance variable must be definitely assigned (§16.9) at the end of every constructor (§8.8) of the class in which it is declared; otherwise a compile-time error occurs.

So, JLS clearly specifies that you should assign your static final fields in static initializer block. You can't assign them in any constructor.

So, if you have a static final field, you should either initialize them in place, or you can use a Static Initializer Block to initialize them.

Moreover, you can assign your final fields in constructor, provided you don't change the assignment later on anywhere..

Answer 5

A static field belongs to the class, which is (should be) initialized right after the class is loaded, and that is when it must be initialized (the value needs to be assigned) if it is final.

Constructor is called when someone needs to instantiate the class. Now if you change the value of that static final field in the constructor means that you are trying to change (or re-assign) it's value.

But doing that in a static initializer ( static { /* assign value here */ } ) should be fine, which is meant for initialization of the class.