Java - final variables

Question

I know that once a final variable has a value assigned to it, it cannot be changed. However I just have a couple of questions regarding this:

• When I have a field, say static final JButton button; outside a class, and then in the main method, try to assign it a value, button = new JButton("OK");, I get an error telling me to remove the final modifier? However since the original button variable does not yet reference an object I was under the impression I could assign it once?

• Secondly, if I completely remove reference to the button so I just have static final JButton button; outside the class, my IDE claims "The blank final field button may not have been initialised." Does this mean that all final fields must be initialised? And if so, must they be initialised there and then as I can't seem to initialise it later.

• Also, silly question, but my initial assumption that when a final variable is referenced to an instance or data type it can not be assigned to anything else is correct, right?

This code is not complete but is provided to illustrate my point:

public class FinalVarTester {

    static final JButton button;

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        Container container = frame.getContentPane();
        container.setLayout(new BorderLayout());
        button = new JButton("OK");
        container.add(button, BorderLayout.SOUTH);
    }

}

Solution 1

You should initialize a static final variable either in a static initializer, or directly. So either

static final JButton button = new JButton();

or

static final JButton button;

static {
  button = new JButton();
}

The Java language specification has some more documentation about it: the section about final variables specifies why you get the compile error:

It is a compile-time error if a final variable is assigned to unless it is definitely unassigned (§16) immediately prior to the assignment.

and chapter 16 talks about the definite assignment

Solution 2

final fields must be initialized indeed, as this will be their value for the rest of the program.

An exception to that is that a final variable may be initialized in the constructor. Since static fields don't belong to an instance they must be initialized (there is no constructor) direct initialization, or static initialization blocks are options for doing that.

Regarding your last question, yes, this is why it's called final.