How to initialize constant such as “public static final Integer” in java from args[] in main method (or any)

Question

I'm basically a beginner in Java and I am trying to figure out this problem:

my project uses (integer)constant through many classes and I need to set up this constant from file/argument of programm and I have no idea how. I could remove "final" statement but that is against all conventions.

How to resolve this? What is best way to avoid it? Please help me :)

short example:

public class App {
    public static final int k;  
    public static void main( String[] args ) {
        k = Integer.parseInt(args[0]); // does not work ... sure but how? 
    }
}

EDIT: public STATIC void main ... (it was missing static)

Answer 1

You can only use this kind of functionality within the static { } block.

I'd recommend making the constant itself private, and accessible only through public static getter methods. This should be a suitable architecture.

Answer 2

You need to use a static{} block:

public class App {
    public static final int k;

    static {
        String kvalue = "";
        try
        {
            // Insert some code to open a file and read a value from it.
            kvalue = "<value from file>";
        }
        catch( Exception e )
        {
            // handle any exceptions opening the file
        }
        finally
        {
            k = Integer.parseInt( kvalue );
        }
    }

    public static void main( final String[] args ) {
        // do stuff
    }
}

Answer 3

Alternatively, you could have each class that requires this value access to a shared pool of data, the ApplicationContext , where the value X would reside as a constant:

public final class ApplicationContext {

    public static abstract class Worker {

        private final ApplicationContext mApplicationContext;

        public Worker(final ApplicationContext pApplicationContext) {
            this.mApplicationContext = pApplicationContext;
        }


        public final ApplicationContext getApplicationContext() {
            return this.mApplicationContext;
        }

    }

    private final int mX;

    ApplicationContext(final String[] pArgs) {
        this.mX = pArgs[0];
    }


    public final int getX() {
        return this.mX();
    }

}

By having all relevant classes extend ApplicationContext.Worker , we can ensure access to the constant value without having to rely upon a static implementation because all classes will receive a reference to ApplicationContext upon construction.

Answer 4

它不起作用，因为static final变量只能在静态块中初始化。