public class Parser{
    private final HashMap val <String, String>;
    private final File source;

    public Parser(final File path){
        /*initialise stuff*/
        val=new HashMap<>();
        source=path; //throws exception if null but omitted for brevity
        /* and finally parse the file */
        parse();
    }

public void parse() {
    final Scanner sc=new Scanner(source);
    while(sc.hasNextLine()){
        //do stuff
    }
    sc.close();
}

private void parseDecision(final String possibleStr){
    final Scanner sc;

    if(possibleStr==null) //use File
        sc=new Scanner(source); //remember source is a field
    else
        sc=new Scanner(possibleStr);
    /*...*/
}