How to save a reference to a class depending on a conditional statement...

Question

Hi!
I'm rather new to Java, but I'm already in the process of creating a rather big application (big, from my point of view) :).
I have a main class, in which I find out what OS the application is running on (utilizing System.getProperty("os.name")). If the string corresponds to Windows (I'm using regular expressions to match this), I create an instance of another class called WindowsPlatform.java. And vice versa for Unix.
My problem is that I'm not really sure how to save a reference to the Unix or Windows platform class, as the program conditionally decides, depending on the platform, which class to invoke.
The error Java's giving me is:
MainClass.java:9 cannot find symbol
symbol  : method sayHi()
location: class Platform
        platform.sayHi();
                ^
1 errorWhat I want is to have the sayHi() method invoked from whatever class (UnixPlatform or WindowsPlatform) that was earlier in the program used as argument for the setPlatform() method in Initializer.java.
This is the the code involved (class names have been changed to easier reflect their purpose):
### MainClass.java ###
public class MainClass
    public static void main(String[] args) {
        SettingsClass settings = new SettingsClass();
     Initializer init = new Initializer(settings);
     Platform platform = settings.getPlatform();
     platform.sayHi();
### SettingsClass.java ###
public class SettingsClass
    private Platform platform;
    public void setPlatform(Platform _platform) {
        platform = _platform;
    public Platform getPlatform() {
        return platform;
### Initializer.java ###
public class Initializer
    private SettingsClass settings;
    private RegExpr re;
    private String os;
    public Initializer(SettingsClass _settings) {
        settings = _settings;
        re = new RegExpr();
     os = System.getProperty("os.name");
     if (re.find(".*win.*", os, true)) {
            settings.setPlatform(new WindowsPlatform());
     } else {
            settings.setPlatform(new UnixPlatform());
### Platform.java ###
public class Platform
    /* Some generic functions applicable to
     * all platforms
### RegExpr.java ###
import java.util.regex.*;
public class RegExpr
    private String regExpr;
    private String str;
    private boolean iCase;
    public boolean find(String _regExpr, String _str, boolean _iCase) {
        this.regExpr = _regExpr;
        this.str = _str;
     this.iCase = _iCase;
        Pattern pat;
     Matcher matcher;
     if (iCase) {
            pat = Pattern.compile(regExpr, Pattern.CASE_INSENSITIVE);
     } else {
            pat = Pattern.compile(regExpr);
     matcher = pat.matcher(str);
     if (matcher.find()) {
         return true;
     } else {
            return false;
### UnixPlatform.java ###
public class UnixPlatform extends Platform
    public void sayHi() {
     System.out.println("Hi, I'm Unix!");
### WindowsPlatform.java ###
public class WindowsPlatform extends Platform
    public void sayHi() {
     System.out.println("Hi, I'm Windows!");
}I would greatly appreciate any help I could get on this issue!
Thanks for your time!
Regards, ridgero

Answer 1

Dear Ridgero, your method must exist in your Platform class. Now you can only call this method from UnixPlatform or WindowsPlatform. The two easiest ways to resolv this would be something like:
abstract class Platform
    /* Some generic functions applicable to
     * all platforms
    abstract void sayHi();
}or this:
public class Platform
    /* Some generic functions applicable to
     * all platforms
    abstract void sayHi() {}
}Good luck.