Varför java visar MalformedURLexception?

public class SoundPlayer extends Frame{
    URL u;
    AudioClip a;
    
    SoundPlayer (String filename){
        try{
            u = new URL(filename);
        }
        catch(MalformedURLException e){
            System.out.println("URL error:" + filename +"Could not play the audiofile");
            e.getStackTrace();
        }
        
        a = Applet.newAudioClip(u);
        a.loop();
        
        setSize(400,200);
        add(new Label("Play" + filename));
        setVisible(true);
    }
    
    public static void main (String arg[]){
        new SoundPlayer("hello.mp3");
    }
}

Exception in thread "main" java.lang.NullPointerException
	at sun.applet.AppletAudioClip.<init>(AppletAudioClip.java:65)
	at java.applet.Applet.newAudioClip(Applet.java:313)
	at Exempel_Kap15.SoundPlayer.<init>(SoundPlayer.java:25)
	at Exempel_Kap15.SoundPlayer.main(SoundPlayer.java:34)