Help with File() in java

1

I want to check many file in my Folder.
It is very slow.
I want Increase a speed.


 for(f1='a';f1<='z';f1++)
                {
                        for(f2='a';f2<='z';f2++)
                        {
                                for(f3='a';f3<='z';f3++)
                                {       
                                        for(f4='a';f4<='z';f4++)
                                        {       
                                        
                                                File f=new File("image/"+String.valueOf(f1)+String.valueOf(f2)+String.valueOf(f3)+String.valueOf(f4)+".jpg");
                                                if(f.canRead())
                                                {
                                                if(i<=19)
                                                {
                                                no*=("image/"+String.valueOf(f1)+String.valueOf(f2)+String.valueOf(f3)+String.valueOf(f4)+".jpg");
                                                i++;
                                                }
                                                }
                                        }
                                }       
                        }
                }

Help me.