longest common substring

public static String lcs(String a, String b){
   int aLen = a.length();
   int bLen = b.length();
   if(aLen==0 || bLen==0)
        return "";

   if(a.charAt(aLen-1)==b.charAt(bLen-1))
   {
       return lcs(a.substring(0,aLen-1),b.substring(0.bLen-1))+a.charAt(aLen-1);
   }else{
       String x = lcs(a.substring(0,aLen-1),b);
       String y = lcs(a,b.substring(0,bLen-1));
       return (x.length()>=y.length()) ? x : y;
   }

}