Синтаксический анализатор

import java.util.StringTokenizer;
class MyAvt{
String letters[]={"A","B","C","D","E"};
int digit[]={1,2,3,4,5,6,7,8,9,0};
public boolean isLetter(String sIn){
for(String l:letters){
if(l.equals(sIn)) return true;
}
return false;
}
public boolean isDigit(String sIn){
for(int d:digit){
try{
if(d==Integer.parseInt(sIn)) return true;
 
}catch(Exception e){
System.out.println("Oshibka-ne chislo");
 
}
 
 
}
return false;
}
 
public void oper(String sIn){
StringTokenizer st=new StringTokenizer(sIn,"=",false);
while(st.hasMoreTokens()){
String StringStmp=st.nextToken();
if(isLetter(StringStmp))
  System.out.println("Mi vveli bykvu :"+StringStmp);
 
else if(isDigit(StringStmp))
  System.out.println("Mi vveli chislo :"+StringStmp);
else
  System.out.println("Oshibka-nevernaya stroka :"+StringStmp);
}
}
}
class Main{
public static void main(String args[]){
MyAvt a=new MyAvt();
a.oper("A=7");
a.oper("C=9");
a.oper("*=7");
}
}

catch (NumberFormatException e) {
        return false;
    }

private static boolean isDigit(String s) throws NumberFormatException {
    try {
        Integer.parseInt(s);
        return true;
    } catch (NumberFormatException e) {
        return false;
    }
}