reading by keyboard of a registration of eight characters, the first being obligatory a letter

public ExMatricula() {
    }
     ExMatricula(String s){
         super(s);
     }
     ExMatricula(int mal_formada){
         this();mal_formada=mal_formada;
     }
     public int DimeProblema(){
         return mal_formada;
     }
    private boolean UnaLetra(String matricula){
        return matricula.substring(0,1).matches("[A-Za-Z]");
    }

public void caracteres(String  matricula)throws ExPropia{
    try{
     if (matricula.length()!=8) {
        throw new ExMatricula(ExMatricula.mal_tamaño);
    }else{
         if (!UnaLetra(matricula)) {
             throw new ExMatricula(ExMatricula.mal_letra);
         }
     }
    }catch(ExMatricula e) { 

 switch (e.DimeProblema()) { 

 case ExMatricula.mal_tamaño: 
 System.out.println("Tamanio incorrecto"); 
 break; 
 case ExMatricula.mal_letra: 
 System.out.println("Letra inicial no incluida"); 
 break; 
 default: 
 System.out.println("Matricula correcta"); 
 break; 
              } 
}
}