java.lang.NumberFormatException: null When I try to read a number from a .txt -

try {         bufferedreader in = new bufferedreader(new filereader("encuestas/encuesta_"+s+".txt"));         try {             this.id = integer.valueof(in.readline());             this.genero = in.readline();             this.fecha = in.readline();             this.n_preguntas = integer.valueof(in.readline());             for(int = 0; < this.n_preguntas; ++i){                 integer tip = integer.valueof(in.readline());                 string aux = "";                  aux = in.readline(); 

when try read integer tip = integer.valueof(in.readline()); doesn't work , don't understand why... same before , working. error:

5. leer encuestajava.lang.numberformatexception: null @ java.lang.integer.parseint(integer.java:542) @ java.lang.integer.valueof(integer.java:766) @ prop.dominio.encuesta.leer(encuesta.java:134) @ prop.dominio.driver_encuesta.main(driver_encuesta.java:219) 

simply check this,

for(int = 0; < this.n_preguntas; ++i){        string line = null;     if((line = in.readline()) != null){         integer tip = integer.valueof(line);         string aux = "";     } 

if there ten lines in file, when reading 11th line got error