如何在读取文件时跳过空行（java）？

Question

I am trying to read this file, and then put it into a List:

23333
Manuel Oliveira
19

222222
Mário Santos
18

using Scanner. This is how I've done if Ignore the empty line between 19 and 222222.

public List<Aluno> lerFicheiro ( String nomeF) throws FileNotFoundException{
    List<Aluno> pauta = new LinkedList<Aluno>();
try{
    Scanner s = new Scanner ( new File ( nomeF));
    int cont = 0;
    String numero = null;
    String nome = null;;
    String nota;
    while ( s.hasNextLine())
    {
        if ( cont ==0)
        {
             numero = s.nextLine();
        }
        else
        {  
          if ( cont == 1) 
             nome = s.nextLine();
          else
          {   
              if ( cont == 2) 
              {
                  nota = s.nextLine();
                  cont =-1;
                  pauta.add( new Aluno(Integer.valueOf(numero), nome, nota)); 
              }
           }
        }
        cont++;
    }
    s.close();
}
catch ( FileNotFoundException e)
{
    System.out.println(e.getMessage());
}
return pauta;
}

But I've no idea how to still read it with the empty line. Thank you.

Answer 1

For a beginner, you have a good start of a rudimentary state machine there. There are two ways you can think about the empty line:

1. You're not interested in any empty line. So you just ignore them and don't change your cont .
2. You are specifically not interested in an empty line after the "nota". In this case, you should add this as a fourth state.

The advantage of the first approach is that even if there is more than one empty line in the file, or an empty line after the name, it will be ignored. The second approach is better if other empty lines that don't come after "nota" are useful.

I think it would be more elegant to read the line, and then decide what to do with it based on the cont . So for the first approach (ignore any empty line):

while ( s.hasNextLine()){

    // First read
    String theLine = s.nextLine();

    // Ignore it if it's empty. That is, only do something if
    // the line is not empty

    if ( ! theLine.isEmpty() ) {

        if ( cont == 0){
             numero = theLine; // You use the line you have read
        }else if ( cont == 1) {
             nome = theLine;
        }else {
             nota = theLine;
             cont =-1;
             pauta.add( new Aluno(Integer.valueOf(numero), nome, nota)); 
        }

        cont++;
    }

}

When you read an empty line, the state ( cont ) is not changed. So it's as if that line never existed.

Note that it would be nicer to use constants instead of just 0 , 1 , 2 , so that they will be meaningful.

Also note the way to properly write if...else if...else . You don't need to put them in curly braces all the time.

Answer 2

You could set delimiter to regex representing one or more line separators like (\\r?\\n)+ (or since Java 8 \\R+ ) and simply use next() method to read lines.

BTW: you should not be placing close() inside try block but in finally block. Or better use try-with-resources.

So if you are sure that file will always contain tokens build from 3 lines then your method can look like (I changed return type to List<String> for simplicity, but you should be able to change it back quite easily)

public static List<String> lerFicheiro(String nomeF)
        throws FileNotFoundException {

    List<String> pauta = new LinkedList<>();
    try (Scanner s = new Scanner(new File(nomeF))){
        s.useDelimiter("\\R+");

        while (s.hasNext()) {
            //since delimiter is line separator `next` will read lines
            String numero = s.next();
            String nome = s.next();
            String nota = s.next();
            pauta.add(numero + ":" + nome + ":" + nota);
        }
    } catch (FileNotFoundException e) {
        System.out.println(e.getMessage());
    }
    return pauta;
}