Error Converting String to Numbers in Java

Question

In my last post (do/while repeats the expression twice) was having an input overlay problem using the Scanner class, and according to the links placed in the answer, we cannot use nextLine() after nextInt().

Now I’m making another algorithm and falling into the same problem. According to the link solution used as a response, I used Integer.parseint() to solve the problem. It turns out that java.lang.Numberformatexception: For input string: ""

From what I understand searching in English is because the String is empty. Ok, it is. But it is because it returns the error even before I can type, and has a row before asking for the input of a value in the String.

As you can see in the image above, after I insert the third note it already gives the error, before I put the frequency.

Follows the code:

public static void main(String[] args) {

    Scanner read = new Scanner(System.in);
    DecimalFormat df = new DecimalFormat("0.00");

    String matricula;
    String resultado;
    String frequencia = "0";
    int freq;
    double nota1, nota2, nota3, notaFinal;
    double maiorNota;
    double menorNota;
    double mediaGeral = 0;
    int totalRep = 0;
    double percentRepFreq;

    for (int i = 1; i <= 5; i++) {

        System.out.print("Entre com a matrícula do aluno " + i + ": ");
        matricula = read.nextLine();

        System.out.print("Digite as 3 notas do aluno " + i + "\nNota 1: ");
        nota1 = read.nextDouble();
        System.out.print("Nota 2: ");
        nota2 = read.nextDouble();
        System.out.print("Nota 3: ");
        nota3 = read.nextDouble();

        System.out.print("Digite a frequência do aluno " + i + ": ");
        frequencia = read.nextLine();

        freq = Integer.parseInt(frequencia);
        if (freq < 40) {
            totalRep += 1;
        }

        notaFinal = (nota1 + nota2 + nota3) / 3;
        mediaGeral += notaFinal;

        if (notaFinal >= 6 && freq >= 40) {
            resultado = "Aprovado.";
        } else {
            resultado = "Reprovado.";
        }

        System.out.println("Aluno " + i + ", com matrícula " + matricula + ", teve frequência de " + frequencia
                + ", nota final: " + notaFinal + " e foi " + resultado);

        System.out.println(" ");
    }

    percentRepFreq = (int) totalRep / 5 * 100;

    System.out.println("Media geral da turma: " + mediaGeral);
    System.out.println("Total de alunos reprovados: " + totalRep);
    System.out.println(percentRepFreq + "% " + "dos alunos foram reprovados por frequência.");

}

}

Answer 1

This happens because nextDouble() (or nextInt(), nextFloat() etc) do not function in the same way as nextLine().

The nextLine() rescues the next line of your input, now nextDouble() rescues the next double of its input, and this need not necessarily be the entire content of the line.

For example: if in creating the Scanner you do:

Scanner read = new Scanner(System.in).useDelimiter(" ");

And in execution, when reporting the first note, instead of passing a single value, pass three values separated by space (e.g.: 1 2 3 (has one last space after the 3)), notes 2 and 3 will be captured automatic precisely because these methods do not read the whole line but only the next Double.

---

To solve the problem there are basically two options:

• Call the method nextLine() after each call from nextDouble()

• Or change from nextDouble() for nextLine() and do the conversions "in hand" (something like Double.parseDouble(val))

Answer 2

use next(); invez de nextLine() when picking up frequency

frequencia = read.next();