错误！！！ NumberFormatException：对于输入字符串：“”

Question

I'm getting these issues:

java.lang.NumberFormatException: For input string: ""
Exception in thread "main" java.lang.NullPointerException

System.out.println("Amount of elements to calculate: ");
try
{
   x = Integer.parseInt(br.readLine()); 
}
catch(NumberFormatException | IOException z)
{
   System.out.println("Error!!!"+z);
}

int [] n = new int[x];

this is how I'm reading the values of the array:

for(int i=0; i<n.length; i++)
    n[i] = Integer.parseInt(br.readLine()); 

this is how I call the method and send the array as a parameter

obj.asignar(n); 

and this is the method in my class in which I load the array:

private int[] num;

public void asignar(int n[])
{

    for(int i=0; i<n.length; i++)
    {
        num[i] = n[i];
    }
}

Answer 1

You cannot get an integer from an empty String :

Integer.parseInt(br.readLine());   // when br.readLine() is ""

but you can use 2 common approaches:

1. Skip the empty String s:

    String line = br.readLine(); if ( !"".equals(line)) { x = Integer.parseInt(line); }

2. Use a default value for the empty String - like 0 :

    String line = br.readLine(); x = "".equals(line) ? 0 : Integer.parseInt(line);

   You can also generalize this approach by writing a new method:

    static int parseInt(String s, int defaultValue) { try { return Integer.parseInt(s); } catch (NumberFormatException e) { return defaultValue; } }

   that can be called using:

    x = parseInt(br.readLine(), 0);

---

Related to your comment, it's easy to read as many elements as you want. Just define a total and use it in a for loop:

public static void main(String[] args) throws IOException {

    int total = 3;
    int n[] = new int[total];
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

    for (int i = 0; i < total; i++) {
        n[i] = Integer.parseInt(br.readLine()); // use the approaches above
    }

    System.out.println(Arrays.toString(n));
}

Eg:

If your input is

1
2
3

the output will be

[1, 2, 3]