如何读取整数并将其存储在Java中的数组中

Question

Sorry if this is an obvious problem.

I'm trying to read integers from users and store them in an array.

The thing is that I want to use arraylist, because the size of the input is not for sure

If I know the size then I know a way to do that, which is

class Test1 
{
    public static void main(String[] args) 
    {
        Scanner reader = new Scanner(System.in);
        System.out.println("Please input your numbers");

        int num;       // integer will be stored in this variable

        ArrayList<Integer> List = new ArrayList<Integer>();

        // for example if I know the size of the input is 5, 
        // then I read one single number and put it into the arraylist.
        for (int i = 0; i <= 4; i++) 
        {
            num = reader.nextInt();
            List.add(num);
        }
        System.out.println(List);
    }
}

How to do this if I don't know the size? Except for reading one number in each loop, is there any better way to do that? Can I use BufferedReader instead of Scanner?

Thanks a lot for any help!

Answer 1

You could change this

for (int i = 0; i <= 4; i++) 
{
  num = reader.nextInt();
  List.add(num);
}

to use Scanner.hasNextInt() with something like

while (reader.hasNextInt()) 
{
  num = reader.nextInt();
  List.add(num);
}

Answer 2

You cannot instanciate an array if you do not know its size.

Therefore your approach is the correct one: start with an ArrayList and when done adding to it, you can convert it to an array.

Answer 3

You can use the hasNextInt() in a while loop to keep going until there are no more numbers to read.

  while (reader.hasNextInt()) {
      List.add(reader.nextInt());
  }