程序不将值存储在数组中

Question

Was trying to make a sorting program of integers entered by user and wanted it to take as many integer user wants, without first asking how many int he wanna sort , but while taking input it just crashes , basically the problem is it is not storing values in array

import java.util.Scanner;
class apple {
    public static void main(String[] args){
        Scanner scan = new Scanner(System.in);
        int j = 0;
        int[] arr;
        arr = new int[j];
        while (true) {
            arr[j] = scan.nextInt();
            if (arr[j]==(int)'\n'){break;}
            j++;
            arr = new int[j];
        }
        for(j = 0 ;j <  arr.length ;j++){
            System.out.print(arr[j] + "    ");
        }

    }
}

Answer 1

Arrays

First, use Scanner.hasNextInt() in conjunction with Scanner.nextInt() . Next, you are creating an array of size 0 ; it can't store any values unless you first copy it ¹ (because Java arrays are fixed length). You can use Arrays.copyOf(int[], int) to do that. And, you can use Arrays.toString(int[]) to display your array after you fill it. Something like,

Scanner scan = new Scanner(System.in);
int j = 0;
int[] arr = new int[j];
while (scan.hasNextInt()) {
    arr = Arrays.copyOf(arr, ++j);
    arr[j - 1] = scan.nextInt();
}
System.out.println(Arrays.toString(arr));

Collections

Java Collections (like ArrayList ² ) are more flexible, allowing a variable number of elements. Sadly they cannot operate directly with primitive types ³ , but can be used to store the corresponding wrapper classes . Something like,

List<Integer> al = new ArrayList<>();
while (scan.hasNextInt()) {
    al.add(scan.nextInt()); // <-- int will be autoboxed to Integer
}
// int j = al.size(); // <-- like array.length, if you want to know how
// many numbers were read.
System.out.println(al); // ArrayList, unlike arrays, overrides toString

^{1 _{Which has a cost.}
2 _{A collection backed by an array.}
3 _{Although primitive types can be autoboxed .}}

Answer 2

@Elliot Frisch explains how to do this with an array that you "grow" each time you want to add element by reallocating and copying the array.

A simpler idea is to replace the int[] arr; with

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

and then you can just use list.add(scan.nextInt()); to add the integer to the list. The ArrayList takes care of "growing" the storage behind the scenes. (A LinkedList would work just as well here.)