Convert an integer to an array of digits

I try to convert an integer to an array. For example, 1234 to int[] arr = {1,2,3,4};.

I've written a function:

public static void convertInt2Array(int guess)  {
    String temp = Integer.toString(guess);
    String temp2;
    int temp3;
    int [] newGuess = new int[temp.length()];
    for(int i=0; i<=temp.length(); i++) {
        if (i!=temp.length()) {
            temp2 = temp.substring(i, i+1);
        } else {
            temp2 = temp.substring(i);
            //System.out.println(i);
        }
        temp3 =  Integer.parseInt(temp2);
        newGuess[i] = temp3;
    }

    for(int i=0; i<=newGuess.length; i++) {
        System.out.println(newGuess[i]);
    }
}

But an exception is thrown:

Exception in thread "main" java.lang.NumberFormatException: For input string: ""
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)

at java.lang.Integer.parseInt(Integer.java:504)
at java.lang.Integer.parseInt(Integer.java:527)
at q4.test.convertInt2Array(test.java:28)
at q4.test.main(test.java:14)
Java Result: 1

How can I fix this?

The immediate problem is due to you using <= temp.length() instead of < temp.length(). However, you can achieve this a lot more simply. Even if you use the string approach, you can use:

String temp = Integer.toString(guess);
int[] newGuess = new int[temp.length()];
for (int i = 0; i < temp.length(); i++)
{
    newGuess[i] = temp.charAt(i) - '0';
}

You need to make the same change to use < newGuess.length() when printing out the content too - otherwise for an array of length 4 (which has valid indexes 0, 1, 2, 3) you'll try to use newGuess[4]. The vast majority of for loops I write use < in the condition, rather than <=.

You don't need to convert int to String. Just use % 10 to get the last digit and then divide your int by 10 to get to the next one.

int temp = test;
ArrayList<Integer> array = new ArrayList<Integer>();
do{
    array.add(temp % 10);
    temp /= 10;
} while  (temp > 0);

This will leave you with ArrayList containing your digits in reverse order. You can easily revert it if it's required and convert it to int[].