error when adding string value to array index [duplicate]

As I was saying in the comments, temp2 is the wrong type. You declared it as an array of ints, so println, of course, is treating it as an array of numbers, not of printable characters.

Just change temp2's type to char[]:

public class aa {
    public static void main(String[] args) {
        int number = 521;
        String temp1 = "" + number;
        int result = 0;
        char[] temp2 = new char[temp1.length()];
        for(int i=0; i<temp1.length(); i++){
            int len = temp1.length();
            temp2[i] = temp1.charAt(len-i-1);
            System.out.println(temp2[i]);
            System.out.println(temp1.charAt(len-i-1));
        }

        // print the whole reversed number on one line
        for(char c : temp2) {
            System.out.print(c);
        }

        System.out.println();
    }
    
}

Output

1
1
2
2
5
5
125

Of course this is not the optimal solution, just a way to fix the code you wrote so that it works. See Alex Rudenko's comment for a link with better solutions to the problem of reversing the digits of a number.

As temp2 is an array of int values, the following assignment needs to be fixed:

temp2[i] = temp1.charAt(len-i-1);

because it assign an ASCII value of char.

This can be done by subtracting a 0 or by using Character::getNumericValue

temp2[i] = temp1.charAt(len-i-1) - '0';
// or
temp2[i] = Character.getNumericValue(temp1.charAt(len-i-1));

A non-negative integer number may be reversed without using any String conversion:

1. The "length" of temp2 array is defined by Math.log10
2. Each digit is calculated with modulo operator % and dividing the input while it is greater than 0

public static void main(String[] args) {
    int number = 521;

    int len = number < 2 ? 1 : (int) Math.ceil(Math.log10(number));
    
    int[] temp2 = new int[len];
    
    for (int i = 0, n = number; n > 0; n /= 10, i++) {
        
        temp2[i] = n % 10;
        
        System.out.println(temp2[i]);
    }
    for (int i : temp2) {
        System.out.print(i);
    }
    System.out.println();
}

Output

1
2
5
125

You question is not very clear about if you are asking for opinion. But a query that "when I add that string value to array, it goes wrong way."

So , String is a reference type, while your array is of int type. You need to parse the value from String to int

Integer.parseInt(String)