How do I read a file in Java char by char into an integer array?

As @Scary Wombat suggest, both of the max size of String and max size of array are Integer.MAX_VALUE. We can refer to String max size, Array max size and List max size. Note, the specific max size should be Integer.MAX_VALUE - 1 or -2 or -5 is irrelevant with this topic. For insurance purpose, we can use Integer.MAX_VALUE - 6.

I suppose your number is very large and the amount of character in the file may exceed the maximum of Integer.MAX_VALUE according to

I tried reading the file as an String then step through it char by char into an array but i think the file exceeded the String size limit.

To handle the maximum, I suggest you create another List to hold the integer. The core concept of it is like dynamic array but there are some differences. For dynamic array, you are applying for another memory space and copy current elements into that space. You can refer to the code below:

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.util.ArrayList;

public class ReadFile {
    public static void main(String args[]){
        try{        
            File file = new File("number.txt");
            FileReader fileReader = new FileReader(file);
            BufferedReader reader = new BufferedReader(fileReader);
            ArrayList<ArrayList<Integer>> listContainer = new ArrayList<ArrayList<Integer>>();
            ArrayList<Integer> list = new ArrayList<Integer>();
            int item;
            while((item = reader.read()) != -1){
                /*
                 * I assume you want to get the integer value of the char but not its ascii value
                 */
                list.add(item - 48);
                /*
                 * Reach the maximum of ArrayList and we should create a new ArrayList instance to hold the integer
                 */
                if(list.size() == Integer.MAX_VALUE - 6){
                    listContainer.add(list);
                    list = new ArrayList<Integer>();
                }
            }
            reader.close();
        }catch(Exception e){
            e.printStackTrace();
        }
    }
}