You need to read more about Reading from input streams. You can start here, but there are other wonderful tutorials out there.

Not exactly answering your question (because it's still ambiguous), but in the following I've read 5 characters from input and output the sorted character array. This will help you get started.

public static void main(String[] args) throws IOException {

        char arr[] = new char[5];
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            System.out.println("Enter input characters and press enter: "); 
        for(int i=0; i<arr.length; i++) {
            arr[i] = (char) br.read();
        }       
        br.close();
        Arrays.sort(arr);
            System.out.println("Sorted Array: "+String.valueOf(arr));   
    }

Console I/O:

Enter input characters and press enter:
zeros
Sorted Array: eorsz

Further as Rohit has already mentioned you should look up the String class and its extensive utilities functions. Also, as you also mention that you don't probably know the number of elements in your array, you might want to look at it as a List, in which case you should use List<Character>.

You just must convert the character to String and then use something like this:

int sum = Integer.parseInt(stringNum1) + Integer.parseInt(stringNum2);

Your fundamental problem seems to be that you don't seem to understand how arrays work in Java.

In Java, arrays have a fixed length, and each location can hold only one of the declared data type. So for instance, if you have this char[]: [w, h, a, t, s, , u, p], its length will permanently be fixed at 8.

You can replace the character in a given location, but if you do that more than once, you will just overwrite the value.

    char[] characterStr = {'w','h','a','t','s',' ','u','p'};
    characterStr[5] = 'h'; // [w, h, a, t, s, h, u, p]
    characterStr[5] = 'i'; // [w, h, a, t, s, i, u, p]

Also, you cannot use += to try to append a value:

    char[] characterStr = {'w','h','a','t','s',' ','u','p'};
    characterStr += 'h'; // compile error! operator += is undefined for these arguments

You have to assign a value to a given location, using the arrayname[index] = value; syntax.

What you need to do (as mentioned in the comments), is first calculate the size of the array you are trying to create, by counting the number of spaces in the original array. You are already looping through and looking for spaces, so you just need to replace the code where you try to modify the array with a count, and create the output array based on that size:

    int size = input.length;
    for (char ch: input) {
        if (ch == ' ') {
            size += 1;
        }
    }
    char[] total = new char[size];

Also note that I didn't just declare total as a variable of type char[], but I used the new keyword to actually create it. Like other objects, arrays in Java need to be explicitly created, or you will end up with a NullPointerException when you try to use it.

Once you have created your new array, you can loop through your original input array and copy the non-space characters over (or 'h' and 'i' when you see a space).

You won't want to use a for-each loop for this, since you'll need to manually keep track of the array indices. (Hint: the index into the input array and the index into the output array won't be the same once you see a space).

There is no add method for arrays. Arrays are not resizeable. If you indeed want to use a char[] array, you need to allocate one that is large enough, e.g. by counting the uppercase letters or simply by allocating a array that is surely large enough (twice the String length minus 1).

String input = ...

String outputString;

if (input.isEmpty()) {
    outputString = "";
} else {
    char[] output = new char[input.length() * 2 - 1];
    output[0] = input.charAt(0);
    int outputIndex = 1;
    for (int i = 1; i < input.length(); i++, outputIndex++) {
        char c = input.charAt(i);
        if (Character.isUpperCase(c)) {
            output[outputIndex++] = ' ';
            output[outputIndex] = Character.toLowerCase(c);
        } else {
            output[outputIndex] = c;
        }
    }
    outputString = new String(output, 0, outputIndex);
}

System.out.println(outputString);

Or better still use a StringBuilder

String input = ...

String outputString;

if (input.isEmpty()) {
    outputString = "";
} else {
    StringBuilder sb = new StringBuilder().append(input.charAt(0));

    for (int i = 1; i < input.length(); i++) {
        char c = input.charAt(i);
        if (Character.isUpperCase(c)) {
            sb.append(' ').append(Character.toLowerCase(c));
        } else {
            sb.append(c);
        }
    }
    outputString = sb.toString();
}

System.out.println(outputString);

Using arrays in that case is not the best choice because arrays have a fixed size and you cannot dynamically add and remove items, maybe rather choose lists.

If you need a simple introduction, here you are:
https://www.geeksforgeeks.org/list-interface-java-examples/

Here's how you convert a String to a char array:

String str = "someString"; 
char[] charArray = str.toCharArray();

I'd recommend that you use an IDE when programming, to easily see which methods a class contains (in this case you'd be able to find toCharArray()) and compile errors like the one you have above. You should also familiarize yourself with the documentation, which in this case would be this String documentation.

Also, always post which compile errors you're getting. In this case it was easy to spot, but when it isn't you won't be able to get any answers if you don't include it in the post.