How to format this output?

Easy and short-sighted solution: just trim the string before adding it to the ArrayList:

        maxarray.add(num.trim());

With this change the output is:

[1, 2 2, 3 3 3, 4 4 4 4, 5 5 5 5 5]

trim() removes trailing spaces (and leading, had there been any), but not the spaces between the numbers.

Produce correct output from the outset: However, rather than producing not-quite-right output and then correcting it it’s probably less confusing in the end to produce correct output from the outset. That is, without the space after the last number. This is also what the answer by Berto99 does. My preferred way of doing this is: take one iteration out of the inner loop and refrain from adding the space there.

    for (i = 1; i <= max; i++) {
        num = "";
        for (j = 1; j < i; j++) { // stop one number before i
            num = num + i + " ";
        }
        num = num + i; // add no space here
        maxarray.add(num);
    }

Output still is:

[1, 2 2, 3 3 3, 4 4 4 4, 5 5 5 5 5]

I find this way easier to read than the version with the extra if statement in the answer by Berto99.

For the sake of precision: maxarray is not an int array, it as an ArrayList of strings. And what we needed to trim was not the ArrayList but each string in it.

PS Stream version: For readers who know and like streams here’s a stream version. If you don’t want to learn about streams yet, just ignore.

    List<String> maxarray = IntStream.rangeClosed(1, max)
            .mapToObj(i -> IntStream.rangeClosed(1, i)
                    .mapToObj(j -> String.valueOf(i))
                    .collect(Collectors.joining(" ")))
            .collect(Collectors.toList());

One of the convenient things is that Collectors.joining(" ") puts spaces between the numbers without putting any space before the first or after the last, which solves the problem asked about.

As you can see from here:

ArrayList < String > maxarray = new ArrayList < String > ();
int i, j;
String num = "";

for (i = 1; i <= max; i++) {
    num = "";
    for (j = 1; j <= i; j++) {
        num = num + i + " ";
                        ^^^^-- here
    }
    //System.out.print(i);
    maxarray.add(num);
}
return maxarray;

you add a space after every number, so you only need to check if you are not in the last number, in order not to add the space in that case:

ArrayList < String > maxarray = new ArrayList < String > ();
int i, j;
String num = "";

for (i = 1; i <= max; i++) {
    num = "";
    for (j = 1; j <= i; j++) {
        num = num + i;
        if(j < i) // <--- check if you are not in the last iteration
           num = num + " ";
    }
    //System.out.print(i);
    maxarray.add(num);
}
return maxarray;