Convert an array of Integer into a single String

Strings are immutable.. You can't append element to it

In this case, i would use StringBuilder

With StringBuilder you can append element :

    int[] myArr = {4,3,4,53,2};
    StringBuilder myStringB = new StringBuilder();
    String myString = new String();

    for (int i : myArr) {
        myStringB.append(i);
    }

    myString = myStringB.toString();

You were almost ready with the solution, just that you need to supply the proper arguments.

If you look at the public static String join(CharSequence delimiter, Iterable<? extends CharSequence> elements) method signature it takes a delimiter and an iterable. You were almost close to joining the strings using this method, all you had to do is to provide an Iterable<String>.

Since you start with a List<Integer> you need to convert it to an Iterable<String> to do that you can use the below code:

Iterable<String> iterable = ids.stream().map(String::valueOf).collect(Collectors.toList());

Where ids is a List<Integer>. Note that you need to do a map(String::valueOf) as you start with a List<Integer> not with a List<String>. Now that you have an Iterable<String> you can use that as the second argument of String.join().

So the complete solution is:

List<Integer> ids = new ArrayList<>();
ids.add(1);
ids.add(2);
ids.add(3);

Iterable<String> iterable = ids.stream().map(String::valueOf).collect(Collectors.toList());
String result = String.join(" ", iterable);
System.out.println(result);

Output:

1 2 3

If the ids may contain null then you can filter it out before applying the map():

Iterable<String> iterable = ids.stream()
                               .filter(Objects::nonNull)
                               .map(String::valueOf)
                               .collect(Collectors.toList());

If you do not want to use StringBuilder then you can you the following simple for loop:

    List<Integer> ids = new ArrayList<Integer>();
    ids.add(3);
    ids.add(22);
    ids.add(1);
    ids.add(5);

    String st = "";

    for(int i = 1; i <= ids.size(); i++) {
        st += String.valueOf(ids.get(i-1));
        if(i != ids.size()) {
            st += " ";
        }
    }

    System.out.println(st);

If performance matters for you or if you have a very large list I would recommend using a StringBuilder and Java Streams to collect all numbers:

String joined = ids.stream()
        .collect(StringBuilder::new, 
                (s, i) -> s.append(" ").append(i), 
                (s1, s2) -> s1.append(" ").append(s2))
        .substring(1);

This prepends a " " to all numbers. To get the final result we can use StringBuilder.substring() to cut off the first white space.

To improve the performance on large lists you can also use List.parallelStream() or Stream.parallel() to create a parallel stream.

Try this

String numberString = ids.stream().map(String::valueOf).collect(Collectors.joining(" "));