How to get String values from ArrayList and store them in a single string separated by commas, in Java 8?

Question

I have an ArrayList with some Strings. I want to store that list of numbers from the ArrayList in a single string separated by a comma like the following.

String s = "350000000000050287,392156486833253181,350000000000060764"

This is my list:

   List<String> e = new ArrayList<String>();

    e.add("350000000000050287");
    e.add("392156486833253181");
    e.add("350000000000060764");

I have been trying to do it the following way:

     StringBuilder s = new StringBuilder();
    for (String id : e){

        s.append(id+",");

    }

The only problem with this is that this adds a comma to the end and I do not want that.What would be the best way to this?

Thanks

Answer 1

The easiest solution is to use String.join:

List<String> list = new ArrayList<String>();

list.add("11");
list.add("22");
list.add("33");

String joined = String.join(",", list);

System.out.println(joined);
//prints "11,22,33"

Note that this requires Java 8.

---

However if you want to support older versions of Java, you could fix your code using an iterator:

StringBuilder sb = new StringBuilder();

Iterator<String> iterator = list.iterator();

// First time (no delimiter):
if (iterator.hasNext()) {
    sb.append(iterator.next());

    // Other times (with delimiter):
    while (iterator.hasNext()) {
        sb.append(",");
        sb.append(iterator.next());
    }
}

Or simply use a boolean to determine the first time:

StringBuilder sb = new StringBuilder();

boolean firstTime = true;

for (String str : list) {

    if (firstTime) {
        firstTime = false;
    } else {
        sb.append(",");
    }

    sb.append(str);
}

But the latter should obviously be less performant than using an iterator comparing the generated bytecode per method. However, this might not be true as Tagir Valeev pointed out: this benchmark shows us that using a flag is more performant with a number of iterations starting from 10.

If anyone could explain why this is the case, I'd be glad to know.