How do I get string values from ArrayList and store them in a comma separated single string in Java 8?

I have an ArrayList with some strings I want to store the list of numbers in ArrayList in a single string separated by commas, as shown below

String s = "350000000000050287,392156486833253181,350000000000060764"

Here is my list:

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

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

I have been trying to achieve this by:

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

        s.append(id+",");

    }

The only problem is that this will add a comma at the end, which I don't want What is the best way?

thank you

Solution

The simplest 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 can fix the code using iterators:

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 just use Boolean values 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 is obviously not as good as using iterators to compare the bytecode generated by each method However, as tagir valeev pointed out, this may not be true: this benchmark shows us that the performance of using flags is higher, with multiple iterations starting from 10

If anyone can explain why, I'm glad to know