Using Java 8 to convert a list of objects into a string obtained from the toString() method
One simple way is to append your list items in a StringBuilder
List<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);
list.add(3);
StringBuilder b = new StringBuilder();
list.forEach(b::append);
System.out.println(b);
you can also try:
String s = list.stream().map(e -> e.toString()).reduce("", String::concat);
Explanation: map converts Integer stream to String stream, then its reduced as concatenation of all the elements.
Note: This is normal reduction which performs in O(n2)
for better performance use a StringBuilder or mutable reduction similar to F. Böller's answer.
String s = list.stream().map(Object::toString).collect(Collectors.joining(","));
Ref: Stream Reduction
There is a collector joining in the API.
It's a static method in Collectors.
list.stream().map(Object::toString).collect(Collectors.joining(","))
Not perfect because of the necessary call of toString, but works. Different delimiters are possible.
Just in case anyone is trying to do this without java 8, there is a pretty good trick. List.toString() already returns a collection that looks like this:
[1,2,3]
Depending on your specific requirements, this can be post-processed to whatever you want as long as your list items don't contain [] or , .
For instance:
list.toString().replace("[","").replace("]","")
or if your data might contain square brackets this:
String s=list.toString();
s = s.substring(1,s.length()-1)
will get you a pretty reasonable output.
One array item on each line can be created like this:
list.toString().replace("[","").replace("]","").replaceAll(",","\r\n")
I used this technique to make html tooltips from a list in a small app, with something like:
list.toString().replace("[","<html>").replace("]","</html>").replaceAll(",","<br>")
If you have an array then start with Arrays.asList(list).toString() instead
I'll totally own the fact that this is not optimal, but it's not as inefficient as you might think and is pretty straightforward to read and understand. It is, however, quite inflexible--in particular don't try to separate the elements with replaceAll if your data might contain commas and use the substring version if you have square brackets in your data, but for an array of numbers it's pretty much perfect.