Using Java 8 to convert a list of objects into a string obtained from the toString() method

Question

There are a lot of useful new things in Java 8. E.g., I can iterate with a stream over a list of objects and then sum the values from a specific field of the Object

Answer 1

There is a method in the String API for those "joining list of string" usecases, you don't even need Stream.

List<String> myStringIterable = Arrays.asList("baguette", "bonjour");

String myReducedString = String.join(",", myStringIterable);

// And here you obtain "baguette,bonjour" in your myReducedString variable

Answer 2

Can we try this.

public static void main(String []args){
        List<String> stringList = new ArrayList<>();
        for(int i=0;i< 10;i++){
            stringList.add(""+i);
        }
        String stringConcated = String.join(",", stringList);
        System.out.println(stringConcated);

    }

Answer 3

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

Answer 4

List<String> list = Arrays.asList("One", "Two", "Three");
    list.stream()
            .reduce("", org.apache.commons.lang3.StringUtils::join);

Or

List<String> list = Arrays.asList("One", "Two", "Three");
        list.stream()
                .reduce("", (s1,s2)->s1+s2);

This approach allows you also build a string result from a list of objects Example

List<Wrapper> list = Arrays.asList(w1, w2, w2);
        list.stream()
                .map(w->w.getStringValue)
                .reduce("", org.apache.commons.lang3.StringUtils::join);

Here the reduce function allows you to have some initial value to which you want to append new string Example:

 List<String> errors = Arrays.asList("er1", "er2", "er3");
            list.stream()
                    .reduce("Found next errors:", (s1,s2)->s1+s2);

Answer 5

String actual = list.stream().reduce((t, u) -> t + "," + u).get();

Answer 6

The other answers are fine. However, you can also pass Collectors.toList() as parameter to Stream.collect() to return the elements as an ArrayList.

System.out.println( list.stream().map( e -> e.toString() ).collect( toList() ) );