Convert a List<String> in a String with all the values of the List comma separated using Java 8 is really straightforward. Let’s have a look on how to do that.

Java 8 (or above)

We simply can write String.join(..), pass a delimiter and an Iterable, then the new StringJoiner will do the rest:

List<String> cities = Arrays.asList("Milan",
                                    "London",
                                    "New York",
                                    "San Francisco");

String citiesCommaSeparated = String.join(",", cities);

System.out.println(citiesCommaSeparated);
// prints 'Milan,London,New York,San Francisco' to STDOUT.

In case we are working with a stream we can write as follow and still have the same result:

String citiesCommaSeparated = cities.stream()
                                    .collect(Collectors.joining(","));

System.out.println(citiesCommaSeparated);

//Output: Milan,London,New York,San Francisco

Note: you can statically import java.util.stream.Collectors.joining if you prefer just typing joining(“,”).

Java 7

For the old times’ sake, let’s have a look at Java 7 implementation.

private static final String SEPARATOR = ",";

public static void main(String[] args) {
  List<String> cities = Arrays.asList(
                                "Milan",
                                "London",
                                "New York",
                                "San Francisco");

  StringBuilder csvBuilder = new StringBuilder();

  for(String city : cities){
    csvBuilder.append(city);
    csvBuilder.append(SEPARATOR);
  }

  String csv = csvBuilder.toString();
  System.out.println(csv);
  //OUTPUT: Milan,London,New York,San Francisco,

  //Remove last comma
  csv = csv.substring(0, csv.length() - SEPARATOR.length());

  System.out.println(csv);
  //OUTPUT: Milan,London,New York,San Francisco

As you can see it’s much more verbose and easier to make mistakes like forgetting to remove the last comma. You can implement this in several ways - for example by moving the logic that removes the last comma inside the for loop - but none will be so explicative and immediate to understand as the declarative solution expressed in Java 8.

The focus should be on what you want to do - joining a List of String - not on how.

Java 8: Manipulate String before joining

Before joining you can manipulate your String as you prefer by using map() or cutting some String out by using filter().  I’ll cover those topics in further articles. Meanwhile, this a straightforward example on how to transform the whole String in upper-case before joining them.

Java 8: From List to upper-case String comma separated

String citiesCommaSeparated = cities.stream()
                                    .map(String::toUpperCase)
                                    .collect(Collectors.joining(","));

//Output: MILAN,LONDON,NEW YORK,SAN FRANCISCO

If you want to find out more about stream, I strongly suggest this cool video of Venkat Subramaniam.

Let’s play

The best way to learn it’s playing! Copy this class with all the implementations discussed and play with that. There is already a small test for each of them 🙂

package net.reversecoding.examples;

import static java.util.stream.Collectors.joining;
import static org.junit.Assert.assertEquals;

import java.util.Arrays;
import java.util.List;

import org.junit.Test;

public class CsvUtil {
    private static final String SEPARATOR = ",";

    public static String toCsv(List<String> listToConvert){
        return String.join(SEPARATOR, listToConvert);
    }

    @Test
    public void toCsv_csvFromListOfString(){
        List<String> cities = Arrays.asList(
                "Milan", "London", "New York", "San Francisco");

        String expected = "Milan,London,New York,San Francisco";

        assertEquals(expected, toCsv(cities));
    }


    public static String toCsvStream(List<String> listToConvert){
        return listToConvert.stream()
                    .collect(joining(SEPARATOR));
    }

    @Test
    public void toCsvStream_csvFromListOfString(){
        List<String> cities = Arrays.asList(
                "Milan", "London", "New York", "San Francisco");

        String expected = "Milan,London,New York,San Francisco";

        assertEquals(expected, toCsv(cities));
    }

    public static String toCsvJava7(List<String> listToConvert){
        StringBuilder csvBuilder = new StringBuilder();

        for(String s : listToConvert){
            csvBuilder.append(s);
            csvBuilder.append(SEPARATOR);
        }

        String csv = csvBuilder.toString();

        //Remove last separator
        if(csv.endsWith(SEPARATOR)){
            csv = csv.substring(0, csv.length() - SEPARATOR.length());
        }

        return csv;
    }

    @Test
    public void toCsvJava7_csvFromListOfString(){
        List<String> cities = Arrays.asList(
                "Milan", "London", "New York", "San Francisco");

        String expected = "Milan,London,New York,San Francisco";

        assertEquals(expected, toCsvJava7(cities));
    }

    public static String toUpperCaseCsv(List<String> listToConvert){
        return listToConvert.stream()
                    .map(String::toUpperCase)
                    .collect(joining(SEPARATOR));
    }

    @Test
    public void toUpperCaseCsv_upperCaseCsvFromListOfString(){
        List<String> cities = Arrays.asList(
                "Milan", "London", "New York", "San Francisco");

        String expected = "MILAN,LONDON,NEW YORK,SAN FRANCISCO";

        assertEquals(expected, toUpperCaseCsv(cities));
    }
}