Java How to join List String with commas

In Java, we can use String.join(",", list) to join a List String with commas.

1. Java 8

1.1 String.join

package com.mkyong;
import java.util.Arrays;
import java.util.List;
public class JavaStringExample1 {
    public static void main(String[] args) {
        List<String> list = Arrays.asList("a","b","c");
        String result = String.join(",", list);
        System.out.println(result);

Output

a,b,c

1.2 Stream Collectors.joining

package com.mkyong;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class JavaStringExample2 {
    public static void main(String[] args) {
        List<String> list = Arrays.asList("a", "b", "c");
        String result = list.stream().collect(Collectors.joining(","));
        System.out.println(result);

Output

a,b,c

2. In Old Days

Create a custom method to join the String with separator manually.

package com.mkyong;
import java.util.Arrays;
import java.util.List;
public class JavaStringExample3 {
    public static void main(String[] args) {
        System.out.println(join(",", Arrays.asList("a")));
        System.out.println(join(",", Arrays.asList("a", "b")));
        System.out.println(join(",", Arrays.asList("a", "b", "c")));
        System.out.println(join(",", Arrays.asList("")));
        System.out.println(join(",", null));
    private static String join(String separator, List<String> input) {
        if (input == null || input.size() <= 0) return "";
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < input.size(); i++) {
            sb.append(input.get(i));
            // if not the last item
            if (i != input.size() - 1) {
                sb.append(separator);
        return sb.toString();

Output

a,b
a,b,c
//empty

References

1. Java 8 – StringJoiner example