Here you can find the source of join(List
public static String join(List<String> list, String glue)
//package com.java2s; import java.util.List; public class Main { public static String join(List<String> list, String glue) { return join(list.toArray(new String[0]), glue); }// ww w. ja v a 2s . c o m public static String join(String[] s, String glue) { int k = s.length; if (k == 0) return null; StringBuilder out = new StringBuilder(); out.append(s[0]); for (int x = 1; x < k; ++x) out.append(glue).append(s[x]); return out.toString(); } }