Konkatenacja bez użycia substringa

import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        
        List<String> companiesList = companiesList();
        String string = "";
        int length = 0;
        for (int i = 0; i < companiesList.size(); i++) {
            string = string + companiesList.get(i) + ", ";
            length = string.length();
        }
        System.out.println(string.substring(0, length - 2));
    }
    private static List<String> companiesList() {
        List<String> companies = new ArrayList<>();
        companies.add("DELL");
        companies.add("APPLE");
        companies.add("SAMSUNG");
        companies.add("SONY");
        companies.add("ALCATEL");
        return companies;
    }
}

// Util method for strings and other char sequences
List<String> strs = Arrays.asList("1", "2", "3");
String listStr1 = String.join(",", strs);

// For any type using streams and collectors
List<Object> objs = Arrays.asList(1, 2, 3);
String listStr2 = objs.stream()
    .map(Object::toString)
    .collect(joining(",", "[", "]"));

// Using the new StringJoiner class
StringJoiner joiner = new StringJoiner(", ", "[", "]");
joiner.setEmptyValue("<empty>");
for (Integer i : objs) {
  joiner.add(i.toString());
}
String listStr3 = joiner.toString();

package com;

import java.util.*;

public class Main {

    public static void main(String[] args) {

        List<String> companiesList = companiesList();
        String string = "";
        for (int i = 0; i < companiesList.size() - 1; i++) {
            string = string + companiesList.get(i) + ", ";
            if (i == companiesList.size() - 2) {
                string = string + companiesList.get(i + 1);
            }
        }
        System.out.println(string);
    }
    private static List<String> companiesList() {
        List<String> companies = new ArrayList<>();
        companies.add("DELL");
        companies.add("APPLE");
        companies.add("SAMSUNG");
        companies.add("SONY");
        companies.add("ALCATEL");
        return companies;
    }
}

import java.util.*;

public class WriteOnlyPL {
    public static void main(String[] args) {
        final List<String> companiesList = companiesList();
        final String value = myJoin(companiesList);
        System.out.println(value);
    }
    public static String myJoin(List<String> list) {
        final int size = list.size();
        if (size == 0) return "";
        if (size == 1) return list.get(0);
        
        final StringBuffer buffer = new StringBuffer(list.get(0));
        for (int i = 1; i < size; ++i) {
            buffer.append(", ").append(list.get(i));
        }
        return buffer.toString();
    }
    private static List<String> companiesList() {
        List<String> companies = new ArrayList<>();
        companies.add("DELL");
        companies.add("APPLE");
        companies.add("SAMSUNG");
        companies.add("SONY");
        companies.add("ALCATEL");
        return companies;
    }    
}