ArrayList, String

We often use ArrayLists to store data in Java programs. We add strings and Integers to these collections. We can convert these lists into strings.

For an ArrayList of Strings, we can use String.join. But for other types like Integers, a StringBuilder is a clearer approach. We can append and add delimiters.

Example, String.join

Here we have an ArrayList collection that contains String elements. We add three simple strings to it (cat, dog and bird).

import java.util.ArrayList;

public class Program {
    public static void main(String[] args) {
        // Create an ArrayList and add three strings to it.
        ArrayList<String> arr = new ArrayList<>();
        arr.add("cat");
        arr.add("dog");
        arr.add("bird");

        // Convert the ArrayList into a String.
        String res = String.join(",", arr);
        System.out.println(res);
    }
}
cat,dog,bird

Join, empty delimiter

Sometimes we want to combine just the strings in an ArrayList, with no delimiters between them. We use an empty string literal as the delimiter.

import java.util.ArrayList;

public class Program {

    public static void main(String[] args) {

        // Append three Strings to the ArrayList.
        ArrayList<String> list = new ArrayList<>();
        list.add("abc");
        list.add("DEF");
        list.add("ghi");

        // Join with an empty delimiter to concat all strings.
        String result = String.join("", list);
        System.out.println(result);
    }
}
abcDEFghi

StringBuilder, integers

For an ArrayList containing numbers, not strings, we can convert to a string with a StringBuilder. We use a for-loop.

import java.util.ArrayList;

public class Program {

    static String convertToString(ArrayList<Integer> numbers) {
        StringBuilder builder = new StringBuilder();
        // Append all Integers in StringBuilder to the StringBuilder.
        for (int number : numbers) {
            builder.append(number);
            builder.append(":");
        }
        // Remove last delimiter with setLength.
        builder.setLength(builder.length() - 1);
        return builder.toString();
    }

    public static void main(String[] args) {
        // Create an ArrayList of three ints.
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(10);
        numbers.add(200);
        numbers.add(3000);

        // Call conversion method.
        String result = convertToString(numbers);
        System.out.println(result);
    }
}
10:200:3000

For a simple string ArrayList, String.join is the best method to use. But for more complex objects or even numbers, a StringBuilder loop is a good option.