public class Sample {
    public static void main(String[] args) {
        String[] arr = {"A","B","C","D"};

        // for 문 활용
        String s = "";
        for (String str : arr) {
            s += str;
        }
        System.out.println(s);
        // ABCD

        // String.join 활용
        String str = String.join("", arr);
        System.out.println(str);
        // ABCD

    }
}