Почему не работает со StringWriter или StingBuilder ?

public class Solution {
    public static volatile BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

    public static void main(String[] args) throws InterruptedException {
        Read3Strings t1 = new Read3Strings();
        Read3Strings t2 = new Read3Strings();

        t1.start();
        t1.join();

        t2.start();
        t2.join();

        t1.printResult();
        t2.printResult();
    }

    public static class Read3Strings extends Thread {
        public static StringWriter sw = new StringWriter();

        @Override
        public void run() {
            try {
                sw.append(reader.readLine() + " ");
                sw.append(reader.readLine() + " ");
                sw.append(reader.readLine());

            } catch (IOException e) {
                e.printStackTrace();
            }

        }

        public void printResult() {
            System.out.println(sw);
        }
    }
}