How to synchronize usage of stream providing direct reference on it? One of possible solutions

Sample.java

import java.io.PrintStream;

public class Sample {
    public static final int COUNT = 10000;

    public static void main(String[] args) {
        final StreamHolder holder = new StreamHolder(System.out);

        new Thread(new Runnable() {
            @Override
            public void run() {
                threadOne(holder);
            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                threadTwo(holder);
            }
        }).start();
    }

    private static void threadOne(StreamHolder holder) {
        final SampleWorkerOne worker = new SampleWorkerOne();

        for (int i = 0, n = COUNT; i < n; ++i) {
            worker.head = "(my head " + i + ")";
            worker.body = "(my body " + i + ")";
            holder.doWithStream(worker);

            sleep(20);
        }
    }

    private static void threadTwo(StreamHolder holder) {
        final SampleWorkerTwo worker = new SampleWorkerTwo();

        for (int i = 0, n = COUNT; i < n; ++i) {
            worker.buffer = "(my buffer " + i + ")";
            holder.doWithStream(worker);

            sleep(30);
        }
    }

    private static void sleep(long millis) {
        try {
            Thread.sleep(millis);
        } catch (Throwable ex) {
            // ignore
        }
    }
}

interface StreamWorker {
    void work(PrintStream out);
}

class StreamHolder {
    private PrintStream mOut;

    public StreamHolder(PrintStream out) {
        mOut = out;
    }

    public synchronized void doWithStream(StreamWorker worker) {
        worker.work(mOut);
    }
}

class SampleWorkerOne implements StreamWorker {
    String head;
    String body;

    @Override
    public void work(PrintStream out) {
        out.print("Head: ");
        out.print(head);
        out.print(" Body: ");
        out.print(body);
        out.print(" : ");

        for (int i = 0, n = 20; i < n; ++i) {
            out.print(i);
            out.print('.');
        }

        out.println();
    }
}

class SampleWorkerTwo implements StreamWorker {
    String buffer;

    @Override
    public void work(PrintStream out) {
        out.print("Buffer: ");
        out.print(buffer);
        out.print(" : ");

        for (int i = 0, n = 20; i < n; ++i) {
            out.print(i);
            out.print('.');
        }

        out.println();
    }
}

This is example output without synchronized keyword:

Buffer: (my buffer 48) : 0.1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.
Head: (my head 73) Body: (my body 73) : 0.1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.
Head: (my head 74) Body: (my body 74) : 0.1.2.3.4.5.6.7.8.9.10Buffer: (my buffer 49) : .11.12.130.1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18..14.15.16.1719..
18.19.
Head: (my head 75) Body: (my body 75) : 0.1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.
Buffer: (my buffer 50) : 0.1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.
Head: (my head 76) Body: (my body 76) : 0.1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.