Bad example: Use SimpleDateFormat with multi threads

BreakDateFormat.java

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Collections;
import java.util.Date;
import java.util.HashSet;
import java.util.Set;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class BreakDateFormat {
    static SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSZ");

    public static void main(String[] args) throws ParseException, InterruptedException {
        final int numThreads = 10;
        final int numLoops = 10;
        final String dateString = "2001-07-04T12:08:56.235-0700";
        final Set<Date> parsedDates = Collections.synchronizedSet(new HashSet<>());

        ExecutorService executorService = Executors.newFixedThreadPool(numThreads);
        for (int i = 0; i < numLoops; i++) {
            executorService.submit(new Runnable() {
                @Override
                public void run() {
                    try {
                        for (int j = 0; j < numLoops; j++) {
                            parsedDates.add(dateFormat.parse(dateString));
                        }
                    } catch (ParseException e) {
                        e.printStackTrace();
                    }
                }
            });
        }
        executorService.shutdown();
        executorService.awaitTermination(5, TimeUnit.SECONDS);

        System.out.println("size=" + parsedDates.size());
        System.out.println("values:");
        for (Date date : parsedDates) {
            System.out.println(dateFormat.format(date));
        }
    }
}