A behavior mismatch in AbstractCollection.toArray(T[] )

Sean Chou zhouyx at linux.vnet.ibm.com
Tue Dec 13 07:21:46 UTC 2011

Hi all,

    When I was reading the code of AbstractCollection.toArray(T[] ), I
found its
behavior maybe different from the spec in multithread environment. The spec
says "If the collection fits in the specified array, it is returned
therein. Otherwise,
a new array is allocated with the runtime type of the specified array and
size of this collection."  However, in multithread environment, it is not
to tell if the collection fits in the specified array, because the items
may be
removed when toArray is copying.

    Here is a testcase:

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class CollectionToArrayTest {

    static volatile Map<String, String> map = new
TConcurrentHashMap<String, String>();
    static volatile boolean gosleep = true;

    static class TConcurrentHashMap<K, V> extends ConcurrentHashMap<K, V> {
        public int size() {
            int oldresult = super.size();
            System.out.println("map size before concurrent remove is "
                    + oldresult);
            while (gosleep) {
                try {
                    // Make sure the map is modified during toArray is
                    // between getsize and being iterated.
                    // System.out.println("size called, size is " +
oldresult +
                    // " take a sleep to make sure the element is deleted
before size is returned.");
                } catch (Exception e) {
            return oldresult;

    static class ToArrayThread implements Runnable {
        public void run() {
            for (int i = 0; i < 5; i++) {
                String str = Integer.toString(i);
                map.put(str, str);
            String[] buffer = new String[4];
            String[] strings = map.values().toArray(buffer);
            // System.out.println("length is " + strings.length);
            if (strings.length <= buffer.length) {
                System.out.println("given array size is "
                                + buffer.length
                                + " \nreturned array size is "
                                + strings.length
                                + ", \nbuffer should be used according to
spec. Is buffer used : "
                                + (strings == buffer));

    static class RemoveThread implements Runnable {
        public void run() {
            String str = Integer.toString(0);
            gosleep = false;

    public static void main(String args[]) {
        CollectionToArrayTest app = new CollectionToArrayTest();

    public void test_concurrentRemove() {

System.out.println("//////////////////////////////////////////////\n" +
                "The spec says if the given array is large\n "  +
                        "enough to hold all elements, the given array\n" +
                        "should be returned by toArray. This \n" +
                        "testcase checks this case. \n" +

        Thread[] threads = new Thread[2];
        threads[0] = new Thread(new ToArrayThread());
        threads[1] = new Thread(new RemoveThread());


        try {
            // Take a sleep to make sure toArray is already called.
        } catch (Exception e) {



TConcurrentHashMap is used to make sure the collection is modified
during toArray is invoked. So the returned array fits in the specified
array, but a new array is used because toArray checks the size
before copying.

Is this a bug ?

Best Regards,
Sean Chou

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