ToArrayTest.java

gistfile1.java

package test;

import java.util.concurrent.ConcurrentLinkedQueue;

/**
 * Illustrates a case when Collection.size() cannot reliably indicate how many elements were dumped by
 * Collection.toArray().
 *
 * Assuming that the array is reused and was previously filled with unrelated values, the fact that toArray()
 * writes null after the last element is the only way to find out how many elements were actually dumped.
 *
 * PS: ConcurrentLinkedQueue does not permit null elements.
 */
public class ToArrayTest {
  
  static final ConcurrentLinkedQueue<Integer> queue  = new ConcurrentLinkedQueue<Integer>();
  
  static Thread filler = new Thread() {
    @Override public void run() {
      try {
        while (!isInterrupted()) {
          queue.offer(0);
          sleep(1);
        }
      }
      catch (InterruptedException e) {} // exit
    }
  };
  
  public static void main(String[] args) throws InterruptedException {
    Integer[] dest = new Integer[100000];
    
    filler.start();
    Thread.sleep(1000);
    
    int sizeBefore = 0, dumpedSize = 0;
    while (sizeBefore == dumpedSize) {
      sizeBefore = queue.size();
      queue.toArray(dest);
      dumpedSize = countUntilNull(dest);
    }
    
    filler.interrupt();
    
    System.out.println("Size before : " + sizeBefore);
    System.out.println("Dumped size : " + dumpedSize);
  }
  
  private static int countUntilNull(Integer[] array) {
    int size = -1;
    while (array[++size] != null);
    return size;
  }
}