Use CountDownLatch as a CyclicBarrier

Following is a program to use CountDownLatch as a CyclicBarrier: (However, unlike CyclicBarrier, CountDownLatch cannot be restarted!) Just to understand how/if CountDownLatch can be used instead of CyclicBarrier.

public class CountDownLatchTry {

      private static CountDownLatch latch = new CountDownLatch(2);

      /**

      * @param args

      */

      public static void main(String[] args) {

            // TODO Auto-generated method stub

            Runnable r1 = new Runnable() {

                  public void run() {

                        System.out.println("T1: Before countDown()");

                        latch.countDown();

                        System.out.println("T1: After countDown()... sleeping");

                        try {

                              Thread.sleep(2000);

                        } catch (InterruptedException e1) {

                              // TODO Auto-generated catch block

                              e1.printStackTrace();

                        }

                        try {

                              latch.await();

                        } catch (InterruptedException e) {

                              // TODO Auto-generated catch block

                              e.printStackTrace();

                        }

                        System.out.println("T1: After await()");

                  }

            };

           

            Runnable r2 = new Runnable() {

                  public void run() {

                        System.out.println("T2: Before countDown()");

                        latch.countDown();

                        System.out.println("T2: After countDown()... sleeping");

                        try {

                              Thread.sleep(2000);

                        } catch (InterruptedException e1) {

                              // TODO Auto-generated catch block

                              e1.printStackTrace();

                        }

                        try {

                              latch.await();

                        } catch (InterruptedException e) {

                              // TODO Auto-generated catch block

                              e.printStackTrace();

                        }

                        System.out.println("T2: After await()");

                  }

            };

            Thread t1 = new Thread(r1, "T1");

            Thread t2 = new Thread(r2, "T2");

           

            t1.start();

            t2.start();

            System.out.println("Done!");

      }

}

Corresponding CyclicBarrier example:

public class CyclicBarrierTry {

      private static CyclicBarrier cb = new CyclicBarrier(2);

      /**

      * @param args

      */

      public static void main(String[] args) {

            // TODO Auto-generated method stub

           

            Runnable r1 = new Runnable() {

                  public void run() {

                        System.out.println("T1: Before await()");

                        try {

                              cb.await();

                        } catch (InterruptedException e) {

                              // TODO Auto-generated catch block

                              e.printStackTrace();

                        } catch (BrokenBarrierException e) {

                              // TODO Auto-generated catch block

                              e.printStackTrace();

                        }

                        System.out.println("T1: After await()");

                  }

            };

           

            Runnable r2 = new Runnable() {

                  public void run() {

                        System.out.println("T2: Before await()");

                        try {

                              cb.await();

                        } catch (InterruptedException e) {

                              // TODO Auto-generated catch block

                              e.printStackTrace();

                        } catch (BrokenBarrierException e) {

                              // TODO Auto-generated catch block

                              e.printStackTrace();

                        }

                        System.out.println("T2: After await()");

                  }

            };

            Thread t1 = new Thread(r1, "T1");

            Thread t2 = new Thread(r2, "T2");

           

            t1.start();

            t2.start();

            System.out.println("Done!");

      }

}

Potential perf issues with String.substring()

This post is relevant for Oracle’s java implementation of 1.6!

Following is the implementation of substring method in String class:

    public String substring(int beginIndex, int endIndex) {

      if (beginIndex < 0) {

          throw new StringIndexOutOfBoundsException(beginIndex);

      }

      if (endIndex > count) {

          throw new StringIndexOutOfBoundsException(endIndex);

      }

      if (beginIndex > endIndex) {

          throw new StringIndexOutOfBoundsException(endIndex - beginIndex);

      }

      return ((beginIndex == 0) && (endIndex == count)) ? this :

          new String(offset + beginIndex, endIndex - beginIndex, value);

    }

Now, if you take a look at the highlighted part of the code, it says a new String is created with same char sequence (‘value’), but with different offset!

Let’s take a scenario where we have a huge string, say of 100MB and we take a substring containing last 100 chars of that string.

String s100MB = <100MB String>; //memory occupied is 100MB

String substring = s100MB.subString(s100MB.length() – 100); //does not occupy any additional memory for the chars of substring as it uses the same char array as the parent string

s100MB = null; //‘substring’ still occupies 100MB of memory where as what it requires is only 200k (for 100 chars)

Now, in real world this may not be a very serious issue as GC is not as instantaneous but cases where ‘substring’ hangs around in memory for very long time – this can be unnecessary wastage of memory! (Worse…. think of the original string being 1GB instead of 100MB!)

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