Write a thread safe data structure such that there could be only one writer at a time but there could be n readers reading the data. You can consider that incrementing or decrementing a variable is an atomic operation. If more than one threads try to write simultaneously then just select one randomly and let others wait

 // http://www.careercup.com/question?id=6751316000899072

 import java.util.concurrent.Semaphore;

 public class FooBar {

     public int n = 100;

     private Semaphore sharedSemaphore;

     private Semaphore exclusiveSemaphore;

     public FooBar(int n) {

         // TODO Auto-generated constructor stub

         this.n = n;

         this.sharedSemaphore = new Semaphore(n);

         this.exclusiveSemaphore = new Semaphore(1);

     }

     public void reader() {

         // The reader here is not to return a value, but to perform read()

         // action. Thus it is 'void reader()'.

         while (exclusiveSemaphore.availablePermits() < 1) {

             try {

                 Thread.sleep(50);

             } catch (InterruptedException e) {

                 // TODO Auto-generated catch block

                 e.printStackTrace();

             }

         }

         try {

             sharedSemaphore.acquire();

             System.out.println("Performing read() operation.");

             sharedSemaphore.release();

         } catch (InterruptedException e) {

             // TODO Auto-generated catch block

             e.printStackTrace();

         }

     }

     public void writer() {

         while (exclusiveSemaphore.availablePermits() < 1

                 && sharedSemaphore.availablePermits() < n) {

             try {

                 Thread.sleep(50);

             } catch (InterruptedException e) {

                 // TODO Auto-generated catch block

                 e.printStackTrace();

             }

         }

         try {

             exclusiveSemaphore.acquire();

             System.out.println("Performing write() operation.");

             exclusiveSemaphore.release();

         } catch (InterruptedException e) {

             // TODO Auto-generated catch block

             e.printStackTrace();

         }

     }

     public static void main(String[] args) {

         FooBar fooBar = new FooBar(100);

         fooBar.reader();

         fooBar.writer();

     }

 }