Musings, Thoughts and Cogitations on Java

Java7 has introduced an improved hash function for Map and Map derived implementations ( since 7u6 ). It is applicable for keys of type String . Following collections can make use of this new hash function. HashMap Hashtable HashSet LinkedHashMap LinkedHashSet WeakHashMap ConcurrentHashMap The new hash function improves the performance when there are large number of collisions .  It can be enabled by setting the System property  jdk.map.althashing.threshold . If jdk.map.althashing.threshold=k  , This means that java will use alternative hashing function for maps with capacity greater than k entries. Usage Example - java -Djdk.map.althashing.threshold=512 MyAppMain Why it is disabled by default and caveat before using - This feature is disabled by default to ensure backward compatibility.  The iteration order of keys, values and entries will change after using the new Hash function. According to Java team, many applications have depend...

Java 7 has introduced a new random number generator - ThreadLocalRandom Normally to generate Random numbers, we either do Create an instance of java.util.Random  OR Math.random() - which internally creates an instance of java.util.Random on first invocation.  However in a concurrent applications usage of above leads to contention issues - Random is thread safe for use by multiple threads. But if multiple threads use the same instance of Random, the same seed is shared by multiple threads. It leads to contention between multiple threads and so to performance degradation.  ThreadLocalRandom is solution to above problem. ThreadLocalRandom has a Random instance per thread and safeguards against contention. From the api docs -  Usages of this class should typically be of the form:  ThreadLocalRandom.current().nextX(...)  (where  X  is  Int ,  Long , etc). When all usages are of this form, it is never possible to accidently...

All of us have seen the following idiom  numerous times in java code. //Sleep for 6 minutes Thread.sleep(6*60*1000); However the above is not readable. When someone is writing code, he has to perform the conversion to milliseconds. While trying to understand the code also, someone has to perform the transformation from milliseconds to familiar seconds and minutes. java.util.concurrent .TimeUnit class also provides mechanism to sleep which circumvents the above difficulties. Following is an example of sleep using TimeUnit. //Sleep for 6 minutes TimeUnit.MINUTES.sleep(6); //Sleep for 10 seconds TimeUnit.SECONDS.sleep(10); //Sleep for 2 hours TimeUnit.HOURS.sleep(2); TimeUnit supports following time units for sleep. DAYS HOURS MICROSECONDS MILLISECONDS MINUTES NANOSECONDS  SECONDS Internally TimeUnit also invokes Thread.sleep() for sleeping. Following is the code used by sleep() method in TimeUnit public void sleep(long timeout) throws Interrupte...