Top Most Difference between notify and notifyAll in Java - When and How to use

notify vs notifyAll in Java

What is difference between notify and notifyAll method is one of the tricky Java question, which is easy to answer but once Interviewer ask followup questions, you either got confused or not able to provide clear cut and to the point answers. Main difference between notify and notifyAll is that notify method will only notify one Thread and notifyAll method will notify all Threads  which are waiting on that monitor or lock. By the way this is something you have been reading in all over places and to be frank,  this statement despite being correct is not complete and its very difficult to understand difference between notify vs notifyAll by just reading this statement. Lot of questions comes in mind like

Which thread will be notified if I use notify()?

How do I know how many threads are waiting, so that I can use notifyAll() ?

How to call notify()?

What are these thread waiting for being notified etc.

Actually discussion of notify and notifyAll is incomplete without discussing wait method in Java and I had touched based on this on my earlier article why wait and notify must be called from synchronized context. In order to get answer of those questions and understand difference between notify and notifyAll we will use a simple Java Thread example using wait and notify code :

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Top Most How Volatile in Java works ? Example of volatile keyword in Java

How to use Volatile keyword in Java

What  is Volatile variable in Java  and when to use Volatile variable in Java is famous multi-threading interview question in Java interviews. Though many programmer knows what is a volatile variable but they fail on second part i.e. where to use volatile variable in Java as its not common to have clear understanding and hands-on on volatile in Java. In this tutorial we will address this gap by providing simple example of volatile variable in Java and discussing some when to use Volatile variable in Java. Any way  Volatile keyword in Java is used as an indicator to Java compiler and  Thread that do not cache value of this variable and always read it from main memory. So if you want to share any variable in which read and write operation is atomic by implementation e.g. read and write in int or boolean variable you can declare them as volatile variable. From Java 5 along with major changes like Autoboxing, Enum, Generics and Variable arguments ,  Java introduces some change in Java Memory Model (JMM),  Which  guarantees visibility of changes made by one thread to another also as "happens-before" which solves the problem of memory writes that happen in one thread can "leak through" and be seen by another thread. Java volatile keyword cannot be used with method or class and it can only be used with variable. Java volatile keyword also guarantees visibility and ordering , after Java 5 write to any volatile variable happens before any read into volatile variable. By the way use of volatile keyword also prevents compiler or JVM from reordering of code or moving away them from synchronization barrier.

This Java tutorial on Volatile keyword is in continuation of my article How HashMap works in Java  and difference between HashMap and Hashtable in Java  , How Garbage collection works in Java and How Synchronization works in Java if you haven’t read already you may find some useful information based on my experience in Java .

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Top Most Prefer TimeUnit Sleep over Thread.Sleep - Java Coding Tips

What is TimeUnit in Java
TimeUnit in Java is a class on java.util.concurrent package, introduced in Java 5 along with CountDownLatch, CyclicBarrier, Semaphoreand several other concurrent utilities. TimeUnit provides a human readable version of Thread.sleep() method which can be used in place of former. From long time Thread's sleep() method is standard way to pause a Thread in Java and almost every Java programmer is familiar with that. In fact sleep method itself is a very popular and has appeared on many Java interviews. Difference between wait and sleep is one of the tough Java question to answer. If you have used Thread.sleep() before and I am sure you do, you might be familiar with fact like it's an static method, it doesn't release lock when pausing Thread and it throws InterruptedException. But what many of us doesn't consider a potential issue is readability. Thread.sleep() is an overloaded method and accept long millisecond and longnanosecond, which makes it hard for programmers to find out exactly how many seconds, minutes, hours or day current Thread is sleeping. Look at below example of Thread's sleep() method:

Thread.sleep(2400000);

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