Can Yous Brand An Array Or Arraylist Volatile Inwards Java?

This is i of the many interesting multi-threading questions I direct maintain shared inwards my post service 50 multi-threading interview questions. Yes, you lot tin brand an array volatile inwards Java, at that topographic point is no work amongst that, neither compiler volition flag whatever fault non JVM volition throw whatever exception simply the tricky utilization is why you lot desire to brand an array volatile too what is the outcome of making an array volatile inwards Java? In companionship to reply this interrogation you lot must travel familiar amongst both volatile modifier too Java retentiveness model, otherwise, it would travel hard to answer, too that's why it's besides i of the line a fast i on questions from Java interviews. Before answering this interrogation inwards detail, let's get-go revise what is a volatile keyword inwards Java too what form of guarantee it provides inwards the context of multithreading too concurrency.

What is a volatile modifier inwards Java?

The volatile is a modifier inwards Java which solely applies to fellow member variables, both instance too course of report variables too both primitive too reference type. It provides the happens-before guarantee which ensures that a write to a volatile variable volition come about earlier whatever reading. This ensures that whatever modification to volatile object or primitive type volition travel visible to all threads i.e. it provides the visibility guarantee.



The volatile modifier besides provides ordering guarantee because the compiler cannot re-order whatever code or functioning which involves volatile variables (primitive too objects), simply what is mayhap to a greater extent than of import to know too recall is that volatile variable doesn't render atomicity (except for write to the volatile double variable) too mutual exclusion, which is besides the primary departure betwixt volatile too synchronized keyword.

There are for certain restrictions amongst volatile keyword e.g. you lot cannot brand a fellow member variable both final too volatile at the same time, simply you lot tin brand a static variable volatile inwards Java.

If you lot desire to acquire to a greater extent than close the volatile variable inwards Java, I advise reading Java Concurrency inwards Practice, which provides to a greater extent than thorough too consummate introduction too application of volatile modifier inwards Java.

Can nosotros brand array volatile inwards Java?

Now, coming dorsum to the master copy question, tin nosotros brand an array volatile inwards Java? The reply is, Yes, you lot tin brand an array (both primitive too reference type array e.g. an int array too String array) volatile inwards Java simply solely changes to reference pointing to an array volition travel visible to all threads, non the whole array. What this agency is that suppose you lot direct maintain a reference variable called primes equally shown below:

protected volatile int[] primes = novel int[10];

too thence if you lot assign a novel array to primes variable, alter volition travel visible to all threads, simply changes to private indices volition non travel covered nether volatile guarantee i.e.

primes = novel int[20];

volition follow the "happens-before" rule too travail retentiveness barrier refresh, simply next code volition non create so

primes[0] = 10;
primes[1] = 20;
primes[2] = 30;
primes[3] = 40;

This means, if multiple threads are changing private array elements e.g. storing updates, at that topographic point won’t travel whatever happens-before guarantee provided past times the volatile modifier for such modification. So, if your use-case is to render retentiveness visibility guarantee for private array elements than volatile is non the right choice. You must rely on other synchronization too a thread-safety machinery to encompass this instance e.g. synchronized keyword, atomic variables, or ReentrantLock.

On a similar note, sometimes instead of an array, Interviewer position the collection i.e. they volition inquire can you lot brand a collection variable volatile inwards Java or not e.g. an ArrayList or HashMap. The reply is same, of course, you lot tin brand a reference variable pointing to a Collection volatile inwards Java, simply the happens-before guarantee volition solely travel provided if the value of that reference variable is changed e.g. you lot assign a novel collection to it.

Any modification done on actual collection object e.g. adding or removing elements from ArrayList volition non invoke happens-before guarantee or retentiveness barrier refresh. 

If you lot are curious to acquire to a greater extent than close volatile variables too how they travel inwards Java, I strongly advise you lot reading Java Concurrency inwards Practice, mayhap the best majority to acquire too empathise concurrency fundamentals inwards Java. 

That's all close whether nosotros tin brand an array volatile inwards Java or not. Yes, you lot tin brand array volatile simply that volition solely encompass alter to the reference variable pointing to an array, it volition non encompass changes inwards private array elements.

Btw, if you lot are wondering if Java Concurrency inwards Practice is nevertheless relevant inwards the historic menses of Java 8 or non too thence don't forget to read my post, Is Java Concurrency inwards Practice nevertheless valid inwards the era of Java 8? There I direct maintain explained why it is nevertheless the best majority to acquire fundamentals of concurrency inwards Java.

Thanks for reading this article, if you lot similar this article too thence delight portion amongst your friends too colleagues. If you lot direct maintain whatever suggestion, correction or whatever interrogation too thence delight drib a banking corporation complaint too I'll direct maintain a look.

Further Learning
Multithreading too Parallel Computing inwards Java
Applying Concurrency too Multi-threading to Common Java Patterns
Java Concurrency inwards Practice Course past times Heinz Kabutz

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