Volatile in Java

In general each thread has its own copy of variable, such that one thread is not concerned with the value of same variable in the other thread. But sometime this may not be the case. Consider a scenario in which the count variable is holding the number of times a method is called for a given class irrespective of any thread calling, in this case irrespective of thread access the count has to be increased. In this case the count variable is declared as volatile. The copy of volatile variable is stored in the main memory, so every time a thread access the variable even for reading purpose the local copy is updated each time from the main memory. The volatile variable also have performance issues.

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.


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.

Example of volatile keyword in Java:

To Understand example of volatile keyword in java let’s go back to Singleton pattern in Java and see double checked locking in Singleton with Volatile and without volatile keyword in java.

/**
 * Java program to demonstrate where to use Volatile keyword in Java.
 * In this example Singleton Instance is declared as volatile variable to ensure
 * every thread see updated value for _instance.
 *
 * @author Javin Paul
 */
public class Singleton{
private static volatile Singleton _instance; //volatile variable

public static Singleton getInstance(){

   if(_instance == null){
            synchronized(Singleton.class){
              if(_instance == null)
              _instance = new Singleton();
            }

   }
   return _instance;

}

If you look at the code carefully you will be able to figure out:

1) We are only creating instance one time

2) We are creating instance lazily at the time of first request comes.

If we do not make _instance variable volatile then Thread which is creating instance of Singleton is not able to communicate other thread, that instance has been created until it comes out of the Singleton block, so if Thread A is creating Singleton instance and just after creation lost the CPU, all other thread will not be able to see value of _instance as not null and they will believe its still null.

Why because reader threads are not doing any locking and until writer thread comes out of synchronized block, memory will not be synchronized and value of _instance will not be updated in main memory. With Volatile keyword in Java this is handled by Java himself and such updates will be visible by all reader threads.

 

When to use Volatile variable in Java

1) You can use Volatile variable if you want to read and write long and double variable atomically. long and double both are 64 bit data type and by default writing of long and double is not atomic and platform dependence. Many  platform perform write in long and double variable 2 step, writing 32 bit in each step, due to this its possible for a Thread to see 32 bit from two different write. You can avoid this issue by making long and double variable volatile in Java.

2) Volatile variable can be used as an alternative way of achieving synchronization in Java in some cases, like Visibility. with volatile variable its guaranteed that all reader thread will see updated value of volatile variable once write operation  completed, without volatile keyword different reader thread may see different values.

3) volatile variable can be used to inform compiler that a particular field is subject to be accessed by multiple threads, which will prevent compiler from doing any reordering or any kind of optimization which is not desirable in multi-threaded environment. Without volatile variable compiler can re-order code, free to cache value of volatile variable instead of always reading from main memory. like following example without volatile variable may result in infinite loop

private boolean isActive = thread;
public void printMessage(){
  while(isActive){
     System.out.println("Thread is Active");
  }
} 

without volatile modifier its not guaranteed that one Thread see the updated value of isActive from other thread. compiler is also free to cache value of isActive instead of reading it from main memory in every iteration. By making isActive a volatile variable you avoid these issue.

4) Another place where volatile variable can be used is to fixing double checked locking in Singleton pattern.

 

 

Important points on Volatile keyword in Java

1. volatile keyword in Java is only application to variable and using volatile keyword with class and method is illegal.

2. volatile keyword in Java guarantees that value of volatile variable will always be read from main memory and not from Thread's local cache.

3. In Java reads and writes are atomic for all variables declared using Java volatile keyword (including long and double variables).

4. Using Volatile keyword in Java on variables reduces the risk of memory consistency errors, because any write to a volatile variable in Java establishes a happens-before relationship with subsequent reads of that same variable.

5. From Java 5 changes to a volatile variable are always visible to other threads. What’s more it also means that when a thread reads a volatile variable in java, it sees not just the latest change to the volatile variable but also the side effects of the code that led up the change.

6. Reads and writes are atomic for reference variables are for most primitive variables (all types except long and double) even without use of volatile keyword in Java.

7. An access to a volatile variable in Java never has chance to block, since we are only doing a simple read or write, so unlike a synchronized block we will never hold on to any lock or wait for any lock.

8. Java volatile variable that is an object reference may be null.

9. Java volatile keyword doesn't means atomic, its common misconception that after declaring volatile ++ will be atomic, to make the operation atomic you still need to ensure exclusive access using synchronized method or block in Java.

10. If a variable is not shared between multiple threads no need to use volatile keyword with that variable.

 

 

Difference between synchronized and volatile keyword in Java

 

Difference between volatile and synchronized is another popular core Java question asked in multi-threading and concurrency interviews. Remember volatile is not a replacement of synchronized keyword but can be used as an alternative in certain cases. Here are few differences between volatile and synchronized keyword in Java.

1. Volatile keyword in java is a field modifier, while synchronized modifies code blocks and methods.

2. Synchronized obtains and releases lock on monitor’s java volatile keyword doesn't require that.

3. Threads in Java can be blocked for waiting any monitor in case of synchronized, that is not the case with volatile keyword in Java.

4. Synchronized method affects performance more than volatile keyword in Java.

5. Since volatile keyword in Java only synchronizes the value of one variable between Thread memory  and "main" memory  while synchronized synchronizes the value of all variable between thread memory and "main" memory and locks and releases a monitor to boot. Due to this reason synchronized keyword in Java is likely to have more overhead than volatile.

6. You can not synchronize on null object but your volatile variable in java could be null.

7. From Java 5 Writing into a volatile field has the same memory effect as a monitor release, and reading from a volatile field has the same memory effect as a monitor acquire

 

Interview Question on Singleton Design Pattern

What is Singleton class? Have you used Singleton before?
Singleton is a class which has only one instance in whole application and provides a getInstance() method to access the singleton instance. There are many classes in JDK which is implemented using Singleton pattern like java.lang.Runtime which provides getRuntime() method to get access of it and used to get free memory and total memory in Java

What is double checked locking in Singleton?One of the most hyped question on Singleton pattern and really demands complete understanding to get it right because of Java Memory model caveat prior to Java 5. If a guy comes up with a solution of using volatile keyword with Singleton instance and explains it then it really shows it has in depth knowledge of Java memory model and he is constantly updating his Java knowledge.

Answer: Double checked locking is a technique to prevent creating another instance of Singleton when call to getInstance() method is made in multi-threading environment. In Double checked locking pattern as shown in below example, singleton instance is checked two times before initialization.

public static Singleton getInstance(){
      if(_INSTANCE == null){
          synchronized(Singleton.class){
          //double checked locking - because second check of Singleton instance with lock
                if(_INSTANCE == null){
                    _INSTANCE = new Singleton();
                }
            }
          }
      return _INSTANCE;
}

Double checked locking should only be used when you have requirement for lazy initialization otherwise use Enum to implement singleton or simple static final variable.

How do you prevent for creating another instance of Singleton using clone() method?This type of questions generally comes some time by asking how to break singleton or when Singleton is not Singleton in Java.
Answer: Preferred way is not to implement Clonnable interface as why should one wants to create clone() of Singleton and if you do just throw Exception from clone() method as  “Can not create clone of Singleton class”. 

How many ways you can write Singleton Class in Java?
Answer:  I know at least four ways to implement Singleton pattern in Java
1) Singleton by synchronizing getInstance() method
2) Singleton with public static final field initialized during class loading.
3) Singleton generated by static nested class, also referred as Singleton holder pattern.
4) From Java 5 on-wards using Enums

What is lazy and early loading of Singleton and how will you implement it?This is another great Singleton interview question in terms of understanding of concept of loading and cost associated with class loading in Java. Many of which I have interviewed not really familiar with this but its good to know concept.

Answer: As there are many ways to implement Singleton like using double checked locking or Singleton class with static final instance initialized during class loading. Former is called lazy loading because Singleton instance is created only when client calls getInstance() method while later is called early loading because Singleton instance is created when class is loaded into memory.


Is it better to make whole getInstance() method synchronized or just critical section is enough? Which one you will prefer?This is really nice question and I mostly asked to just quickly check whether candidate is aware of performance trade off of unnecessary locking or not. Since locking only make sense when we need to create instance and rest of the time its just read only access so locking of critical section is always better option. read more about synchronization on How Synchronization works in Java
Answer: This is again related to double checked locking pattern, well synchronization is costly and when you apply this on whole method than call to getInstance() will be synchronized and contented. Since synchronization is only needed during initialization on singleton instance, to prevent creating another instance of Singleton,  It’s better to only synchronize critical section and not whole method. Singleton pattern is also closely related to factory design pattern where getInstance() serves as static factory method.

Java Banking Interview Questions

What is the difference between creating String as new() and literal?

When we create string with new() Operator, it’s created in heap and not added into string pool while String created using literal are created in String pool itself which exists in PermGen area of heap.

String str = new String("Test");
 
does not  put the object str in String pool , we need to call String.intern() method which is used to put  them into String pool explicitly. its only when you create String object as String literal e.g. String s = "Test" Java automatically put that into String pool. By the way there is a catch here, Since we are passing arguments as "Test", which is a String literal, it will also create another object as "Test" on string pool. This is the one point, which has gone unnoticed, until knowledgeable readers of Javarevisited blog suggested it

What is the difference when String is gets created using literal or new() operator ?
When we create string with new() operator,  its created in heap only and not added into string pool, while String created using literal are created in String pool itself which exists in PermGen area of heap. You can put such string object into pool by calling intern() method. If you happen to create same String object multiple times, intern() can save some memory for you.

What will be the problem if you don't override hashcode() method ?
If you don't override equals method, than contract between equals and hashcode will not work, according to which, two object which are equal by equals() must have same hashcode. In this case other object may return different hashcode and will be stored on that location, which breaks invariant of HashMap class, because they are not supposed to allow duplicate keys. When you add object using put() method, it iterate through all Map.Entry object present in that bucket location, and update value of previous mapping, if Map already contains that key. This will not work if hashcode is not overridden

When do you override hashcode and equals() ?
Whenever necessary especially if you want to do equality check  based upon business logic rather than object equality e.g. two employee object are equal if they have same emp_id, despite the fact that they are two different object, created by different part of code. Also overriding both these methods are must if you want to use them as key in HashMap. Now as part of equals-hashcode contract in Java, when you override equals, you must overide hashcode as well, otherwise your object will not break invariant of classes e.g. Set, Map which relies on equals() method for functioning properly.

How does substring () inside String works?

Another good Java interview question, I think answer is not sufficient but here it is “Substring creates new object out of source string by taking a portion of original string”.  This question was mainly asked to see if developer is familiar with risk of memory leak, which substring can create. Until Java 1.7, substring holds reference of original character array, which means even a substring of 5 character long, can prevent 1GB character array from garbage collection, by holding a strong reference. This issue is fixed in Java 1.7, where original character array is not referenced any more, but that change also made creation of substring bit costly in terms of time. Earlier it was on the range of O(1), which could be O(n) in worst case on Java 7.

Data Structures and Algorithm Interview Questions

Question 1 : How to find middle element of linked list in one pass?

One of the most popular question from data structures and algorithm, mostly asked on telephonic interview. Since many programmer know that, in order to find length of linked list we need to first traverse through linkedlist till we find last node, which is pointing to null, and then in second pass we can find middle element by traversing only half of length. They get confused when interviewer ask him to do same job in one pass. In order to find middle element of linked list in one pass you need to maintain two pointer, one increment at each node while other increments after two nodes at a time, by having this arrangement, when first pointer reaches end, second pointer will point to middle element of linked list. See this trick to find middle element of linked list in single pass for more details.

Question 2 : How to find if linked list has loop ?

This question has bit of similarity with earlier algorithm and data structure interview question. I mean we can use two pointer approach to solve this problem. If we maintain two pointers, and we increment one pointer after processing two nodes and other after processing every node, we are likely to find a situation where both the pointers will be pointing to same node. This will only happen if linked list has loop.

 

Question 3 : In an integer array, there is 1 to 100 number, out of one is duplicate, how to find ?

This is a rather simple data structures question, especially for this kind of. In this case you can simply add all numbers stored in array, and total sum should be equal to n(n+1)/2. Now just subtract actual sum to expected sum, and that is your duplicate number. Of course there is a brute force way of checking each number against all other numbers, but that will result in performance of O(n^2) which is not good. By the way this trick will not work if array have multiple duplicates or its not numbers forming arithmetic progression.

Immutable Class in Java

What is immutable class in Java

 Immutable classes are those class, whose object can not be modified once created, it means any modification on immutable object will result in another immutable object. best example to understand immutable and mutable objects are, String and StringBuffer. Since String is immutable class, any change on existing string object will result in another string e.g. replacing a character into String, creating substring from String, all result in a new objects. While in case of mutable object like StringBuffer, any modification is done on object itself and no new objects are created.

 

How to write immutable class in Java

Despite of few disadvantages, Immutable object still offers several benefits in multi-threaded programming and it’s a great choice to achieve thread safety in Java code. here are few rules, which helps to make a class immutable in Java :

1. State of immutable object can not be modified after construction, any modification should result in new immutable object.

2. All fields of Immutable class should be final.
3. Object must be properly constructed i.e. object reference must not leak during construction process.
4. Object should be final in order to restrict sub-class for altering immutability of parent class.

By the way, you can still create immutable object by violating few rules, like String has its hashcode in non final field, but its always guaranteed to be same. No matter how many times you calculate it, because it’s calculated from final fields, which is guaranteed to be same.

 

Benefits of Immutable Classes in Java

 

1) Immutable objects are by default thread safe, can be shared without synchronization in concurrent environment.

2) Immutable object simplifies development, because its easier to share between multiple threads without external synchronization.

3) Immutable object boost performance of Java application by reducing synchronization in code.
4) Another important benefit of Immutable objects is reusability, you can cache Immutable object and reuse them, much like String literals and Integers.  You can use static factory methods to provide methods like valueOf(), which can return an existing Immutable object from cache, instead of creating a new one.

Apart from above advantages, immutable object has disadvantage of creating garbage as well. Since immutable object can not be reused and they are just a use and throw. String being a prime example, which can create lot of garbage and can potentially slow down application due to heavy garbage collection, but again that's extreme case and if used properly Immutable object adds lot of value.

Final In Java

Final is a keyword or reserved word in java and can be applied to member variables, methods, class and local variables in Java. Once you make a reference final you are not allowed to change that reference and compiler will verify this and raise compilation error if you try to re-initialized final variables in java.

 

Final and Immutable Class in Java

 

Final keyword helps to write immutable class. Immutable classes are the one which can not be modified once it gets created and String is primary example of immutable and final class . Immutable classes offer several benefits one of them is that they are effectively read-only and can be safely shared in between multiple threads without any synchronization overhead. You can not make a class immutable without making it final and hence final keyword is required to make a class immutable in java.

Important points on final in Java

1. Final keyword can be applied to member variable, local variable, method or class in Java.

2. Final member variable must be initialized at the time of declaration or inside constructor, failure to do so will result in compilation error.

3. You can not reassign value to final variable in Java.

4. Local final variable must be initializing during declaration.

5. Only final variable is accessible inside anonymous class in Java

 

6. Final method can not be overridden in Java.

7. Final class can not be inheritable in Java.

8. Final is different than finally keyword which is used on Exception handling in Java.

 

9. Final should not be confused with finalize() method which is declared in object class and called before an object is garbage collected by JVM.

 

10. All variable declared inside java interface are implicitly final.

11. Final and abstract are two opposite keyword and a final class can not be abstract in java.

 

12. Final methods are bonded during compile time also called static binding.

13. Final variables which is not initialized during declaration are called blank final variable and must be initialized on all constructor either explicitly or by calling this(). Failure to do so compiler will complain as "final variable (name) might not be initialized".

14. Making a class, method or variable final in Java helps to improve performance because JVM gets an opportunity to make assumption and optimization.

15. As per Java code convention final variables are treated as constant and written in all Caps e.g.

private final int COUNT=10;

16. Making a collection reference variable final means only reference can not be changed but you can add, remove or change object inside collection. For example:

private final List Loans = new ArrayList();

list.add(“home loan”);  //valid

list.add("personal loan"); //valid

loans = new Vector();  //not valid

 

HashMap Working in Java

HashMap accept null while Hashtable doesn't, HashMap is not synchronized, HashMap is fast and so on along with basics like its stores key and value pairs etc.

"Do you Know how HashMap works in Java” or "How does get () method of HashMap works in Java"

 

"HashMap works on principle of hashing, we have put(key, value) and get(key) method for storing and retrieving Objects from HashMap. When we pass Key and Value object  to put() method on Java HashMap, HashMap implementation calls hashCode method on Key object and applies returned hashcode into its own hashing function to find a bucket location for storing Entry object, important point to mention is that HashMap in Java stores both key and value object as Map.Entry in bucket which is essential to understand the retrieving logic. If people fails to recognize this and say it only stores Value in the bucket they will fail to explain the retrieving logic of any object stored in Java HashMap .

"What will happen if two different objects have same hashcode?”

 

Now from here onwards real confusion starts, Some time candidate will say that since hashcode is equal, both objects are equal and HashMap  will throw exception or not store them again etc, Then you might want to remind them about equals() and hashCode() contract  that two unequal object in Java can have same hashcode. Some will give up at this point and few will move ahead and say "Since hashcode is same, bucket location would be same and collision will occur in HashMap, Since HashMap use LinkedList to store object, this entry (object of Map.Entry comprise key and value )  will be stored in LinkedList. Great this answer make sense though there are many collision resolution methods available this is simplest and HashMap in Java does follow this.

"How will you retrieve Value object  if two Keys will have same hashcode?”

Interviewee will say we will call get() method and then HashMap uses Key Object's hashcode to find out bucket location and retrieves Value object but then you need to remind him that there are two Value objects are stored in same bucket , so they will say about traversal in LinkedList until we find the value object , then you ask how do you identify value object because you don't  have value object to compare ,Until they know that HashMap  stores both Key and Value in LinkedList node or as Map.Entry they won't be able to resolve this issue and will try and fail.

But those bunch of people who remember this key information will say that after finding bucket location , we will call keys.equals() method to identify correct node in LinkedList and return associated value object for that key in Java HashMap . Perfect this is the correct answer.

Why String, Integer and other wrapper classes are considered good keys ?

String, Integer and other wrapper classes are natural candidates of HashMap key, and String is most frequently used key as well because String is immutable and final,and overrides equals and hashcode() method. Other wrapper class also shares similar property. Immutabiility is required, in order to prevent changes on fields used to calculate hashCode() because if key object return different hashCode during insertion and retrieval than it won't be possible to get object from HashMap. Immutability is best as it offers other advantages as well like thread-safety, If you can  keep your hashCode same by only making certain fields final, then you go for that as well. Since equals() and hashCode() method is used during reterival of value object from HashMap, its important that key object correctly override these methods and follow contact. If unequal object return different hashcode than chances of collision will be less which subsequently improve performance of HashMap.

Can we use any custom object as key in HashMap ?

This is an extension of previous questions. Ofcourse you can use any Object as key in Java HashMap provided it follows equals and hashCode contract and its hashCode should not vary once the object is inserted into Map. If custom object is Immutable than this will be already taken care because you can not change it once created.

How HashMap  works in Java

HashMap  works on principle of hashing, we have put() and get() method for storing and retrieving object form HashMap .When we pass an both key and value to put() method to store on HashMap , it uses key object hashcode() method to calculate hashcode and they by applying hashing on that hashcode it identifies bucket location for storing value object. While retrieving it uses key object equals method to find out correct key value pair and return value object associated with that key. HashMap  uses linked list in case of collision and object will be stored in next node of linked list.

Also HashMap  stores both key+value tuple in every node of linked list.

What will happen if two different HashMap  key objects have same hashcode?

They will be stored in same bucket but no next node of linked list. And keys equals () method will be used to identify correct key value pair in HashMap .

Difference Between Stack and Heap in Java

In general both stack and heap are part of memory, a program is allocated and used for different purposes. Java program runs inside JVM which is launched as a process by "java" command. Java also uses both stack and heap memory for different needs

 

1) Main difference between heap and stack is that stack memory is used to store local variables and function call, while heap memory is used to store objects in Java. No matter, where object is created in code e.g. as member variable, local variable or class variable,  they are always created inside heap space in Java.

2) Each Thread in Java has there own stack which can be specified using -Xss JVM parameter, similarly you can also specify heap size of Java program using JVM option -Xms and -Xmx where -Xms is starting size of heap and -Xmx is maximum size of java heap.

 

3) If there is no memory left in stack for storing function call or local variable, JVM will throw java.lang.StackOverFlowError, while if there is no more heap space for creating object, JVM will throw java.lang.OutOfMemoryError: Java Heap Space.

 

4) If you are using Recursion, on which method calls itself, You can quickly fill up stack memory. Another difference between stack and heap is that size of stack memory is lot lesser than size of  heap memory in Java.

5) Variables stored in stacks are only visible to the owner Thread, while objects created in heap are visible to all thread. In other words stack memory is kind of private memory of Java Threads, while heap memory is shared among all threads.

JAVA HEAP MEMORY

What is Heap space in Java?

When a Java program started Java Virtual Machine gets some memory from Operating System. Java Virtual Machine or JVM uses this memory for all its need and part of this memory is call java heap memory. Heap in Java generally located at bottom of address space and move upwards. whenever we create object using new operator or by any another means object is allocated memory from Heap and When object dies or garbage collected ,memory goes back to Heap space.

 

 

How to increase size of Java Heap

Default size of Heap space  in Java is 128MB on most of 32 bit Sun's JVM but its highly varies from JVM to JVM.

 

you can increase size of java heap space based on your application need and I always recommend this to avoid using default JVM heap values. if your application is large and lots of object created you can change size of heap space by using JVM options -Xms and -Xmx.  Xms denotes starting size of Heap while -Xmx denotes maximum size of Heap in Java. There is another parameter called -Xmn which denotes Size of new generation of Java Heap Space. Only thing is you can not change the size of Heap in Java dynamically, you can only provide Java Heap Size parameter while starting JVM.

 

OutOfMemoryError in Java Heap

When JVM starts JVM heap space is equal to the initial size of Heap specified by -Xms parameter, as application progress more objects get created and heap space is expanded to accommodate new objects. JVM also run garbage collector periodically to reclaim memory back from dead objects. JVM expands Heap in Java some where near to Maximum Heap Size specified by -Xmx and if there is no more memory left for creating new object in java heap , JVM throws  java.lang.OutOfMemoryError and  your application dies. Before throwing OutOfMemoryError No Space in Java Heap, JVM tries to run garbage collector to free any available space but even after that not much space available on Heap in Java it results into OutOfMemoryError.

 

10 Points about Java Heap Space

1. Java Heap Memory is part of memory allocated to JVM by Operating System.

2. Whenever we create objects they are created inside Heap in Java.

3. Java Heap space is divided into three regions or generation for sake of garbage collection called New Generation, Old or tenured Generation or Perm Space. Permanent generation is garbage collected during full gc in hotspot JVM.

4. You can increase or change size of Java Heap space by using JVM command line option -Xms, -Xmx and -Xmn. don't forget to add word "M" or "G" after specifying size to indicate Mega or Gig. for example you can set java heap size to 258MB by executing following command java -Xmx256m HelloWord. 

5. You can use either JConsole or Runtime.maxMemory(), Runtime.totalMemory(), Runtime.freeMemory() to query about Heap size programmatic in Java.

6. You can use command "jmap" to take Heap dump in Java and "jhat" to analyze that heap dump.

7. Java Heap space is different than Stack which is used to store call hierarchy and local variables.

8. Java Garbage collector is responsible for reclaiming memory from dead object and returning to Java Heap space

9. Don’t panic when you get java.lang.OutOfMemoryError, sometimes its just matter of increasing heap size but if it’s recurrent then look for memory leak in Java.

10. Use Profiler and Heap dump Analyzer tool to understand Java Heap space and how much memory is allocated to each object.