Frequntly asked Interview questions related to Multi Threading in Java

                       I believe, Multi Threading is always an interesting topic. Here, it is the list of frequently asked interview questions related to it. 

1.) What is the difference between Process and thread? 
- How thread switching is good than process switching?
- Life cycle of thread.
- Advantages of multi threading.
- What is difference between user Thread and daemon Thread?

2.) How can we create thread in Java? 
- What is the better way? Extending Thread class or to implement Runnable interface?
- How to create Demon thread in Java?

3.) Difference between start() and run()
- What will happen if we don’t override Thread class run() method?
- Can we call run() without start()? What will happen at that time?
- Can we call run() method of a Thread class?

4.) What is Thread.join() ?
- What do you understand about Thread Priority?
- There are three threads T1, T2, and T3? How do you ensure sequence T1, T2, T3 died before main thread in Java?
- How can we make sure main() is the last thread to finish in Java Program?

5.) wait() vs notify() vs notifyAll() 
- Why they are part of Object class but not Thread class?
- Why these methods have to be called from synchronized method or block?
- What is the difference between calling wait() and sleep() method in Java multi-threading?

6.) What is the difference between Thread.sleep() and Thread.yield() ?
- Why Thread sleep() and yield() methods are static?

7.) What is the difference between the intrupt(), interrupted() and isInterrupted() method in Java?

8.)  What is the difference between Runnable and Collable in Java?
- What is FutureTask in Java?

9.) How to stop a thread in java?
- Why stop(), suspend() and resume() were deprecated.?

10.) What happens when an Exception occurs in a thread? 
- What happens if a thread throws an Exception inside synchronized block?

11.) What is race condition? Give an example.
- What is the solution for that?

12.) What is busy spin in multi-threading?

13.) How can we achieve thread safety in Java?
- Which is more preferred – Synchronized method or Synchronized block?

14.) What is ThreadLocal?
- What is Thread Group? Why it’s advised not to use it?

15.) What is Thread Pool? 
- How can we create Thread Pool in Java?
- What is the difference between the submit() and execute() method thread pool in Java? 
- What happens if you submit a task when the queue of the thread pool is already filled?

16.) What is Thread Scheduler and Time Slicing?
- How can we pause the execution of a Thread for specific time?

17.) How does thread communicate with each other?

18.) How do you share data between two thread in Java?

19.) What is volatile keyword in Java?
- What is the difference between the volatile and atomic variable in Java?

20.) What is Java Thread Dump. How can we get Java Thread dump of a Program?
- Which JVM parameter is used to control stack size of a thread?

21.) What is Java Memory model?

22.) How do you check if a Thread holds a lock or not?

23.) What is blocking method in Java?

24.) What is dead lock? 
- What is Live lock?
- How can we detect that its a dead lock? 
- How to break deadlock? 
- What are the situations which we need to take care so that it can not occur? 
- A sample program which creates a deadlock as well as Live lock.

25.) What is Semaphore in Java? 

26.)  What is the difference between CyclicBarrier and CountDownLatch in Java?

27.) What is ReadWriteLock in Java?

28.) What is the difference between synchronized and ReentrantLock in Java?

29.) Write code to solve Producer Consumer problem in Java? 

30.) What is the fork-join framework in Java? 
- What are new things in JDK 7 related to multi threading?

31.) List down 3 multi-threading best practice you follow?
Answers: 
- Always give meaningful name to your thread.
- Avoid locking or Reduce scope of Synchronization
- Prefer Synchronizers over wait and notify
- Prefer Concurrent Collection over Synchronized Collection

Garbage Collection in Java

Question: What is responsiveness and throughput parameter for any web application?

Responsiveness	Throughput
Refers to how quickly an application or system responds with a requested piece of data.	Focuses on maximizing the amount of work by an application in a specific period of time.
Long pause times are not acceptable for applications that focus on throughput.	High pause times are acceptable for applications that focus on throughput.
The focus is on responding in short periods of time.	Since high throughput applications focus on benchmarks over longer periods of time, quick response time is not a consideration
E.g. how quickly UI responds, how fast a website returns a page, how fast a database query is returned etc.	The number of transactions/jobs/queries/etc completed in a given time.

Question: Which things are stored in Heap and in stack or diff between Heap Space and Stack Space?

Java Heap space	Java Stack space
It is used by java runtime to allocate memory to objects and JRE classes. (It is used by all the parts of the application.)	It is used for execution of a thread. (It is used only by single thread of execution.)
New objects are created in Heap space. (Stack memory contains the references to it.)	It stores Method specific vales, local primitives and references to other objects in the heap that are getting refereed from the method.
Objects stored in the heap are globally accessible.	Stored values cannot be accessible by other threads.
GC runs on the heap memory to free the memory used by objects that does not have any references. (Heap memory lives from the start till the end of the application/freed by GC)	In the end of the execution of the method, the unused memory becomes available for next method. (Stack memory is short-lived)
Heap space is bigger than stack space. (Heap memory is divided into parts like young generation, old generation and perm gen.) In comparison with each other, access from Heap memory is slower than Stack memory.	Stack size is very small compared to Heap memory. Because of the simple design (LIFO) and less size, stack memory is faster than Heap.
When Heap memory is exhausted, java runtime throws java.lang.OtOfMemoryError	When stack memory is full, java runtime throws java.lang.StackOverFlowError
For tuning, we can mainly use following parameters. –Xms(define the startup sizeof the Heap) –Xmx(define maximum size of the Heap)	For tuning, we can mainly use following parameters. - Xss(define the stack memory size)

Question: What is Automatic Garbage Collection? When does an object become eligible for garbage collection?

Unlike other programming languages, process of deallocating memory in Java is not manual. It is handled by Garbage Collector- a system thread. Automatic garbage collection is the process of looking at heap memory, identifying which objects are in use and which are not, and deleting the unused objects.

An object becomes eligible for GC when there is no live reference for that object or it can not be reached by any live thread. If two objects have cyclic dependency on each other and both have no live reference then also both objects are eligible for GC.

Question: What are Young (New) Generation, Old Generation, Permanent Generation, Eden memory, S0 Survivor memory, S1 Survivor memory, Minor Garbage collection, Major Garbage Collection and “Stop the world” events? Which things are stored in these areas? What is permanent Generation? What are the changes in JDK 1.8? What is Metaspace?

Young Generation: Young generation is the place where all the new objects are created. When young generation is filled completely, garbage collection is performed. This garbage collection is known as Minor GC. Young generation is also known as New Generation. Young Generation is divided into following sections: Eden Memory and two Survivor Memory (S0 Survivor memory and S1 Survivor memory) spaces.

Eden Memory:  Most of the newly created objects are located in the Eden memory space. When it is filled with objects, minor GC is performed and objects are moved to one of the survivor space.

Survivor Memory: Survivor memory has two sections S0 Memory space and S1 Memory space which are also known as From-space and To-space. Each minor GC done in the young generation increments the age of objects in the survivor spaces. When an object has survived a sufficient number of minor GCs (Defaults vary but normally start at 15) it will then be promoted to the Old Generation.

Old Generation: The Old Generation is usually much larger than the New Generation and it contains the objects that are long lived and survived after many rounds of Minor GC. Usually garbage collection is performed in Old Generation memory when it’s full. Old Generation Garbage Collection is called Major GC and usually takes longer time.

Stop the World events: All application threads are stopped during the execution of GC. This event is known as Stop the World event. Minor GC and major GC both are this type of events. Often a major collection is much slower because it involves all live objects.

Permanent generation (PermGen): It contains metadata required by the JVM to describe the classes and methods used in the application. Java SE classes and methods are also stored in it. Runtime, it is populated by JVM, based on classes used in the application. The permanent generation is included in a full garbage collection.

Java 8: PermGen is no longer exists in Java 8. Java designers have redesigned the architecture and they removed PermGen and introduced new area so called Metaspace. 

Commonly asked differences during Java interview.

Map vs Set vs List

ArrayList vs Vector
HashMap vs HashTable
HashMap vs HashSet

Iterator vs Enumeration
Fail-fast and Fail-safe Iterators
Iterator vs ListIterator
Why ListIterator has added() method but Iterator does not have it?
Comparable and Comparator Interface

Working example of compareTo(), comparable, equals(), hashCode().

Synchronized Collection vs Concurrent collection
ArrayList vs Vector vs CopyOnWriteArrayList
HashMap vs HashTable vs ConcurrentHashMap
Can we replace Hashtable with ConcurrentHashMap?
Queue - BlockingQueue
HashTable vs ConcurrentHashMap (putIfAbsent)
What is lock striping in java? 


poll() vs remove()

Singleton Design Pattern
Iterator Design Pattern.

Commonly asked interview questions:

1.) How Hashmap works in java? 
2.) How Hasmap get() and remove() works in java?
3.) How Hashset works in java?
4.) How does Linkedlist implemented in java? Is it singly linkedlist or doubly linked list?

5.) What is Marker interface or Tag interface? Examples of it(Serializable, Clonnable, RMI and Remote interfaces, RandomAccess...)

6.) Which is better to use? Annotations or Marker interface?
7.) What is Clonnable interface? Why it is use? When it is used? How it is used?
8.) What is Serializable interface? Why it is use? When it is used? How it is used?

9.) What is TreeMap, Naviagable Map and WeakHashMap, IdentityHashMap?
10.) WeakReference vs SoftReference vs PhantomReference vs Strong reference

11.) How can an Arraylist be synchronized without using Vector?
Ans) Arraylist can be synchronized using:
Collections.synchronizedList(List list)
Collections.synchronizedMap(Map map)
Collections.synchronizedCollection(Collection c)

12.)  How do you remove an entry from a Collection? OR 
What is difference between remove() method of Collection and remove() method of Iterator?
Which one you will use, while removing elements during iteration.

Thread:

What is the difference between start and run method in Java Thread? 
( thread.start() vs thread.run() )
How to avoid deadlock?
Write producer-consumer problem?
Solve it by using wait() and notify(). Solve it by using BlockinQueue. Which is better?

What are the other thread scheduler? like quartz, threadpool Executors?

List	Set	Map
Ordered collection (It preserves the order on which an element is inserted. ) (Also known as Dynamic Array.)	Unordered collection (It does not preserve the order.) (We can have SortedSet which offers to sort functionality e.g: TreeSet.) (Also we can change/override it by using Comparator and Comparable. )	Key-Value based collection
List allows duplicate values. It can store multiple times null as value.	Set does not allow duplicate values. So it can store only one time null as value. (Duplication of object is detected by using equals() ) (If two objects are equal using equals() then the later object will replace the former object.) (In case of SortedSet like TreeSet, compareTo() is used to compare object and decide whether an object   is duplicate or not. )	Duplicate keys are not allowed. It can contain only one null key but can contain duplicate values.
e.g. ArrayList, LinkList, Vector	e.g HashSet	e.g HashMap, LinkedHashMap,   TreeMap

Ant and triangle problem

Three ants are sitting at the three corners of an equilateral triangle. 
Each ant starts randomly picks a direction and starts to move along the edge of the triangle. 
What is the probability that none of the ants collide?
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Answer: 0.25 or 25%
Probability of no collision = Probability of all ants go in a clockwise direction + Probability of all goes in an anti-clockwise direction) 
Probability of no collision = (0.5 * 0.5 * 0.5) + (0.5 * 0.5 * 0.5) 
Probability of no collision = 0.25

100 door puzzle

You have 100 doors in a row that are all initially closed. you make 100 passes by the doors starting with the first door every time.

The first time through you visit every door and toggle the door (if the door is closed, you open it, if its open, you close it). The second time you only visit every 2nd door (door #2, #4, #6). The third time, every 3rd door (door #3, #6, #9), etc, until you only visit the 100th door.

Question: what state are the doors in after the last pass? which are open which are closed?

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Answer:

Only perfect square doors will be open at the end.
( If all doors are initially opened then only perfect square doors will be close at the end. )