Top 5 Interview Questions on BlockingQueue in Java

1) What is BlockingQueue ? Under which package of JDK its available ?

Ans- A blocking queue is an interface. BlockingQueue implementations are thread-safe. It helps to handle multi threaded execution , specially its for producer and consumer problem.
The queue that blocks when you try to dequeue from it and the queue is empty, or if you try to enqueue items to it and the queue is already full. 

A thread trying to dequeue from an empty queue is blocked until some other thread inserts an item into the queue. 

There are few implementation for this BlockingQueue as below, all these classes available under java.util.concurrent package.

• ArrayBlockingQueue
• SyncronousBlockingQueue
• PriorityBlockingQueue
• LinkedBlockingQueue
• DelayQueue

2) What is the use of these methods peek(), poll(), take() and remove() ?

Ans - 
peek() :- This retrieves, but does not remove, the head of this queue,or returns null if this queue is empty. It doesn't throw any exception.

poll() :- This retrieves and removes the head of this queue,or returns null if this queue is empty.It doesn't throw any exception.

take() :- This retrieves and removes the head of this queue, waiting if necessary until an element becomes available. This method waits for certain time , if its interrupted then it throws InterruptedException. 

remove() :- This retrieves and removes the head of this queue. This method differs from poll() only in that it throws an exception (NoSuchElementException ) if this queue is empty.

Apart from the above difference take() method is provided by BlockingQueue i.e. java.util.concurrent.BlockingQueue.take(). Where as other methods provided by Queue i.e.  java.util.Queue.poll(), java.util.Queue.peek(), java.util.Queue.remove()


3) Is this possible to declare BlockingQueue implementation with ZERO/0 size?

Ans- Yes, if its unbounded implementation. But,if its bounded then we have to provide a capacity. The capacity must be greater than ZERO (i.e. capacity > 0).If we create a BlockingQueue with ZERO capacity then this will throw java.lang.IllegalArgumentException.

4) Write a program for demonstrating producer & consumer problem using blocking
 queue.

Ans- Find the answer here.

5) What is the difference between ArrayBlockingQueue and LinkedBlockingQueue ?

Ans- ArrayBlockingQueue is a bounded blocking queue backed by an array of objects. LinkedBlockingQueue is an optionally-bounded blocking queue based on linked nodes. 
Linked queues typically have higher throughput than array-based queues but less predictable performance in most concurrent applications.Linked nodes are dynamically 
created upon each insertion unless this would bring the queue capacity (Integer.MAX_VALUE). 

5 Common Java Interview Questions About Objects and Object Class

These are some common questions asked in Java/J2EE interviews. These questions are based on Java objects and their behavior.

Q.1. How can you define an object? How is it related to the real world and how does it behave? With Example.

Ans: This question is often asked in Java interviews for candidates with 1-8 years of experience. As we know, an object is a real-world entity. It has three basic properties i.e. State, Behavior and Identity. Simply we can say that a class is a blueprint for creating objects, and an object is an instance of a class. Anything around you can be represented as an object. But yes, it has the above three properties by which we can define an object.

State: what the object has. Dog has a name, age, color, etc. It is implemented by variables in Java.

Behavior: what the object does. Dog can bark, bite, run, etc. It is implemented by methods in Java.

Identity: what makes the object unique. Identity is what makes one object distinguishable from another object. It does not necessarily mean that every object must have a unique id variable. Different objects can also have the same hash code.

Example - Suppose there are many dogs. But, you can't say that is an object. You need to take one dog which has name - Dick, age - 8, color - white and it can be distinguished from other objects. And now we can say Dick is a dog object. And this Dick can bark, bite and run. These are the behaviors that the object can do, and they are implemented by methods.

Q.2. How many constructors & methods are in Object class?

Ans: The Object class has one constructor and several methods. The exact list depends on the Java version. It is available inside the java.lang package.

Constructor :-

public Object()

Methods :-

clone() - This creates and returns a copy of the object.

equals() - This is used to compare whether some other object is equal to this object or not.

toString() - Converts and returns a string representation of the object.

finalize() - An old mechanism associated with garbage collection. It is deprecated and should not be relied upon for resource cleanup.

getClass() - This returns the runtime class of this object.

hashCode() - Returns a hash code for this object. The hash code is not guaranteed to be unique.

notify() - Wakes up a single thread that is waiting on this object's monitor.

notifyAll() - Wakes up all threads that are waiting on this object's monitor.

wait() - This makes the current thread wait until another thread notifies it.

Q.3. Why are wait() and notify() methods inside Object class instead of Thread class?

Ans: This is a FAQ for any Java interview. The wait() and notify() methods are used in a multi-threaded environment. These methods are inside the Object class because we are doing the monitoring work over an object. Whenever we are calling these methods, we are calling them on an object, not on a thread.

The wait() and notify() methods are used for thread coordination through an object's monitor.

Q.4. How many ways can we create an object in Java?

Ans: There are basic four ways for creating an object in Java as below:-

By using new keyword -

Object obj = new Object();

By using Class.forName() -

Dog dogObj = (Dog) Class.forName("jdeveloperguide.lab.Dog")
        .getDeclaredConstructor()
        .newInstance();

By using deserialization -

ObjectInputStream inStream = new ObjectInputStream(anInputStream);
Dog dogObject = (Dog) inStream.readObject();

By using clone() -

Dog dogObj = new Dog();
Dog dogObj2 = dogObj.clone();

The class must implement the Cloneable interface and provide appropriate cloning support.

Q.5. How does JVM allocate objects in memory? What is heap and stack memory?

Ans: Normally, objects are allocated on heap memory. Heap memory is a dynamic memory area used for objects during runtime.

Each thread has its own stack for method calls and local variables. Objects are generally allocated on the heap, while references and local variables may be stored in the stack depending on the JVM implementation and optimization.

Every thread creates a stack (call stack), and when the thread completes, its stack is no longer needed and its memory can be reclaimed.

Stack operations are generally simpler and faster than heap allocation, but the actual behavior depends on the JVM and its optimizations.

You can share your interview experience with asked questions and answers for publishing on this blog.

Wells Fargo Java interview questions for 4-8 years experience

Java/J2ee interview questions asked by Wells Fargo for 4-8 years of experience. These interview questions had asked in the 1st round of technical interview.


1. About your current project.
2. What is Abstract class and interface, when we need to use them.
3. What is the significance of concurrency API ? What are the API you have used ?
4. What is difference between Hashmap and Hashset.
5. What is use of Linked list over ArrayList
6. What is Concurrent Hashmap? Give me the internal implementation of Concurrent HashMap.
7. Can we declare final inside abstract class and interface both.
8. Tell some critical situation in your current project or solution,you have handled. 
9. Give a design level understanding on abstract class and interface implementation.While design, you need consider the system scalability, robustness , few software designing principle.

10. Write a utility using some design pattern ?
11. Write your own hibernate DAO and IMPL for persist and update the record.
12. Which Jar we need for spring annotation.

13. What is importance of @RestControllor ?

14. What is the underlined design pattern implemented in hibernate ?

Sapient Interview Questions and Answers for Java 2 - 6 Year Experience

Sapient Interview questions for Java experience candidate - 2 -6 years.....

Q1) Tell me about yourself!!!

Ans – Give your brief introduction with full confidence.

Q2) What are the types of class loaders in Java ?

Ans –

 Q3)  How to read and write image from a file ?

Ans – By the use of ImageIO.read() and ImageIO.write()  method of javax.imageio package.

 Q4) What is difference between static and init block in Java ?

Ans –

Q5) What is ConcurrentMap and how it works?

Ans – It’s a Map which providing thread safety and atomicity guarantees. For Memory consistency it works with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentMap as a key or value happen-before actions subsequent to the access or removal of that object from the ConcurrentMap in another thread.

       Q6) Can a static block throw exception?

Ans –  Yes. We can throw checked exception.

Q7) What is difference between iterator access and index access?

Ans - Basically iterator access process the traverse operation through each element, where index access process access direct the element by using the index.

Q8) Why character array is better than string for storing password in Java?

Ans – Security !!! Because, character array stores data in encrypted format which is not readable by human. But, the string stores the data in human readable format which is not secure.

Q9) What is the difference between Serializable and Externalizable interfaces ?

Ans - Both interfaces are used for implement serialization. But, the basic

difference is Serializable interface does not have any method (it’s a

marker interface ) and Externalizable interface having 2 methods such as

 readExternal() and writeExternal(). Serializable interface is the super

interface for Externalizable interface. 

 Q10)   Which Interface is used to make duplicate of Objects ?

Ans – Cloneable interface.

 Q11) What are some advantages and disadvantages of Java Sockets?

Ans – The main advantage is its flexible and very efficient during low network bandwidth. Also its helpful for debugging and some kind of testing. But, security is the most disadvantages. Its always recommended to be careful when authenticating.

Q12)  When can an object reference be cast to an interface reference?

Ans –Yes its possible, when that interface is implemented by that class.

Example – MyInterface obj=new MyClass();

                obj.callMethod();

Q13)  How does Java allocate stack and heap Memory?

Ans – As we know stack memory is not dynamic and its follows LIFO order. Java provides similar implementation for memory allocation. Normally all local variables are created in Stack area (memory) and objects (reference types) are created in heap memory (heap area) .Even all primitive types allocated in stack memory. Heap area is dynamic and handled by JVM runtime. Heap memory is cleaned by garbage collector at runtime.

Q14)   What is memory leak in Java?

Ans – Usually memory leak leads to waste of memory. In general memory leak defines the unavailability of referenced memory. It causes the Garbage collector to fail to collect that object.

Q15) Can we throw exception from finally block in Java?

Ans – Yes, but you need to mentioned “throws” on the method head.

Example –

        
public static void main(String[] args) throws Exception {
// TODO Auto-generated method stub
TestInterface2 interObj=new InterfaceObject();

try{
interObj.display();
}catch(Exception e){
System.out.println("Exception handled...");
}finally{
throw new Exception("This is exception");
}
}

Q16)  How does Java handle integer overflows and underflows?

Ans – Yes, java handles overflows and underflows very intelligently. When it overflows it will go to MIN_VALUE (i.e. -2³¹ ) and when it underflows to will go to MAX_VALUE (i.e. 2³¹-1) .

Q18)  What is casting?

Ans – Casting means conversion. Basically we need casting to type cast the type. There are two types of casting by java i.e. explicit casting and implicit casting.

Q19)  What is new in JSP?

Ans – As per my knowledge JSP 2.1 has many new functionalities. It support resource injection via annotation. It has few extended support for java standard tag library. Also literal expression is supported by JSP 2.1 EL  and many other additional features.

Q20) What do you mean by Java Reflection ?

Ans – It provides runtime access to JVM.  Reflection is one of the most powerful API  which help to work with classes, methods and variables dynamically at runtime. Basically it inspects the class attributes at runtime. Also we can say it provides a metadata about the class. 

Q21) Why does the InputStreamReader class has a read() ?

Ans – Because, an InputStreamReader is a bridge from byte streams to character streams: It reads bytes and decodes them into characters using a specified charset. The charset that it uses may be specified by name or may be given explicitly, or the platform's default charset may be accepted.  It has two overloaded read() method . Each invocation of one of an InputStreamReader's read() methods may cause one or more bytes to be read from the underlying byte-input stream. To enable the efficient conversion of bytes to characters, more bytes may be read ahead from the underlying stream than are necessary to satisfy the current read operation.

Q22)   Describe a problem you faced and how did you solve that?

           Ans  - You can describe any issue you faced during your project work in the                          organization. And what the solution you have implemented for that issue. 

IBM Java Interview Questions for 3–8 Years of Experience

Q. What is Difference between interface and abstract class?

Ans : Interface and Abstract class both are looks similar from declaration point of view. The major difference is for implementing multiple inheritance you need interface, because java does not support multiple inheritance. All methods and  variables of interface are public. Interface can contain only abstract method (without method body)   , but abstract class can contains abstract and non-abstract methods. Abstract class must be inherited by sub class. These are few major points you can answer for interview. For more in interface read here.

Q. Can you write final method  in interface?

Ans : No , because interface methods do not have body . It need to be implement in implemented class.

Q. Is this possible to write final method  in abstract class?

Ans :  Yes, you can do this by using non abstract methods.

Q. What is Difference between Iterator and  ListIterator?

Ans : Both are used to retrieve the data from collection. Iterator only traverse in forward direction, but ListIterator can traverse in both forward and backward direction. ListIterator has hasPrevious() and previous() method which is not available in Iterator.

Q. Difference between synchronized  block and synchronized method ?

Ans : Synchronized Method defines a self contained block and  its easy to handle in multi-threaded environment. Both act as similar; there is no major advantage over each other. The difference is that a synchronized block can choose which object it synchronizes on. A synchronized method can only use 'this' (or the corresponding Class instance for a synchronized class method).

Q. Difference between String and StringBuffer class?

Ans :  String is immutable, but StringBuffer is not. StringBuffer used to represent characters that can be modified.  StringBuffer is faster when performing concatenations. This is because when you concatenate a String, you are creating a new object (internally) every time since String is immutable.

Q. How to get the session object in jsp?

Ans : By using implicit object “session”. You can use session.getAttributes() methods.

Q. How to get the session factory object in hibernate?

Ans : By the use of buildSessionFactory() method  .Syntax as below.

  SessionFactory factory= new Configuration().configure().buildSessionFactory();

Q. How to handle the runtime exception in jsp?

Ans : In jsp page directive you can use isErrorPage=true.

Q. Difference between method overloading and overriding?

Ans  :  Both are concept of polymorphism. Method overloading is a form of static binding. Method overriding is a form dynamic binding. Overloading is applied in single class, but overriding is applicable for inherited class. Method overloading is always specific to method signature. It defines number of parameter, type of parameter and sequence of parameter.

Q. Difference between java.util.date and java.sql.date ?

Ans : As we know every database support most three form of entry i.e. date, time and  timestamp. In java JDBC having all those supports using java.sql.Date, java.sql.Time, java.sql.TimeStamp. Internally all these JDBC classes extends java.util.Date.

Q. I have 1 to 100 elements in array unordered, one element missed find that one?

Ans : There is existing old formula. Calculate the sum of all numbers stored in the array of size 11 means it hold 12 data. Then, subtract the sum from (12 * 13)/2

Basic Formula: n * (n + 1) / 2. 

Q. Difference between equals() and hashcode()?

Ans : Both methods are from Object class. Equals() method is used to compare the contents of object or reference. But, hashcode() method is used to get the unique hash code for any object. Hashcode is used for hash implementations like HashMap, HashTable, HashSet etc.

Q. What is Comparator?

Ans : It’s an interface. It has two important methods. But, the frequently used method is compare() . It compares two different objects.

Q. We have equals() to compare two methods then why comparator again ?

Ans : Because , equals() method will only compare the content . It cannot compare two different objects. Comparator interface has compare()  method  which compare two  different object.

Q. What is auto wiring in spring ?

Ans : Autowiring is a mechanism for injecting objects in Spring. By using the annotation @Autowired , you can achieve that.

Its available inside  org.springframework.beans.factory.annotation package.

Q. We can get the container object through Bean factory and Application context what is the difference in both?

Ans : A BeanFactory is used to just instantiates and configures beans. An ApplicationContext also does that, and it provides the supporting infrastructure to enable lots of enterprise-specific features such as transactions and AOP. Read more on Spring Specification.

Q. What is the Difference between get() and load() in hibernate ?

Ans : These two methods are most useful method in Hibernate. Both method s are used to get/fetch the records from database. The  get() method will return null , if the data not found. The load() method always returns proxy.   

Q. Difference between page and page context in jsp ?

Ans : As we know both are implicit object of JSP. Page is a type java.lang.Object and it’s an instance of generated servlet from “this” JSP, but PageContext is a type of javax.servlet.jsp.PageContext. PageContext is used for storing and retrieving page-related information and sharing objects within the same translation unit and same request.

Q. What is application in jsp ?

Ans : It’s a scope. It defines the visibility for data over Application.

Capgemini Interview Questions and Answers for Java 3-8 Year Experience

Experience Java/J2ee interview Questions asked by Capgemini.

1. Tell me about yourself ?

Ans : Give your brief introduction.

2. Explain about your current project?

Ans : It’s quite easy to describe your projects and your key role on this project. But, be careful and get ready about the functionality when you are describing your working module / part of the project. Show your confidence that you have done the major part and you can face challenges in future.

3. How many types of literals are there in JAVA ?

Ans : The literals means the value you are assigning to variable. You can specify the below types of literal in java.As per the primitive data types(int,short,long,float,double,boolean,char ,etc there is respective literal. Some literal needs to be ended with a specific character.Read More.

           long var=20L; //specify L or l for long literal

int var=20; // If not mentioned any character then its can be short or int

char var=’A’;

float var=10.44f; //specify f or F for float literal

double var=10.44;

boolean var=true; //or false

               

4. What is meant by Garbage collection ?

Ans :  Garbage collection is a automatic feature of java for cleaning the unused object from heap. It helps to developer for releasing the reserved memory without any extra effort by developer. It helps developer to save time and extra mental tension for handling object allocation in memory. When there is no reference to an object found, it will clean that object from memory . You can run the garbage collection explicitly by using System.gc() .    

5. Difference between string s= new string (); and string s = "Hi Dude"; ?

Ans : Both statements are different to each other. Always ‘new’ keyword is used to create object.

String s=new String(); // This statement creates new object in heap. S is the object here.

String s=”Hi Dude” ; // This statement do not create object, its creating reference and its storing in String Constant Pool.S is the reference here.

6. What is singleton class? where is it used ?

Ans : View Answer

7. What is the difference between JSP and Servlets ?

Ans : As simple JSP is per-compiled but Servlets are not. JSP is specially use for displaying/populating the data on browser. If we take an example of MVC architecture JSP plays the role of View (V). But, Servlets are used to handle the request and process the business logic. In MVC architecture Servlets are knows as Controller (C) .

8. What is the difference in using request.getRequestDispatcher() and context.getRequestDispatcher()?

Ans : Both are taking String parameter, but request.getRequestDispatcher() will dispatch the request inside the application. Where as context.getRequestDispatcher() will dispatch the request outside the context also. If you are using absolute path for dispatching then both are similar.

9. How the JSP file will be executed on the Server side ?

Ans : First its converted to java file. Then its compiled  by java compiler and creates the .class file.Now once you get the .class file you can execute it. Internally a JSP converts into respective servlet.

Conversion => Compilation => Execution

10. What is ActionServlet ?

Ans : ActionServlet provides the controller in struts application with MVC (Model-View-Controller) Model 2. 

ActionServlet is a sub class of javax.servlet.http.HttpServlet.It has few methods like

doGet(),doPost(),destroy(),etc.

11. What is Struts Validator Framework ?

Ans : Struts provides a convenient way to validate. Basically we are using two xml configuration file for configuring such as Validator.xml and validation-rule.xml . validation-rule.xml defines the rule of validation like number format validation,email validation. Apart from this ActionForm having validate() method which we can implement for validating the form data.

12. What is the difference between the Session and SessionFactory in hibernate ?

Ans : Session and SessionFactory are playing a prominent role in hibernate. SessionFactory is used to create Session  and its created once during starting of the application. You can have only one SessionFactory per application. SessionFactory is also called 2^ndlevel cache. But, Session could be many per application. Session is being created by using SessionFactory object.Session is called 1^st level cache.

13. What is HQL ? 

Ans : HQL stands for Hibernate Query Language. Its fully object oriented and quite similar with SQL.It supports association and joins for effective entity relationship.