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 ?

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.