Core java - Interview Questions and Answers for 'At' | Search Interview Question

Core java - Interview Questions and Answers for 'At' - 391 question(s) found - Order By Newest

Very frequently asked. Among first few questions in almost all interviews. Among Top 5 frequently asked questions. Frequently asked in Indian service companies (HCL,TCS,Infosys,Capgemini etc based on multiple feedback ) and Epam Systems

Q1. Difference between == and .equals() ?

Core Java

Ans. "equals" is the method of object class which is supposed to be overridden to check object equality, whereas "==" operator evaluate to see if the object handlers on the left and right are pointing to the same object in memory.

x.equals(y) means the references x and y are holding objects that are equal. x==y means that the references x and y have same object.

Sample code:

String x = new String("str");
String y = new String("str");

System.out.println(x == y); // prints false
System.out.println(x.equals(y)); // prints true

Sample Code for equals

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java string comparison string object class == equals object equality operator == vs equals equals vs == Asked in 294 Companies basic frequent}

Try 6 Question(s) Test

Advanced level question. Frequently asked in High end product companies. Frequently asked in Google , Cognizant and Deloitte ( Based on 2 feedback )

Q2. Why is String immutable in Java ?

Core Java

Ans. 1. String Pool - When a string is created and if it exists in the pool, the reference of the existing string will be returned instead of creating a new object. If string is not immutable, changing the string with one reference will lead to the wrong value for the other references.

Example -

String str1 = "String1";
String str2 = "String1"; // It doesn't create a new String and rather reuses the string literal from pool

// Now both str1 and str2 pointing to same string object in pool, changing str1 will change it for str2 too

2. To Cache its Hashcode - If string is not immutable, One can change its hashcode and hence it's not fit to be cached.

3. Security - String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Making it mutable might possess threats due to interception by the other code segment.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops string string class immutable immutability advanced Asked in 39 Companies expert frequent}

Try 4 Question(s) Test

Very frequently asked in different variations. Frequently asked in Deloitte ( 2 feedback ) , HCL Tech ( 3 feedback ), TCS and Coginizant (CTS)

Q3. Explain the scenerios to choose between String , StringBuilder and StringBuffer ?

or

What is the difference between String , StringBuilder and StringBuffer ?

Core Java

Ans. If the Object value will not change, use String Class because a String object is immutable.

If the Object value can change and will only be modified from a single thread, use StringBuilder because StringBuilder is unsynchronized(means faster).

If the Object value may change, and can be modified by multiple threads, use a StringBuffer because StringBuffer is thread safe(synchronized).

Sample Code for String

Sample Code for StringBuffer

Sample Code for StringBuilder

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java string class string stringbuilder stringbuffer String vs StringBuffer String vs StringBuilder String vs StringBuilder vs StringBuffer StringBuffer vs stringBuilder Asked in 29 Companies basic frequent}

Try 3 Question(s) Test

Frequently asked to fresh graduates and less experienced developers. Among the first few questions in many interviews.

Q4. What are the difference between composition and inheritance in Java?

Core Java

Ans. Composition - has-a relationship between objects.
Inheritance - is-a relationship between classes.

Composition - Composing object holds a reference to composed objects and hence relationship is loosely bound.
Inheritance - Derived object carries the base class definition in itself and hence its tightly bound.

Composition - Used in Dependency Injection
Inheritance - Used in Runtime Polymorphism

Composition - Single class objects can be composed within multiple classes.
Inheritance - Single class can only inherit 1 Class.

Composition - Its the relationship between objects.
Inheritance - Its the relationship between classes.

Sample Code for inheritance

Sample Code for composition

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java java concepts inheritance object oriented programming (oops) oops concepts composition object oriented programming (oops) oops concepts inheritance object oriented programming (oops) inheritance vs composition object oriented programming (oops) oops concepts Asked in 29 Companies basic frequent}

Try 5 Question(s) Test

Q5. Explain OOPs

or

Explain OOPs Principles

or

Explain OOPs Concepts

or

Explain OOPs features

or

Tell me something about OOPs

Core Java

Ans. OOPs or Object Oriented Programming is a Programming model which is organized around Objects instead of processes. Instead of a process calling series of processes, this model stresses on communication between objects. Objects that all self sustained, provide security by encapsulating it's members and providing abstracted interfaces over the functions it performs. OOP's facilitate the following features

1. Inheritance for Code Reuse
2. Abstraction for modularity, maintenance and agility
3. Encapsulation for security and protection
4. Polymorphism for flexibility and interfacing

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve oops oops features Asked in 260 Companies basic frequent}

Frequently asked at Manhattan Associates ( Based on 2 feedback )

Q6. What is a Lambda Expression ? What's its use ?

Core Java

Ans. Its an anonymous method without any declaration.

Lambda Expression are useful to write shorthand Code and hence saves the effort of writing lengthy Code.

It promotes Developer productivity, Better Readable and Reliable code.

Sample Code for lambda

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java java8 lambda expression architecture Asked in 58 Companies expert frequent}

Try 1 Question(s) Test

Q7. What are different ways to create String Object? Explain.

Core Java

Ans. There are total six ways

1. literals

When we create a String using double quotes, JVM looks in the String pool to find if any other String is stored with same value. If found, it just returns the reference to that String object else it creates a new String object with given value and stores it in the String pool.

2. new keyword

When we use new operator, JVM creates the String object but dont store it into the String Pool. We can use intern() method to store the String object into String pool or return the reference if there is already a String with equal value present in the pool.

3. string buffer
4. string builder
5. System.out.println
6. char to string

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java string class string jvm memory management string pool Asked in 5 Companies basic frequent}

Try 3 Question(s) Test

Q8. Which are the different segments of memory ?

Core Java

Ans. 1. Stack Segment - Contains primitives, Class / Interface names and references.

2. Heap Segment - Contains all created objects in runtime, objects only plus their object attributes (instance variables), Static variables are also stored in heap.

3. Code Segment - The segment where the actual compiled Java bytecodes resides when loaded

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java memory memory management stack memory heap memory code segment memory advanced Asked in 9 Companies expert frequent}

Try 6 Question(s) Test

Very frequently asked. Favorite question in Walk in Drive of many Indian service companies. Frequently asked in HCL Technologies, TCS and Accenture.

Q9. What is the difference between final, finally and finalize() ?

Core Java

Ans. final - constant variable, objects cannot be de-referenced, restricting method overriding, restricting class sub classing.

finally - handles exception. The finally block is optional and provides a mechanism to clean up regardless of what happens within the try block. Use the finally block to close files or to release other system resources like database connections, statements etc.

finalize() - method helps in garbage collection. A method that is invoked before an object is discarded by the garbage collector, allowing it to clean up its state.

Sample Code for final

Sample Code for finally

Sample Code for finalize

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops final finally finalize final vs finally vs finalize Asked in 61 Companies basic frequent}

Try 4 Question(s) Test

Very frequently asked to Fresh graduates.

Q10. What is the difference between Encapsulation and Abstraction?

Core Java

Ans. 1.Abstraction solves the problem at design level while encapsulation solves the problem at implementation level

2.Abstraction is used for hiding the unwanted data and giving relevant data. while Encapsulation means hiding the code and data into a single unit to protect the data from outside world.

3. Abstraction lets you focus on what the object does instead of how it does it while Encapsulation means hiding the internal details or mechanics of how an object does something.

4.For example: Outer Look of a Television, like it has a display screen and channel buttons to change channel it explains Abstraction but Inner Implementation detail of a Television how CRT and Display Screen are connect with each other using different circuits , it explains Encapsulation.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops oops concepts encapsulation object oriented programming (oops) oops concepts abstraction basic interview question encapsulation vs abstraction Asked in 10 Companies basic frequent}

Try 2 Question(s) Test

Q11. Why do we need Inner classes ? Cant we just work with outer classes wherever we implement Inner classes ?

Core Java

Ans. Yes, we can substitute outer classes wherever we need to have inner classes but Inner classes have advantage in certain cases and hence preferred -

Ease - Why to implement a class outside if its objects are only intended to be part of an outer object. Its easy to define the class within another class if the use is only local.

Protection - Making a call an outer exposes a threat of it being used by any of the class. Why should it be made an outer class if its object should only occur as part of other objects.

For example - You may like to have an class address whose object should have a reference to city and by design thats the only use of city you have in your application. Making Address and City as outer class exposes City to any of the Class. Making it an inner class of Address will make sure that its accessed using object of Address.

Sample Code for inner class

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java inner classes classes objects technical lead intermediate}

Try 1 Question(s) Test

Q12. What do you mean by "Java is a statically typed language" ?

Core Java

Ans. It means that the type of variables are checked at compile time in Java.The main advantage here is that all kinds of checking can be done by the compiler and hence will reduce bugs.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java statically typed language variable declaration Asked in 7 Companies basic frequent}

Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.

Q13. What is the difference between ArrayList and LinkedList ?

Core Java

Ans. Underlying data structure for ArrayList is Array whereas LinkedList is the linked list and hence have following differences -

1. ArrayList needs continuous memory locations and hence need to be moved to a bigger space if new elements are to be added to a filled array which is not required for LinkedList.

2. Removal and Insertion at specific place in ArrayList requires moving all elements and hence leads to O(n) insertions and removal whereas its constant O(1) for LinkedList.

3. Random access using index in ArrayList is faster than LinkedList which requires traversing the complete list through references.

4. Though Linear Search takes Similar Time for both, Binary Search using LinkedList requires creating new Model called Binary Search Tree which is slower but offers constant time insertion and deletion.

5. For a set of integers you want to sort using quicksort, it's probably faster to use an array; for a set of large structures you want to sort using selection sort, a linked list will be faster.

Sample Code for ArrayList

Sample Code for LinkedList

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve collections java data structures arraylist linkedlist arraylist vs linkedlist Asked in 61 Companies Basic frequent}

Try 1 Question(s) Test

Q14. What is the difference between declaration, instantiation and initialization ?

Core Java

Ans. Declaration is intimation to the compiler about the nature of Data a reference is going to hold.

For example - List myList;

Instantiation is reservation of memory.

For example

myList = new ArrayList();

Initialization or construction is setting the default values for member elements.

For example

myList = new ArrayList(mySet);

** Example 2nd is both for instantiation as well as initialization. The only difference is that 2nd will initialized the member elements to their default values whereas 3rd will initialized it with the elements from set.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve declaration instantiation initialization construction declaration vs instantiation instantiation vs initialization declaration vs initialization Asked in 1 Companies basic frequent}

Frequently asked in Infosys and HCL Technologies ( Based on 2 feedback )

Q15. What are different ways of object creation in Java ?

Core Java

Ans. Using new operator - new xyzClass()

Using factory methods - xyzFactory.getInstance( )

Using newInstance( ) method - (Class.forName(xyzClass))emp.newInstance( )

By cloning an already available object - (xyzClass)obj1.clone( )

Sample Code for object initialization using clone

Sample Code for object initialization using getInstance

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops object creation new operator class.forname cloning ebay Asked in 9 Companies expert}

Q16. What are the common uses of "this" keyword in java ?

Core Java

Ans. "this" keyword is a reference to the current object and can be used for following -

1. Passing itself to another method.

2. Referring to the instance variable when local variable has the same name.

3. Calling another constructor in constructor chaining.

Sample Code for this keyword

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java this object reference constructor chaining intermediate rare}

Try 3 Question(s) Test

Q17. How can we run a java program without making any object?

Core Java

Ans. By putting code within static method. With Java 6 and earlier versions, even static block can be used.

Sample Code for static block

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops static static method static block Asked in 3 Companies basic frequent}

Try 1 Question(s) Test

Very Frequently asked. Have been asked in HCL Technologies very frequently ( based on 3 feedback ). Among first few questions in many interviews.

Q18. Differences between abstract class and interface ?

Core Java

Ans. Abstract classes can have both abstract methods ( method declarations ) as well as concrete methods ( inherited to the derived classes ) whereas Interfaces can only have abstract methods ( method declarations ).

A class can extend single abstract class whereas it can implement multiple interfaces.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java classes abstract class interfaces abstract class vs interface abstract classes vs interfaces Asked in 82 Companies basic frequent}

Basic and Very Frequently asked.

Q19. What is Polymorphism in Java ?

Core Java

Ans. Polymorphism means the condition of occurring in several different forms.

Polymorphism in Java is achieved in two manners

1. Static polymorphism is the polymorphic resolution identified at compile time and is achieved through function overloading whereas

2. Dynamic polymorphism is the polymorphic resolution identified at runtime and is achieved through method overriding.

Sample Code for overloading

Sample Code for overriding

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve polymorphism object oriented programming (oops) oops concepts oops concepts Asked in 108 Companies Basic frequent}

Try 2 Question(s) Test

Frequently asked.

Q20. If you are given a choice to use either ArrayList and LinkedList, Which one would you use and Why ?

Core Java

Ans. ArrayList are implemented in memory as arrays and hence allows fast retrieval through indices but are costly if new elements are to be inserted in between other elements. LinkedList allows for constant-time insertions or removals using iterators, but only sequential access of elements

1. Retrieval - If Elements are to be retrieved sequentially only, Linked List is preferred.

2. Insertion - If new Elements are to be inserted in between other elements , Linked List is preferred.

3. Search - Binary Search and other optimized way of searching is not possible on Linked List.

4. Sorting - Initial sorting could be pain but lateral addition of elements in a sorted list is good with linked list.

5. Adding Elements - If sufficiently large elements needs to be added very frequently ,Linked List is preferable as elements don't need consecutive memory location.

Sample Code for ArrayList

Sample Code for LinkedList

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections list arraylist linkedlist difference between architecture data structure ebay Asked in 2 Companies basic frequent}

Try 2 Question(s) Test

Frequently asked in all types of companies especially Indian Services companies. Frequently asked in CTS (Based on 2 feedback)

Q21. What is the use of hashcode in Java ?

Core Java

Ans. Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc. The value received from hashcode() is used as bucket number for storing elements. This bucket number is the address of the element inside the set/map. when you do contains() then it will take the hashcode of the element, then look for the bucket where hashcode points to and if more than 1 element is found in the same bucket (multiple objects can have the same hashcode) then it uses the equals() method to evaluate if object are equal, and then decide if contain() is true or false, or decide if element could be added in the set or not.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections hashcode advanced hashtable Asked in 33 Companies intermediate frequent}

Try 1 Question(s) Test

Frequently asked in Infosys India

Q22. What is a String Pool ?

Core Java

Ans. String pool (String intern pool) is a special storage area in Java heap. When a string is created and if the string already exists in the pool, the reference of the existing string will be returned, instead of creating a new object and returning its reference.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops string string class string pool heap memory Asked in 31 Companies intermediate frequent}

Try 2 Question(s) Test

Very Frequently asked to fresh graduates and less experienced. Favorite question in Walk in drives. Frequently asked in Indian Services companies.

Q23. Difference between Overloading and Overriding ?

Core Java

Ans. Overloading - Similar Signature but different definition , like function overloading.

Overriding - Overriding the Definition of base class in the derived class.

Sample Code for overloading

Sample Code for overriding

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops overloading overriding oops concepts basic interview question overloading vs overriding Asked in 86 Companies basic frequent}

Try 1 Question(s) Test

Q24. Can you give a real world example of Encapsulation and Abstraction ?

Core Java

Ans. Car Engine is an example of encapsulation and abstraction. You ignite the car using an interface called starter and least bothered about how the tire actually moves (This is abstraction). The engine encapsulates the complete process to itself only and doesn't allow you to start the other components like the radiator etc ( this is excapsulation )

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve encapsulation object oriented programming (oops) oops concepts abstraction oops oops features java features Asked in 13 Companies}

Frequently asked in Cognizant (CTS)

Q25. What is Serialization ? Why do we need it ?

Core Java

Ans. Storing the state of an object in a file or other medium is called serialization.

Classes can communicate only if they are built together ( as they need Byte code for communication ). What if we need to enable communication between different applications ( i.e they have been built independently or even they reside at different locations ), We need a mechanism that will transfer the Bean state to a Medium than can be transferred to the receiving application.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops serialization Asked in 18 Companies basic frequent}

Try 1 Question(s) Test

Very frequently asked across all types of companies.

Q26. Difference between HashMap and Hashtable?

Core Java

Ans. Hashtable is synchronized whereas HashMap is not.HashMap allows null values whereas Hashtable doesnt allow null values.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java collections hashmap map hashtable hashmap vs hashtable Asked in 14 Companies basic frequent}

Try 2 Question(s) Test

Advanced level question usually asked in High end product companies. Have been asked in Google and Amazon (Based on 1 Feedback)

Q27. Describe, in general, how java's garbage collector works ?

Core Java

Ans. The Java runtime environment deletes objects when it determines that they are no longer being used. This process is known as garbage collection. The Java runtime environment supports a garbage collector that periodically frees the memory used by objects that are no longer needed. The Java garbage collector is a mark-sweep garbage collector that scans Java dynamic memory areas for objects, marking those that are referenced. After all possible paths to objects are investigated, those objects that are not marked (i.e. are not referenced) are known to be garbage and are collected.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java garbage collection java memory management advanced Asked in 21 Companies intermediate frequent}

Try 4 Question(s) Test

Q28. What are the advantages and disadvantages of static variables and static methods ?

Core Java

Ans. Advantages

Can do meta object operations ( like validating something before creating objects , keep count of number of objects )

Can do operations which have nothing to do with objects but still you want them to be tied to Class.

Disadvantages

Commonly used to static variables sometime leads to problems due to access by different objects.

Are not tied to objects so doesn't reflect pure Object Oriented approach.

Needs to be synchronized so as to avoid update conflicts by mutiple objects and threads.

Some limitation in testing as not all frameworks have facility to mock them. Powermock has but Mockito doesnt

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java static static variables static methods}

Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.

Q29. What is a final method ?

Core Java

Ans. Its a method which cannot be overridden. Compiler throws an error if we try to override a method which has been declared final in the parent class.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops final final method Asked in 30 Companies basic frequent}

Try 1 Question(s) Test

Q30. What is casting?

Core Java

Ans. There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java data types casting type casting basic interview question Asked in 8 Companies basic frequent}

Search Interview Questions

Help us and Others Improve. Please let us know the questions asked in any of your previous interview.