Search Interview Questions | More than 3000 questions in repository. There are more than 900 unanswered questions. Click here and help us by providing the answer. Have a video suggestion. Click Correct / Improve and please let us know. |
|
| ||||
Core java - Interview Questions and Answers for 'Abs' - 167 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 | ||||
| ||||
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 ) | ||||
| ||||
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) | ||||
| ||||
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 | ||||
| ||||
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 ) | ||||
| ||||
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 | ||||
Asked multiple times in Capgemini. | ||||
| ||||
Ans. String is immutable in java and stored in String pool. Once it's created it stays in the pool until unless garbage collected, so even though we are done with password it's available in memory for longer duration and there is no way to avoid it. It's a security risk because anyone having access to memory dump can find the password as clear text. | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  java   string class   string   immutable  immutability   string pool   garbage collection   advanced Asked in 6 Companies Expert | ||||
Very frequently asked. Favorite question in Walk in Drive of many Indian service companies. Frequently asked in HCL Technologies, TCS and Accenture. | ||||
| ||||
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. | ||||
| ||||
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 | ||||
| ||||
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. | ||||
| ||||
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 | ||||
Frequently asked in Infosys and HCL Technologies ( Based on 2 feedback ) | ||||
| ||||
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 | ||||
Very Frequently asked. Have been asked in HCL Technologies very frequently ( based on 3 feedback ). Among first few questions in many interviews. | ||||
| ||||
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. | ||||
| ||||
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 in all types of companies especially Indian Services companies. Frequently asked in CTS (Based on 2 feedback) | ||||
| ||||
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 | ||||
Very Frequently asked to fresh graduates and less experienced. Favorite question in Walk in drives. Frequently asked in Indian Services companies. | ||||
| ||||
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 | ||||
| ||||
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 | ||||
Very frequently asked across all types of companies. | ||||
| ||||
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 | ||||
| ||||
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 | ||||
| ||||
Ans. 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 or hacker over internet. Once a String constant is created in Java , it stays in string constant pool until garbage collected and hence stays there much longer than what's needed. Any unauthorized access to string Pool pose a threat of exposing these values. | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  Security  String pool   string immutable  immutability expert   rare | ||||
Advanced level question usually asked to senior developers , leads and architects. | ||||
| ||||
Ans. Volatile is an instruction that the variables can be accessed by multiple threads and hence shouldn't be cached. As volatile variables are never cached and hence their retrieval cannot be optimized. | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  java   java keywords   volatile   synchronization   compiler optimization   variable caching   architecture Asked in 3 Companies expert | ||||
Frequently asked to fresh graduates and less experienced. | ||||
| ||||
Ans. 1. public is the access modifier that makes the method accessible from anywhere, static is the keyword that makes it accessible even without creating any object, void means it doesn't return anything , String args[] is the array of argument that the method receives. 2. If we use main without the string args , it will compile correctly as Java will treat it as just another method. It wont be the method "main" which Java looks for when it looks to execute the class and hence will throw Error: Main method not found in class , please define the main method as: public static void main(String[] args) 3. Main is not a keyword but a special string that Java looks for while initiating the main thread. | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  java   main method Asked in 4 Companies basic   frequent | ||||
Try 1 Question(s) Test | ||||
| ||||
Ans. Because it doesn't make the change in the existing string but would create a new string by concatenating the new string to previous string. So Original string won't get changed but a new string will be created. That is why when we say str1.concat("Hello"); It means nothing because we haven't specified the reference to the new string and we have no way to access the new concatenated string. Accessing str1 with the above code will still give the original string. | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  string  string immutable  immutability  string concat | ||||
Very Frequently asked across all type of companies and across all levels. | ||||
| ||||
Ans. Private - Not accessible outside object scope. Public - Accessible from anywhere. Default - Accessible from anywhere within same package. Protected - Accessible from object and the sub class objects. | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  java   oop   access specifier   public   private   default   protected   public vs private vs default vs protected Asked in 12 Companies basic   frequent | ||||
Try 1 Question(s) Test | ||||
Frequently asked question for intermediate developers. Frequently asked in HCL Technologies and EPAM. | ||||
| ||||
Ans. Volatile is a declaration that a variable can be accessed by multiple threads and hence shouldnt be cached. | ||||
Sample Code for volatile | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  java   oops   synchronization   volatile   java keywords Asked in 42 Companies intermediate   frequent | ||||
Try 1 Question(s) Test | ||||
| ||||
Ans. It in Java is used to indicate that a field should not be serialized. | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  java   oops   serialization   transient   java keywords Asked in 39 Companies intermediate   frequent | ||||
Try 2 Question(s) Test | ||||
| ||||
Ans. All exceptions must be a child of Throwable. If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class. You want to write a runtime exception, you need to extend the RuntimeException class. | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  java   exceptions   exception handling   user defined exceptions   throwable   architecture   library development   technical architect   technical lead | ||||
| ||||
Ans. We can make a class immutable by 1. Making all methods and variables as private. 2. Setting variables within constructor. Public Class ImmutableClass{ private int member; ImmutableClass(int var){ member=var; } } and then we can initialize the object of the class as ImmutableClass immutableObject = new ImmutableClass(5); Now all members being private , you cant change the state of the object. | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  java   oops   immutable  immutability   immutable  immutability class   technical lead Asked in 5 Companies intermediate   frequent | ||||
Try 2 Question(s) Test | ||||
| ||||
Ans. Thread class holds the definition of start method ( This is the method that starts execution of new thread and then calls run method within the scope of new thread ). Interfaces don't hold any definition and so does runnable. So it makes it necessary the usage of Thread class , whatever implementation you choose. When your class extends the thread class, it carries the definition of start method from parent Thread class onto itself and hence new yourClass.start() helps starting a new thread and then executing run method in that new thread scope. When you implement runnable interface , you are just making it sure to the JVM that you have implemented the required method ( run() ) which the Thread start method will look for upon executing start method. | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  java   threads   multithreading   runnable interface   thread class | ||||
| ||||
Ans. HashTable has been deprecated. As an alternative, ConcurrentHashMap has been provided. It uses multiple buckets to store data and hence much better performance than HashTable. Moreover, there is already a raw type HashMap. The only difference between the HashTable and HashMap is that Hashtable is synchronized whereas HashMap is not. Most of the synchronized collections have been deprecated and their raw alternative have been presented as preferred.Synchronization has a cost. Using synchronized collection in places where there is no need of it leads to useless utilization of resources. As these collections are rarely used in a static context or shared among threads, Java might have thought it better to just provide the raw collection and let developers implement synchronization if he feels the need to do so. HashMap is now presented as the default and the preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable. Because of this, Java thought it right to deprecate the use of HashTable.' Synchronization has a cost. Using synchronized collection at a place where there is hardly any need of it would means useless utilization of resources. As these collections are rarely used in static context or shared among threads, Java might have thought it better to just provide the raw collection and let developer implement synchronization if he feels the need to do so. As HashMap has been presented as default and preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable, Java thought it right to deprecate the use of HashTable. | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  hashtable  synchronized collections  Why synchronized collections have been deprecated  Why HashTable has been deprecated  HashTable vs HashMap expert | ||||
Try 3 Question(s) Test | ||||
Very frequently asked. Favorite question in Walk in Drive of many Indian service companies. | ||||
| ||||
Ans. Final variable is a variable constant that cannot be changed after initialization. | ||||
Sample Code for final variable | ||||
Help us improve. Please let us know the company, where you were asked this question : | ||||
Like Discuss Correct / Improve  oops   java   final   final variable   basic interview question Asked in 12 Companies basic   frequent | ||||
Try 1 Question(s) Test | ||||