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Interview Questions and Answers for 'Class' - 209 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
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.
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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).
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
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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.
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.
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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.
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.
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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.
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.
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LikeDiscussCorrect / Improve  declaration   instantiation   initialization   construction   declaration vs instantiation   instantiation vs initialization   declaration vs initialization Asked in 1 Companiesbasic  frequent
Frequently asked in Infosys and HCL Technologies ( Based on 2 feedback )
Q11. What are different ways of object creation in 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.
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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.
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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 )
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Ans. 1. Static class is a class which cannot be instantiated and all its members are static whereas Singleton is the class that only permit creation of single object and then the object is reused.
2. As there is no object in Static class, it cannot participate in runtime Polymorphism.
3. As Static class doesnt allow creating objects and hence it cannot be serialized.
4. Static class body is initialized eagerly at application load time whereas Singleton object can be initiated eagerly using static blocks or lazily on first need.
5. Its not recommended to use pure static class as it fails to use many OOPs concepts.
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Ans. REST or Representational State Transfer is a flexible architecture style for creating web services that recommends the following guidelines -
1. http for client server communication,
2. XML / JSON as formatiing language ,
3. Simple URI as address for the services and,
4. stateless communication.
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Ans. It is a Design Pattern that facilitates loose coupling by sending the dependency information ( object references of dependent object ) while building the state of the object. Objects are designed in a manner where they receive instances of the objects from other pieces of code, instead of constructing them internally and hence provide better flexibility.
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Ans. FALSE. == operator compares object references, a and b are references to two different objects, hence the FALSE. .equals method is used to compare string object content.
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Ans. Bootstrap - Loads JDK internal classes, java.* packages.
Extensions - Loads jar files from JDK extensions directory - usually lib/ext directory of the JRE
System - Loads classes from system classpath.
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LikeDiscussCorrect / Improve  java   jvm   memory management   class loaders   bootstrap   extensions   system  classloaders   advanced   technical lead   technical architect Asked in 7 Companies
Q24. Which access specifiers can be used with top level class ? a. public or default b. public or private c. public or protected d. protected or default
Q25. Which of the following can be marked static ?
a. Methods , Variables and Initialization Blocks.
b. Methods , Variables , Initialization Blocks and Outer Classes and nested Classes.
c. Methods , Variables , Initialization Blocks and Outer Classes.
d. Methods , Variables , Initialization Blocks and nested Classes
Ans. In first case we are trying to initialize Inner class object using the instance of Outer Class whereas in second case we are trying to initialize the Inner class object directly using the Outer class name.
In second case , Inner class is "static inner class" as we cannot access "non static inner class" using Classname alone.
In first case, the inner class could be either "static inner class" or "non static inner class".
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