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Q31. Which elements of a class are ignored during serialization ?

Ans. 1. Objects are serialized and not classes and hence Static variables are ignored.

2. Transient is an explicit declaration to ignore the variable during serialization and hence transient instance variables are ignored too.

3. Base class instance variables if the base class hasn't been declared serializable.

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Q32. Will finally run if we have return statement and it exits the method early ?

Core Java

Ans. finally will execute in all graceful situations - graceful executions as well as graceful exceptions. The only situation when finally block won't execute is when the app is abruptly stopped, killed or unplugged.

Sample Code for finally

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Frequently asked in high end product companies.

Q33. Write code for LRU Cache

Core Java

Ans. https://www.geeksforgeeks.org/lru-cache-implementation/

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Q34. When is the situation when finally section won't execute ?

Core Java

Ans. If the process / app is abruptly killed or terminated.

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Q35. What is database Normalization ?

Database

Ans. https://en.wikipedia.org/wiki/Database_normalization

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Q36. What do you look for when you do code review ?

Core Java

Ans. Modularity - First sign of good code is whether it has been segregated into methods and classes appropriately. I dont mind it in excess because I believe that is forward looking strategy as applications tends to expand and eventually become hard to read.

Self Explanatory - Variables and methods should be named in a way that the code should be self explanatory even without comments. Use of Constant variables to explain use of literal.

Proper Code Reuse - If there is anything being reused , it should be moved to parent classes / methods.

Proper composition calls - Composed hierarchy should not be access in just single line. One or two levels is ok but having multiple levels make it hard to read and debug.

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Frequently asked in Alibaba (Based on 2 feedback)

Q37. What is the best Memory setting for JVM ?

Core Java

Ans. In Java JVM memory settings is done by use the arguments -Xms -Xmx. Use M or G after the numbers for indicating Megs and Gigs of bytes respectively. -Xms indicates the minimum and -Xmx the maximum.

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Very Frequently asked to Senior Software Engineers or Developers.

Q38. Describe some of the Design Patterns you have used ?

Design

Ans. [Open Ended Answer]

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Q39. Difference between Static and Singleton Class ?

Core Java

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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Q40. Write an Algorithm for Graph Traversal ? The Graph has a loop.

Algorithm

Ans. Please not that all such questions can be easily answered through recursion.

Simple recursive implementation could be

traverse(root);

void traverse(Element element){
if(element.hasNext()){
traverse(element.next());
} else {
System.out.println(element);
}
}

but this algo / code lead to endless loop if there is a loop in graph traversal.

So you can keep a collection to keep track of which elements have laready been traversed

static List<Elements> listOfAlreadyTraversedElements = new ArrayList<Elements>();

main(){
traverse(root);
}

void traverse(Element element){
if(element.hasNext()){
traverse(element.next());
} else {
listOfAlreadyTraversedElements.add(element);
System.out.println(element);
}
}

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Q41. How can one determine if JVM is 32-bit or 64-bit from Java Program ?

Core Java

Ans. There is a Java system property "sun.arch.data.model" that can tell if JVM is 32 bit of 64 bit

System.getProperty("sun.arch.data.model") can be used to get that property.

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Usually asked with Questions related to Generics.

Q42. What are Type Erasures in Java ?

Core Java

Ans. Type erasure applies to the use of generics. When generics are used, they're pre compiled into compile time checks and execution-time casts.

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Frequently asked to fresh graduates.

Q43. What is ACID ?

Database

Ans. ACID stands for Atomicity, Consistency, Isolation, Durability is a set of properties of database transactions.

Atomicity means all or nothing. i.e parts of a transaction shouldn't commit if any one of them fails. Either the whole transaction should succeed or it should be complete rollback.

Consistency means that any transaction should lead database from one stabe state to another.

Isolation means that the execution of transaction results in a system state that would be obtained if transactions were executed serially.

Durability means that when a transaction is committed it forms the permanent state of database.

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Frequently asked Design Pattern interview question.

Q44. What is a prototype design pattern ?

Design

Ans. The prototype pattern is a creational design pattern. It is used when the type of objects to create is determined by a prototypical instance, which is cloned to produce new objects. Prototype is used when we need duplicate copies of objects.

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Q45. How to Bulk upload the data into Oracle database?

Database

Ans. We can use external table feature of Oracle.

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Q46. Can we have try statement without catch? If try statement contains return will the finally block be executed? What happens if there is an exception inside finally block?

Core Java

Ans. Yes, with finally.

Yes, finally block will be executed even if there is no exception in try block.

If finally throws an exception, the exception gets thrown to the calling module.

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Q47. Should a DB Insert Request be a POST or PUT request in Rest

Rest

Ans. It depends on whether the request object is persisted as it is

or

its first dismantled, modified or refactored and then inserted into DB.

In first case, it should be a PUT request whereas in second case it should be a POST Request.

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Q48. What are fail-fast Iterators ?

Core Java

Ans. Fail-fast iterators detect illegal concurrent modification during iteration and fail quickly and cleanly rather than risking arbitrary, non deterministic behavior at an undetermined time in future. Example could be of an Iterator failing if it smells ConcurrentModificationException.

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Q49. Does Constructor creates the object ?

Core Java

Ans. New operator in Java creates objects. Constructor is the later step in object creation. Constructor's job is to initialize the members after the object has reserved memory for itself.

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Q50. Does java allow overriding static methods ?

Core Java

Ans. No. Static methods belong to the class and not the objects. They belong to the class and hence doesn't fit properly for the polymorphic behavior.

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Q51. When are static variables loaded in memory ?

Core Java

Ans. They are loaded at runtime when the respective Class is loaded.

Sample Code for static variable

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Q52. Can we serialize static variables ?

Core Java

Ans. No. Only Object and its members are serialized. Static variables are shared variables and doesn't correspond to a specific object.

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Q53. 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

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Frequently asked to fresh graduates and less experienced developers.

Q54. Explain multithreading in Java ?

Core Java

Ans. 1. Multithreading provides better interaction with the user by distribution of task

2. Threads in Java appear to run concurrently, so it provides simulation for simultaneous activities.The processor runs each thread for a short time and switches among the threads to simulate sim-ultaneous execution (context-switching) and it make appears that each thread has its own processor.By using this feature, users can make it appear as if multiple tasks are occurring simultaneously when, in fact, each is running for only a brief time before the context is switched to the next thread.

3. We can do other things while waiting for slow I/O operations.In the java.iopackage, the class InputStreamhas a method, read(), that blocks until a byte is read from the stream or until an IOExceptionis thrown. The thread that executes this method cannot do anything elsewhile awaiting the arrival of another byte on the stream.

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

Q55. 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.

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Q56. What are transient variables in java?

Core Java

Ans. Transient variables are variable that cannot be serialized.

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Frequently asked in Infosys India

Q57. 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.

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Q58. what is the use of cookie and session ? and What is the difference between them ?

Java EE

Ans. Cookie and Session are used to store the user information. Cookie stores user information on client side and Session does it on server side. Primarily, Cookies and Session are used for authentication, user preferences, and carrying information across multiple requests. Session is meant for the same purpose as the cookie does. Session does it on server side and Cookie does it on client side. One more thing that quite differentiates between Cookie and Session. Cookie is used only for storing the textual information. Session can be used to store both textual information and objects.

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

Q59. 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.

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Q60. What is the role of JSON.stringify ?

Json

Ans. JSON.stringify() turns an object into a JSON text and stores that JSON text in a string.

So If we stringfy above notation , it will become

{"name":"xyz","gender":"male";"age":30}

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