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Core java - Interview Questions and Answers for 'At' - 421 question(s) found - Order By Newest

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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 )
  Q1. Difference between == and .equals() ?Core Java
Anonymous
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

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Advanced level question. Frequently asked in High end product companies. Frequently asked in Cognizant and Deloitte ( Based on 2 feedback )
  Q2. Why is String immutable in Java ?Core Java
Anonymous
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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Very frequently asked in different variations. Frequently asked in Deloitte ( 2 feedback ) , HCL Tech ( 3 feedback ) 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
Anonymous
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

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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
Admin
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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

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  Q5. Which are the different segments of memory ?Core Java
Anonymous
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

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 Q6. What are different ways to create String Object? Explain.Core Java
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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




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 Q7. Why do we need Inner classes ? Cant we just work with outer classes wherever we implement Inner classes ?Core Java
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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.

  Sample Code for inner class

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  Q8. What do you mean by "Java is a statically typed language" ?Core Java
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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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Very frequently asked to Fresh graduates.
  Q9. What is the difference between Encapsulation and Abstraction?Core Java
Anonymous
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.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies. Frequently asked in HCL Technologies, TCS and Accenture.
  Q10. What is the difference between final, finally and finalize() ?Core Java
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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

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Frequently asked in Infosys and HCL Technologies ( Based on 2 feedback )
 Q11. What are different ways of object creation in Java ?Core Java
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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

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  Q12. What is a Lambda Expression ? What's its use ?Core Java
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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

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 Q13. What is the difference between declaration, instantiation and initialization ?Core Java2016-05-26 14:17:48

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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 Q14. What are the common uses of "this" keyword in java ?Core Java
Anonymous
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.
 Q15. If you are given a choice to use either ArrayList and LinkedList, Which one would you use and Why ?Core Java
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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

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  Q16. Explain OOPs

or

Explain OOPs Principles

or

Explain OOPs Concepts

or

Explain OOPs features

or

Tell me something about OOPs
Core Java2017-06-13 08:25:45

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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 Q17. How can we run a java program without making any object?Core Java
Anonymous
Ans. By putting code within static method. With Java 6 and earlier versions, even static block can be used.

  Sample Code for static block

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 Q18. Can we use both "this()" and "super()" in a constructor ?Core Java
Anonymous
Ans. No, because both this and super should be the first statement.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q19. What is the difference between ArrayList and LinkedList ?Core Java
admin
info@buggybread.com
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

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Basic and Very Frequently asked.
  Q20. What is Polymorphism in Java ?Core Java2016-11-06 09:10:54

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

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Very frequently asked in HCL Tech ( Based of 4 inputs )
  Q21. Write a Program to reverse a string iteratively and recursivelyCore Java

Ans. Using String method -

new StringBuffer(str).reverse().toString();

Iterative -

Strategy - Loop through each character of a String from last to first and append the character to StringBuilder / StringBuffer

public static String getReverseString(String str){
StringBuffer strBuffer = new StringBuffer(str.length);
for(int counter=str.length -1 ; counter>=0;counter--){
strBuffer.append(str.charAt(counter));
}
return strBuffer;
}

Recursive -

Strategy - Call the method with substring starting from 2nd character recursively till we have just 1 character.

public static String getReverseString(String str){
if(str.length <= 1){
return str;
}
return (getReverseString(str.subString(1)) + str.charAt(0);
}

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  Q22. What is a String Pool ?Core Java
Anonymous
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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 Q23. Why every object constructor automatically call super() in Object before its own constructors?Core Java
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Ans. Derived object carries the body of its class as well as the body of the parent class. Its body ( member elements ) is initialized using its own class constructor whereas the body ( member elements ) carried from the parent class are initialized using super class constructor. So In order to initialize the elements of the parent class before its own elements are even initialized, super is called.

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 Q24. What are the advantages and disadvantages of static variables and static methods ?Core Java
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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

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q25. Difference between TreeMap and HashMap ?Core Java
Anonymous
Ans. They are different the way their elements are stored in memory. TreeMap stores the Keys in order whereas HashMap stores the key value pairs randomly.

  Sample Code for treemap

  Sample Code for hashmap

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Very frequently asked across all types of companies.
  Q26. Difference between HashMap and Hashtable?Core Java
Anonymous
Ans. Hashtable is synchronized whereas HashMap is not.HashMap allows null values whereas Hashtable doesnt allow null values.

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Frequently asked in all types of companies especially Indian Services companies. Frequently asked in CTS (Based on 2 feedback)
  Q27. What is the use of hashcode in Java ?Core Java
Anonymous
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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 Q28. Describe what happens when an object is created in Java ?Core Java
Anonymous
Ans. 1. Memory is allocated from heap to hold all instance variables and implementation-specific data of the object and its superclasses. Implemenation-specific data includes pointers to class and method data.
2. The instance variables of the objects are initialized to their default values.
3. The constructor for the most derived class is invoked. The first thing a constructor does is call the constructor for its superclasses. This process continues until the constructor for java.lang.Object is called,as java.lang.Object is the base class for all objects in java.
4. Before the body of the constructor is executed, all instance variable initializers and initialization blocks are executed. Then the body of the constructor is executed. Thus, the constructor for the base class completes first and constructor for the most derived class completes last.

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Frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q29. What are the methods of Object Class ?Core Java
Anonymous
Ans. clone() - Creates and returns a copy of this object.

equals() - Indicates whether some other object is "equal to" this one.

finalize() - Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.

getClass() - Returns the runtime class of an object.

hashCode() - Returns a hash code value for the object.

toString() - Returns a string representation of the object.

notify(), notifyAll(), and wait() - Play a part in synchronizing the activities of independently running threads in a program.

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Advanced level question usually asked to senior developers , leads and architects.
 Q30. How does volatile affect code optimization by compiler?Core Java
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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.

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 Q31. Difference between Static and Singleton Class ?Core Java2016-11-30 21:00:43

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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Frequently asked to fresh graduates and less experienced.
 Q32. Why do we write public static void main ? Can we use some other syntax too for main ?Core Java
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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.

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Very Frequently asked. Have been asked in HCL Technologies very frequently ( based on 3 feedback ). Among first few questions in many interviews.
  Q33. Differences between abstract class and interface ?Core Java
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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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 Q34. Can you give a real world example of Encapsulation and Abstraction ?Core Java2016-05-29 13:47:31

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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 Q35. Can we declare static variables as transient ?Core Java2017-01-12 11:12:48

Ans. It's weird that compiler doesn't complain if we declare transient with static variable because it makes no sense. At least a warning message saying "transient is useless in this situation" would have helped with code cleaning.

Static variables are never serialized and transient is an indication that the specified variable shouldn't be serialized so its kind of double enforcement not to serialize.

It could be that as it makes no different to the variable behavior and hence using both keywords with a variable are permitted.

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   Like         Discuss         Correct / Improve     static  transient  serialization     Asked in 1 Companies      expert        rare

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 Q36. How is string object immutable if we can concat a string to it ?Core Java2017-08-08 09:50:28

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.

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Very Frequently asked across all type of companies and across all levels.
  Q37. Difference between Public, Private, Default and Protected ?Core Java
Anonymous
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.

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   Like         Discuss         Correct / Improve     java   oop   access specifier   public   private   default   protected   basic interview question     Asked in 11 Companies      basic        frequent

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q38. What is a final method ?Core Java
Anonymous
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.

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

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  Q39. What is casting?Core Java
Anonymous
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

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


Advanced level question usually asked in High end product companies. Have been asked in Google and Amazon (Based on 1 Feedback)
  Q40. Describe, in general, how java's garbage collector works ?Core Java
Anonymous
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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   Like         Discuss         Correct / Improve     java   garbage collection   java memory management   advanced     Asked in 21 Companies      intermediate        frequent

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