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

Core java - Interview Questions and Answers for 'Abs' - 160 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 )

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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Q4. What do you mean by "Java is a statically typed language" ?

Core Java

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

Q5. What are different ways of object creation in Java ?

Core Java

^{Admin
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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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Very frequently asked to Fresh graduates.

Q6. 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.

Q7. What is the difference between final, finally and finalize() ?

Core Java

^{Admin
info@buggybread.com}

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

Q8. 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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Q9. How does making string as immutable helps with securing information ? How does String Pool pose a security threat ?

Core Java

^{2017-06-22 11:32:57}

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.

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Very frequently asked across all types of companies.

Q10. 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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Q11. Why Char array is preferred over String for storing password?

Core Java

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

Q12. 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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Advanced level question usually asked to senior developers , leads and architects.

Q13. How does volatile affect code optimization by compiler?

Core Java

^{Admin
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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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Frequently asked to fresh graduates and less experienced.

Q14. Why do we write public static void main ? Can we use some other syntax too for main ?

Core Java

^{Admin
info@buggybread.com}

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 and using class name only , void means it doesnt return anything , String args[] is the array of argument that this method receives.

2. If I use Main , it will compile correctly as Java will treat it as just another method but 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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Q15. What are the advantages and disadvantages of static variables and static methods ?

Core Java

^{Admin
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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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Q16. How is string object immutable if we can concat a string to it ?

Core Java

^{2017-08-08 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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Q17. What things should be kept in mind while creating your own exceptions in Java?

Core Java

^{Admin
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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.

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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.

Q18. Differences between abstract class and interface ?

Core Java

^{Admin
info@buggybread.com}

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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Q19. Can you give a real world example of Encapsulation and Abstraction ?

Core Java

^{2016-05-29 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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Very Frequently asked across all type of companies and across all levels.

Q20. 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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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.

Q21. What is a Final Variable ?

Core Java

^Anonymous

Ans. Final variable is a variable constant that cannot be changed after initialization.

Sample Code for final variable

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Q22. Can we serialize static variables ?

Core Java

^Anonymous

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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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

^Anonymous

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

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Q24. Difference between Vector and ArrayList ?

Core Java

^Anonymous

Ans. Vectors are synchronized whereas Array lists are not.

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Very frequently asked in phone and walk in interviews.

Q25. What are Marker Interfaces ? Name few Java marker interfaces ?

Core Java

^Anonymous

Ans. These are the interfaces which have no declared methods.Serializable and cloneable are marker interfaces.

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Q26. What are the disadvantages of using arrays ?

Core Java

^{Admin
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Ans. Arrays are of fixed size and have to reserve memory prior to use. Hence if we don't know size in advance arrays are not recommended to use.

Arrays can store only homogeneous elements.

Arrays store its values in contentious memory location. Not suitable if the content is too large and needs to be distributed in memory.

There is no underlying data structure for arrays and no ready made method support for arrays, for every requriment we need to code explicitly

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Q27. Why do we need Thread class even in case we execute thread using runnable interface ?

Core Java

^{Admin
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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.

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Q28. Which of the following Java feature promotes Code Re usability ?

a. Abstraction
b. Encapsulation
c. Inheritance
d. Polymorphism

Core Java

^Anonymous

Ans. Inheritance

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Q29. Why HashTable has been deprecated ?

Core Java

^{2016-07-08 10:01:58}

Ans. HashTable has been Deprecated as an alternative ConcurrentHashMap has been provided which 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 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.

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Q30. What are different types of variables a class can have ?

Core Java

^{2017-01-07 16:58:43}

Ans. 1. Method local variables - These are declared and defined within a method ( instance or static methods ) and their scope is limited to the method itself. They are destructed once the execution of method completes. They are stored in stack memory.

2. Instance variables - These are declared as non static variables as part of the class.They are initialized as part of object creation ( constructor ) and are destructed by java's garbage collection mechanism and hence stored in heap.

3. Static variables - These are declared with the static keyword and are part of the class. They are initialized at the time of class loading and are destructed by java's garbage collection mechanism and hence stored in heap.

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Q31. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?

Core Java

^{2017-05-07 07:26:29}

Ans. Such a class still can have member elements which can be inherited and hence facilitate code reuse. Moreover Abstract class can have non final static elements whereas interfaces are only allowed to have static final elements.

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Q32. When we say final x = 10, is the reference final or the value ?

Core Java

^{2017-07-29 13:27:04}

Ans. final keyword have meaning only to referenced and not the value. It means that the specified reference cannot be dereferenced. It doesn't control the value assigned to the memory that's being referenced. This is the reason that final object references doesn't mean that the object is immutable but means that the reference cannot be changed to point to new object.

In case of primitive types too, when we assign a reference to another, values are passed and not the object reference, and hence a new placeholder is created in memory with the same value. That is why final to that context means that you cannot change the assigned memory and there is no way we can have that memory place have another value.

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Q33. When are static variables loaded in memory ?

Core Java

^Anonymous

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

Sample Code for static variable

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Q34. Can we use null as a key for a map collection ?

Core Java

^Anonymous

Ans. Yes , for HashMap. HashMap implements Map interface. HashMap allows one null key and any number of null values.

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Q35. Difference between Abstract and Concrete Class ?

Core Java

^Anonymous

Ans. Abstract classes are only meant to be sub classed and not meant to be instantiated whereas concrete classes are meant to be instantiated.

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Frequently asked question for intermediate developers.

Q36. What is Volatile keyword used for ?

Core Java

^Anonymous

Ans. Volatile is a declaration that a variable can be accessed by multiple threads and hence shouldnt be cached.

Sample Code for volatile

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Very frequently asked. Usually asked with questions related to String.

Q37. What is an Immutable Object ?

Core Java

^Anonymous

Ans. Object that can't be changed after instantiation.

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Very Frequently asked. Usually asked along with String Class related questions.

Q38. What is an immutable class ?

Core Java

^Anonymous

Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created.

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Q39. How to implement an immutable class ?

Core Java

^Anonymous

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.

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Q40. Does Declaring an object "final" makes it immutable ?

Core Java

^Anonymous

Ans. Only declaring variables as final makes them immutable. Making objects final means that the object handler cannot be used to target some other object but the object is still mutable.

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