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Interview Questions and Answers for 'Class' - 184 question(s) found - Order By Newest

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Very Very frequently asked. Among the first few questions in almost all interviews.Among Top 5 frequently asked questions. Frequently asked in Indian service companies (HCL,TCS,Infosys 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

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Advanced level question. Frequently asked in High end product companies. Frequently asked in Cognizant ( Based on 2 feedbacks )
  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. Why do we need Inner classes ? Cant we just work with outer classes wherever we implement Inner classes ?Core Java
Admin
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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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Frequently asked in Infosys and HCL Technologies ( Based on 2 feedback )
 Q5. 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( )

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 Q6. What are different ways to create String Object? Explain.Core Java
Admin
info@buggybread.com
Ans.
String str = new String("abc");
String str1 = "abc";


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.

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.

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 Q7. 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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 Q8. 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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 Q9. What are different types of classes ?Core Java

Ans. There are different verticals in which Java Classes can be classified.

1. Access - Public , Private , default or Protected.

2. Packaging - System, library or User Defined

3. Structure - Outer or Inner

4. Object Derivation - Abstract Class or Concrete Class.

5. Object Creation - Normal, Singleton , Doubleton , Immutable or Enum.

6. Functionality - String , Util , Stream etc.

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 Q10. Why Char array is preferred over String for storing password?Core Java
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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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 Q11. What will this code print ?

String a = new String ("TEST");
String b = new String ("TEST");
if(a == b) {
System.out.println ("TRUE");
} else {
System.out.println ("FALSE");
}
Core Java
Anonymous
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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Frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q12. 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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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q13. 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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Very Frequently asked. Have been asked in HCL Technologies very frequently ( based on 2 feedback ). Among first few questions in many interviews.
  Q14. Differences between abstract class and interface ?Core Java
Admin
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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 ).

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 Q15. 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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  Q16. 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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Advanced level question. Recently asked in few Indian service companies ( Based on 3 inputs )
 Q17. What are various types of Class loaders used by JVM ?Core Java
Admin
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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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 Q18. Difference Between this() and super() ?
Core Java
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Ans. 1.this is a reference to the current object in which this keyword is used whereas super is a reference used to access members specific to the parent Class.

2.this is primarily used for accessing member variables if local variables have same name, for constructor chaining and for passing itself to some method whereas super is primarily used to initialize base class members within derived class constructor.

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 Q19. What are the Disadvantages of using Collection Classes over Arrays ?Core Java
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Ans. Collections can only hold objects, It can't hold primitive data types.

Collections have performance overheads as they deal with objects and offer dynamic memory expansion. This dynamic expansion could be a bigger overhead if the collection class needs consecutive memory location like Vectors.

Collections doesn't allow modification while traversal as it may lead to concurrentModificationException.

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 Q20. 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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 Q21. What are the contents of Hibernate configuration file ( hibernate.cfg.xml ) ?Hibernate
Admin
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Ans. HBM Files ( Mapping )
DB Connection ( DB Connection String , User Name , Password , Pool Size )
SQL Dialect ( SQL variant to be generated )
Show SQL ( Show / No show SQL on Console )
Auto Commit ( True / False )

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 Q22. 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
Core Java
Anonymous
Ans. Methods , Variables , Initialization Blocks and nested Classes

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Almost sure to be asked in every company using any Dependency Injection framework ( Spring, Guice etc )
  Q23. What is Dependency Injection or IOC ( Inversion of Control ) ?Design

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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 Q24. In which cases , moving methods to utility class could be useful ?Core Java

Ans. It could be worthy to move a method to util class if the method needs to be shared, doesn't require polymorphic behavior and need not be overridden in special cases.

Don't belong to one group through is-a relationship ( You can share through parent class method )

Don't implement a specific interface ( java 8 default methods )

Doesn't involve complex computing as you will be loosing the benefit of object state with just static method.

Doesn't require polymorphic behavior as static methods don't participate in runtime polymorphism.

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 Q25. What is the difference between the following two code lines ?

1. new OuterClass().new InnerClass();

2. new OuterClass.InnerClass();
Core Java2016-06-30 14:45:37

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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 Q26. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?Core Java2017-05-07 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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 Q27. Which of the following are valid declarations

1. void method(int... x){};
2. void method(int.. x){};
3. void method(int.. .x){};
4. void method(int ...x){};
5. void method(int... x){};
6. void method(int ... x){};
7. void method(int x, int... y){};
8. void method(int... x, int y){};
9. void method(int... x,int... y){};
Core Java2017-05-22 21:22:57

Ans. 1st is a valid and standard declaration.

2nd results in compilation error as only 2 dots are there.

3rd results in compilation error as three dots are not consecutive and broken.

4 through 6 may not be standard and ideal way of declarations but they are valid and will compile and work fine.

7 is valid declaration.

8 and 9 will result in compilation error as var args can only be provided to last argument.

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 Q28. What is the difference between these two method declarations ?

private static void method(String[] arg)

and

private static void method(String... arg)
Core Java2017-05-23 08:41:54

Ans. First expects the argument as a string array whereas second expects variable number of string arguments or a string array.

So we can call both by providing string array as an argument but second can be called with 0 to n string arguments which cannot be done for first.

for example - We can call second method with any of following

method();
method("Hello");
method("Hello","World");
method(new String[4]);


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  Q29. What are the different types of inner classes ?Core Java
Anonymous
Ans. Simple Inner Class,
Local Inner Class,
Anonymous Inner Class,
Static Nested Inner Class.

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 Q30. What are the Wrapper classes available for primitive types ?Core Java
Anonymous
Ans. boolean - java.lang.Boolean
byte - java.lang.Byte
char - java.lang.Character
double - java.lang.Double
float - java.lang.Float
int - java.lang.Integer
long - java.lang.Long
short - java.lang.Short
void - java.lang.Void

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 Q31. Which String class does not override the equals() and hashCode() methods, inheriting them directly from class Object?Core Java
Anonymous
Ans. java.lang.StringBuffer.

  Sample Code for StringBuffer

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 Q32. What are wrapper classes ?Core Java
Anonymous
Ans. They are wrappers to primitive data types. They allow us to access primitives as objects.

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 Q33. Which access specifier can be used with Class ?
Anonymous
Ans. For top level class we can only use "public" and "default". We can use private with inner class.

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 Q34. How to display and set the Class path in Unix ?
Anonymous
Ans. To display the current CLASSPATH variable, use these commands in UNIX (Bourne shell):

% echo $CLASSPATH

To delete the current contents of the CLASSPATH variable,

In UNIX: % unset CLASSPATH; export CLASSPATH

To set the CLASSPATH variable,

In UNIX: % CLASSPATH=/home/george/java/classes; export CLASSPATH

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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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Very Frequently asked.Favorite question in Walk in drive for many Indian service companies.
 Q36. What is a Final Class ?
Anonymous
Ans. A Class that cannot be sub classed.

  Sample Code for final class

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Very Frequently asked. Usually asked along with String Class related questions.
 Q37. 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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 Q38. 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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 Q39. Difference between boolean and Boolean ?Core Java
Anonymous
Ans. boolean is a primitive type whereas Boolean is a class.

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 Q40. Explain Autoboxing ?Core Java
Anonymous
Ans. Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes

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 Q41. What are Wrapper Classes ? What are Primitive Wrapper Classes ?
Core Java
Anonymous
Ans. A wrapper class is any class which "wraps" or "encapsulates" the functionality of another class or component. A Wrapper Class that wraps or encapsulates the primitive data type is called Primitive Wrapper Class.

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 Q42. What Design pattern Wrapper Classes implement ?Design
Anonymous
Ans. Adapter.

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 Q43. What is a class loader ? What are the different class loaders used by JVM ?Core Java
Anonymous
Ans. Part of JVM which is used to load classes and interfaces.

Bootstrap , Extension and System are the class loaders used by JVM.

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Very frequently asked. Usually difference between String,StringBuffer and StringBuilder is asked in different variations.
  Q44. Difference between StringBuffer and StringBuilder ?Core Java
Anonymous
Ans. StringBuffer is synchronized whereas StringBuilder is not.

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 Q45. Difference between nested and inner classes ?
Anonymous
Ans. Inner classes are non static nested classes.

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 Q46. What is a nested interface ?
Anonymous
Ans. Any interface declared inside a class or an interface. It is static by default.

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 Q47. What are the design considerations while making a choice between using interface and abstract class ?Core Java
Anonymous
Ans. Keep it as a Abstract Class if its a "Is a" Relationsship and should do subset/all of the functionality. Keep it as Interface if its a "Should Do" relationship.

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 Q48. Will this code give error if i try to add two heterogeneous elements in the arraylist. ? and Why ?

List list1 = new ArrayList<>();
list1.add(5);
list1.add("5");
Admin
info@buggybread.com
Ans. If we don't declare the list to be of specific type, it treats it as list of objects.

int 1 is auto boxed to Integer and "1" is String and hence both are objects.

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 Q49. How are classes loaded by JVM ?Core Java
Admin
info@buggybread.com
Ans. Class loaders are hierarchical. The very first class is specially loaded with the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running.

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 Q50. Difference between static vs. dynamic class loading?Core Java
Admin
info@buggybread.com
Ans. static loading - Classes are statically loaded with Java new operator.

dynamic class loading - Dynamic loading is a technique for programmatically invoking the functions of a class loader at run time.

Class.forName (Test className);

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 Q51. When do you get ClassCastException?
Anonymous
Ans. As we only downcast class in the hierarchy, The ClassCastException is thrown to indicate that code has attempted to cast an object to a subclass of which it is not an instance.

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  Q52. Write a method to check if input String is Palindrome?Core Java
Admin
info@buggybread.com
Ans. private static boolean isPalindrome(String str) {

if (str == null)
return false;

StringBuilder strBuilder = new StringBuilder(str);

strBuilder.reverse();

return strBuilder.toString().equals(str);

}

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 Q53. Write a method that will remove given character from the String?Core Java
Admin
info@buggybread.com
Ans.
private static String removeChar(String str, char c) {    
if (str == null)
return null;

return str.replaceAll(Character.toString(c), "");
}


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 Q54. What does String intern() method do?Core Java
Admin
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Ans. intern() method keeps the string in an internal cache that is usually not garbage collected.

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 Q55. How substring() method of String class create memory leaks?Core Java
Admin
info@buggybread.com
Ans. substring method would build a new String object keeping a reference to the whole char array, to avoid copying it. Hence you can inadvertently keep a reference to a very big character array with just a one character string.

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Very frequently asked in HCL Tech ( Based of 4 inputs )
 Q56. Write a program to reverse a string iteratively and recursively ?Core Java
Admin
info@buggybread.com
Ans. Using String method -

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

Iterative -

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 -

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

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 Q57. Difference between new operator and Class.forName().newInstance() ?
Admin
info@buggybread.com
Ans. new operator is used to statically create an instance of object. newInstance() is used to create an object dynamically ( like if the class name needs to be picked from configuration file ). If you know what class needs to be initialized , new is the optimized way of instantiating Class.

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Very frequently asked in companies using SOA.
  Q58. What are RESTful Web Services ?Rest
Admin
info@buggybread.com
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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 Q59. What are the restrictions for the entity classes ?Hibernate
Admin
info@buggybread.com
Ans. 1. Entity classes should have default constructor.

2. Entity classes should be declared non final.

3. All elements to be persisted should be declared private and should have public getters and setters in the Java Bean style.

4. All classes should have an ID that maps to Primary Key for the table.

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 Q60. How is Abstraction implemented in Java ?Core Java
Admin
info@buggybread.com
Ans. Abstraction is provided in Java by following ways -

Coding to the ( Interfaces / Abstract Classes ) or contracts

By Encapsulating details within classes and exposing the minimal Door ( few public methods )

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 Q61. Why do we need Thread class even in case we execute thread using runnable interface ?Core Java
Admin
info@buggybread.com
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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 Q62. 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
Core Java
Anonymous
Ans. public or default

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 Q63. Which of the following cannot be marked static ?

a. Constructors , Classes ( Outer ) , Classes ( nested ), Interfaces , Local variables , Inner Class methods and instance variables.
b. Constructors , Classes ( Outer ) , Interfaces , Local variables , Class variables , Class Methods , Inner Class methods and instance variables.
c. Constructors , Classes ( Outer ) , Interfaces , Local variables , Inner Class methods and instance variables.
d. Constructors , Classes ( Outer ) , Classes (Nested), Interfaces , Local variables , Inner Class methods and instance variables
Anonymous
Ans. Constructors , Classes ( Outer ) , Interfaces , Local variables , Inner Class methods and instance variables.

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 Q64. Explain System.out.println ?Core Java2016-11-25 09:37:08

Ans. System is a class within java.lang package that contains several useful class fields and methods. It cannot be instantiated and hence can use only statically.even in this case this has been used statically i.e with class name itself and without creating an instance.

out is the static reference of Printstream declared as following in the System Class -

public final static PrintStream out = null;

println is the method of PrintStream class.

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 Q65. If everything is an object , Cant we declare every object as

Object obj = new String();
Core Java2016-12-02 14:45:05

Ans. Yes, we can do that. Compiler wont complain. But using object reference we can only access methods which have been defined for object class i.e clone(), equals(), hashCode(), toString() etc.

We cannot access methods defined in String class or in any class in hierarchy between String and Object.

For example - we cannot do obj.append("abc") as it will now give compile time error.

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 Q66. Can we overload method as following ?

void method(int... x){};
void method(int[] x){};
Core Java2017-05-22 21:27:42

Ans. No. Because java internally treats var args as arrays and hence both method declarations will generate the same byte code and hence would result in ambiguity while determining call binding.

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 Q67. What is servlet Chaining ?Java EE
Anonymous
Ans. Multiple servlets serving the request in chain.

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Very frequently asked. Usually asked in this format or as difference with interfaces / concrete classes.
  Q68. What is an abstract class ?Core Java2017-01-10 13:12:20

Ans. Abstract class is the class that is not supposed to be instantiated. The purpose of the class to only have extension to the derived class.

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 Q69. Difference between long.Class and Long.TYPE ?
Anonymous
Ans. They both represent the long primitive type. They are exactly the same.

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 Q70. Explain use of nested or inner classes ?
Anonymous
Ans. Sometime we just need classes or class objects just to be used as part of a particular class or objects. Making them non nested won't make any difference as far as functionality is concerner but making them Nested provide a level of convenience and protection fro, being used anywhere else. Moreover it helps reducing the Code.

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 Q71. Difference between loadClass and Class.forName ?
Anonymous
Ans. loadClass only loads the class but doesn't initialize the object whereas Class.forName initialize the object after loading it.

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  Q72. what is the difference between collections class vs collections interface ?Core Java
Admin
info@buggybread.com
Ans. Collections class is a utility class having static methods for doing operations on objects of classes which implement the Collection interface. For example, Collections has methods for finding the max element in a Collection.

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 Q73. What is the difference between System.console.write and System.out.println ?
Admin
info@buggybread.com
Ans. System.console() returns null if your application is not run in a terminal (though you can handle this in your application)System.console() provides methods for reading password without echoing charactersSystem.out and System.err use the default platform encoding, while the Console class output methods use the console encoding

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 Q74. Difference between the jsp scriptlet tag and jsp declaration tag?Java EE
Anonymous
Ans. The jsp scriptlet tag can only declare variables not methods whereas jsp declaration tag can declare variables as well as methods.

The declaration of scriptlet tag is placed inside the _jspService() method whereas The declaration of jsp declaration tag is placed outside the _jspService() method.

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 Q75. What is the benefit of inner / nested classes ?Core Java
Admin
info@buggybread.com
Ans. You can put related classes together as a single logical group.

Nested classes can access all class members of the enclosing class, which might be useful in certain cases.

Nested classes are sometimes useful for specific purposes. For example, anonymous inner classes are useful for writing simpler event-handling code with AWT/Swing.

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 Q76. Explain Static nested Classes ?
Admin
info@buggybread.com
Ans. The accessibility (public, protected, etc.) of the static nested class is defined by the outer class.

A static nested class is not an inner class, it's a top-level nested class.

The name of the static nested class is expressed with OuterClassName.NestedClassName syntax.

When you define an inner nested class (or interface) inside an interface, the nested class is declared implicitly public and static.

Static nested classes can be declared abstract or final.

Static nested classes can extend another class or it can be used as a base class.

Static nested classes can have static members.

Static nested classes can access the members of the outer class (only static members, obviously).

The outer class can also access the members (even private members) of the nested class through an object of nested class. If you don’t declare an instance of the nested class, the outer class cannot access nested class elements directly.

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 Q77. Explain Inner Classes ?Core Java
Admin
info@buggybread.com
Ans. The accessibility (public, protected, etc.) of the inner class is defined by the outer class.

Just like top-level classes, an inner class can extend a class or can implement interfaces.

Similarly, an inner class can be extended by other classes, and an inner interface can be implemented or extended by other classes or interfaces.

An inner class can be declared final or abstract.Inner classes can have inner classes, but you will have a hard time reading or understanding such complex nesting of classes.

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 Q78. Explain Method Local Inner Classes ?Core Java
Admin
info@buggybread.com
Ans. You can create a non-static local class inside a body of code. Interfaces cannot have local classes, and you cannot create local interfaces.

Local classes are accessible only from the body of the code in which the class is defined. The local classes are completely inaccessible outside the body of the code in which the class is defined.

You can extend a class or implement interfaces while defining a local class.

A local class can access all the variables available in the body of the code in which it is defined. You can pass only final variables to a local inner class.

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 Q79. Explain about anonymous inner classes ?
Admin
info@buggybread.com
Ans. Anonymous classes are defined in the new expression itself, so you cannot create multiple objects of an anonymous class.

You cannot explicitly extend a class or explicitly implement interfaces when defining an anonymous class.

An anonymous inner class is always created as part of a statement; don't forget to close the statement after the class definition with a curly brace. This is a rare case in Java, a curly brace followed by a semicolon.

Anonymous inner classes have no name, and their type must be either a subclass of the named type or an implementer of the named interface

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 Q80. Can we create an object if a Class doesn't have any constructor ( not even the default provided by constructor ) ?
Admin
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Ans. Yes , using Class.getInstance.

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 Q81. Advantage of Collection classes over Arrays ?
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Ans. Collections are re-sizable in nature. We can increase or decrease the size as per recruitment.
Collections can hold both homogeneous and heterogeneous data's.
Every collection follows some standard data structures.
Collection provides many useful built in methods for traversing,sorting and search.

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 Q82. What is a Class ?Core Java
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Ans. A class is a blue print or Mold using which individual objects are created. A class can contain fields and methods to describe the behavior of an object.

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 Q83. What are the sub classes of Exception class?Core Java
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Ans. The Exception class has two main subclasses : IOException class and RuntimeException Class.

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 Q84. What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?Core Java
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Ans. The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented

  Sample Code for InputStream

  Sample Code for OutputStream

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 Q85. What environment variables do I need to set on my machine in order to be able to run Java programs?
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Ans. CLASSPATH and PATH are the two variables.

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 Q86. Which String class methods are used to make string upper case or lower case?Core Java
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Ans. toUpperCase and toLowerCase

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 Q87. How to convert String to byte array and vice versa?Core Java
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Ans. We can use String getBytes() method to convert String to byte array and we can use String constructor new String(byte[] arr) to convert byte array to String.

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 Q88. Why String is popular HashMap key in Java?Core Java
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Ans. Since String is immutable, its hashcode is cached at the time of creation and it doesnt need to be calculated again. This makes it a great candidate for key in a Map and its processing is fast than other HashMap key objects. This is why String is mostly used Object as HashMap keys.

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 Q89. What will be the output of the following code ?

String s1 = "Buggy Bread";
String s2 = "Buggy Bread";
if(s1 == s2)
   System.out.println("equal 1");
String n1 = new String("Buggy Bread");
String n2 = new String("Buggy Bread");
if(n1 == n2)
   System.out.println("equal 2");
Core Java
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Ans. equal 1

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 Q90. Difference between PATH and CLASSPATH ?Operating System
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Ans. PATH is the variable that holds the directories for the OS to look for executables. CLASSPATH is the variable that holds the directories for JVM to look for .class files ( Byte Code ).

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 Q91. what will be the output ?

class Animal {
public void eat() throws Exception {
}
}

class Dog2 extends Animal {
public void eat(){}
public static void main(){
Animal an = new Dog2();
an.eat();
}
}
Core Java
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Ans. Compile Time Error: Unhandled exception type Exception

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 Q92. Does SQL allow null values ? Can we use it within Where clause ?
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Ans. Yes , we can have null values for columns in SQL. Null value represent that the columns value is unknown or haven't been filled. Yes, We can use it within where clause to get the rows with null values.

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 Q93. Will this code Work ? If not , Why ?

java.util.Calendar c = new java.util.Calendar();
Core Java
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Ans. No. It gives the error "Cannot Instantiate the type Calendar". Calendar is an abstract class and hence Calendar object should be instantiated using Calendar.getInstance().

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 Q94. Is java.util.Date an abstract Class ? Is java.util.Calendar an abstract Class ?Core Java
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Ans. Date is not a abstract class whereas Calendar is.

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 Q95. Which methods of the Pattern class have equivalent methods in the String class? Core Java
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Ans. split() and macthes()

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 Q96. Can we compare Integers by using equals() in Java ?Core Java
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Ans. Yes for the Wrapper class Integer but not for the primitive int.

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 Q97. Difference between Assert and Verify ?
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Ans. Assert works only if assertions ( -ea ) are enabled which is not required for Verify.

Assert throws an exception and hence doesn't continue with the test if assert evaluates to false whereas it's not so with Verify.

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 Q98. Can we declare Entity class as final ?Hibernate
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Ans. Yes but as Hibernate creates the Proxy Classes inherited from the Entity Classes to communicate with Database for lazy initialization. Declaring entity classes as final will prohibit communication with database lazily and hence will be a performance hit.

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 Q99. What is the difference between int[] x; and int x[]; ?Core Java
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Ans. No Difference. Both are the acceptable ways to declare an array.

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 Q100. What is the difference between namenode and datanode in Hadoop? BigData
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Ans. NameNode stores MetaData (No of Blocks, On Which Rack which DataNode is stored etc) whereas the DataNode stores the actual Data.

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