Interview FAQ's

1.What are the principle concepts of OOPS?
There are four principle concepts upon which object oriented design and programming rest. They are:
  • Abstraction
  • Polymorphism
  • Inheritance
  • Encapsulation (i.e. easily remembered as A-PIE).

2.What is Abstraction?
Abstraction refers to the act of representing essential features without including the background details or explanations.

3.What is Encapsulation?
Encapsulation is a technique used for hiding the properties and behaviors of an object and allowing outside access only as appropriate. It prevents other objects from directly altering or accessing the properties or methods of the encapsulated object.

4.What is the difference between abstraction and encapsulation?
  • Abstraction focuses on the outside view of an object (i.e. the interface) Encapsulation (information hiding) prevents clients from seeing it’s inside view, where the behavior of the abstraction is implemented.
  • Abstraction solves the problem in the design side while Encapsulation is the Implementation.
  • Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up your abstraction to suit the developer needs.


5.What is Inheritance?
  • Inheritance is the process by which objects of one class acquire the properties of objects of another class.
  • A class that is inherited is called a superclass.
  • The class that does the inheriting is called a subclass.
  • Inheritance is done by using the keyword extends.
  • The two most common reasons to use inheritance are:
    • To promote code reuse
    • To use polymorphism


6.What is Polymorphism?
Polymorphism is briefly described as "one interface, many implementations." Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts - specifically, to allow an entity such as a variable, a function, or an object to have more than one form.

7.How does Java implement polymorphism?
(Inheritance, Overloading and Overriding are used to achieve Polymorphism in java).
Polymorphism manifests itself in Java in the form of multiple methods having the same name.
  • In some cases, multiple methods have the same name, but different formal argument lists (overloaded methods).
  • In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods).


8.Explain the different forms of Polymorphism.
There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading. Runtime time polymorphism is done using inheritance and interface.Note: From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:
  • Method overloading
  • Method overriding through inheritance
  • Method overriding through the Java interface

9.What is runtime polymorphism or dynamic method dispatch?
In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable.

10.What is Dynamic Binding?
Binding refers to the linking of a procedure call to the code to be executed in response to the call. Dynamic binding (also known as late binding) means that the code associated with a given procedure call is not known until the time of the call at run-time. It is associated with polymorphism and inheritance.

11.What is method overloading?
Method Overloading means to have two or more methods with same name in the same class with different arguments. The benefit of method overloading is that it allows you to implement methods that support the same semantic operation but differ by argument number or type.Note:
  • Overloaded methods MUST change the argument list
  • Overloaded methods CAN change the return type
  • Overloaded methods CAN change the access modifier
  • Overloaded methods CAN declare new or broader checked exceptions
  • A method can be overloaded in the same class or in a subclass


12.What is method overriding?
Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior that’s specific to a particular subclass type.Note:
  • The overriding method cannot have a more restrictive access modifier than the method being overridden (Ex: You can’t override a method marked public and make it protected).
  • You cannot override a method marked final
  • You cannot override a method marked static

13.What are the differences between method overloading and method overriding?

Overloaded Method
Overridden Method
ArgumentsMust changeMust not change
Return typeCan changeCan’t change except for covariant returns
ExceptionsCan changeCan reduce or eliminate. Must not throw new or broader checked exceptions
AccessCan changeMust not make more restrictive (can be less restrictive)
InvocationReference type determines which overloaded version is selected. Happens at compile time.Object type determines which method is selected. Happens at runtime.


14.Can overloaded methods be override too?
Yes, derived classes still can override the overloaded methods. Polymorphism can still happen. Compiler will not binding the method calls since it is overloaded, because it might be overridden now or in the future.

15.Is it possible to override the main method?
NO, because main is a static method. A static method can't be overridden in Java.

16.How to invoke a superclass version of an Overridden method?
To invoke a superclass method that has been overridden in a subclass, you must either call the method directly through a superclass instance, or use the super prefix in the subclass itself. From the point of the view of the subclass, the super prefix provides an explicit reference to the superclass' implementation of the method.

     // From subclass


        super.overriddenMethod();



17.What is super?
super is a keyword which is used to access the method or member variables from the superclass. If a method hides one of the member variables in its superclass, the method can refer to the hidden variable through the use of the super keyword. In the same way, if a method overrides one of the methods in its superclass, the method can invoke the overridden method through the use of the super keyword. Note: You can only go back one level. In the constructor, if you use super(), it must be the very first code, and you cannot access any this.xxx variables or methods to compute its parameters.
18.How do you prevent a method from being overridden?
To prevent a specific method from being overridden in a subclass, use the final modifier on the method declaration, which means "this is the final implementation of this method", the end of its inheritance hierarchy.

                           public final void exampleMethod() {
                         //  Method statements
                         }



19.What is an Interface?
An interface is a description of a set of methods that conforming implementing classes must have.Note: You can’t mark an interface as final. Interface variables must be static. An Interface cannot extend anything but another interfaces.
20.Can we instantiate an interface?
You can’t instantiate an interface directly, but you can instantiate a class that implements an interface.

21.Can we create an object for an interface?
Yes, it is always necessary to create an object implementation for an interface. Interfaces cannot be instantiated in their own right, so you must write a class that implements the interface and fulfill all the methods defined in it.

22.Do interfaces have member variables?
Interfaces may have member variables, but these are implicitly public, static, and final- in other words, interfaces can declare only constants, not instance variables that are available to all implementations and may be used as key references for method arguments for example.

23.What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.

24.What is a marker interface?
Marker interfaces are those which do not declare any required methods, but signify their compatibility with certain operations. The java.io.Serializable interface and Cloneable are typical marker interfaces. These do not contain any methods, but classes must implement this interface in order to be serialized and de-serialized.

25.What is an abstract class?
Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation. Note:
  • If even a single method is abstract, the whole class must be declared abstract.
  • Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
  • You can’t mark a class as both abstract and final.


26.Can we instantiate an abstract class?
An abstract class can never be instantiated. Its sole purpose is to be extended (subclassed).

27.What are the differences between Interface and Abstract class?
Abstract Class
Interfaces
An abstract class can provide complete, default code and/or just the details that have to be overridden.An interface cannot provide any code at all,just the signature.
In case of abstract class, a class may extend only one abstract class.A Class may implement several interfaces.
An abstract class can have non-abstract methods.All methods of an Interface are abstract.
An abstract class can have instance variables.An Interface cannot have instance variables.
An abstract class can have any visibility: public, private, protected.An Interface visibility must be public (or) none.
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.
An abstract class can contain constructors .An Interface cannot contain constructors .
Abstract classes are fast.Interfaces are slow as it requires extra indirection to find corresponding method in the actual class.


28.When should I use abstract classes and when should I use interfaces?
Use Interfaces when…
  • You see that something in your design will change frequently.
  • If various implementations only share method signatures then it is better to use Interfaces.
  • you need some classes to use some methods which you don't want to be included in the class, then you go for the interface, which makes it easy to just implement and make use of the methods defined in the interface.

Use Abstract Class when…
  • If various implementations are of the same kind and use common behavior or status then abstract class is better to use.
  • When you want to provide a generalized form of abstraction and leave the implementation task with the inheriting subclass.
  • Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for nonleaf classes in class hierarchies.


29.When you declare a method as abstract, can other nonabstract methods access it?
Yes, other nonabstract methods can access a method that you declare as abstract.

30.Can there be an abstract class with no abstract methods in it?
Yes, there can be an abstract class without abstract methods.
31.What is Constructor?
  • A constructor is a special method whose task is to initialize the object of its class.
  • It is special because its name is the same as the class name.
  • They do not have return types, not even void and therefore they cannot return values.
  • They cannot be inherited, though a derived class can call the base class constructor.
  • Constructor is invoked whenever an object of its associated class is created.


32.How does the Java default constructor be provided?
If a class defined by the code does not have any constructor, compiler will automatically provide one no-parameter-constructor (default-constructor) for the class in the byte code. The access modifier (public/private/etc.) of the default constructor is the same as the class itself.

33.Can constructor be inherited?
No, constructor cannot be inherited, though a derived class can call the base class constructor.

34.What are the differences between Contructors and Methods?

Constructors
Methods
PurposeCreate an instance of a classGroup Java statements
ModifiersCannot be abstract, final, native, static, or synchronizedCan be abstract, final, native, static, or synchronized
Return TypeNo return type, not even voidvoid or a valid return type
NameSame name as the class (first letter is capitalized by convention) -- usually a nounAny name except the class. Method names begin with a lowercase letter by convention -- usually the name of an action
thisRefers to another constructor in the same class. If used, it must be the first line of the constructorRefers to an instance of the owning class. Cannot be used by static methods.
superCalls the constructor of the parent class. If used, must be the first line of the constructorCalls an overridden method in the parent class
InheritanceConstructors are not inheritedMethods are inherited


35.How are this() and super() used with constructors?
  • Constructors use this to refer to another constructor in the same class with a different parameter list.
  • Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.

36.What are the differences between Class Methods and Instance Methods?
Class Methods
Instance Methods
Class methods are methods which are declared as static. The method can be called without creating an instance of the classInstance methods on the other hand require an instance of the class to exist before they can be called, so an instance of a class needs to be created by using the new keyword.
Instance methods operate on specific instances of classes.
Class methods can only operate on class members and not on instance members as class methods are unaware of instance members.Instance methods of the class can also not be called from within a class method unless they are being called on an instance of that class.
Class methods are methods which are declared as static. The method can be called without creating an  instance of the class.Instance methods are not declared as static.


37.How are this() and super() used with constructors?
  • Constructors use this to refer to another constructor in the same class with a different parameter list.
  • Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.


38.What are Access Specifiers?
One of the techniques in object-oriented programming is encapsulation. It concerns the hiding of data in a class and making this class available only through methods. Java allows you to control access to classes, methods, and fields via so-called access specifiers..

39.What are Access Specifiers available in Java?
Java offers four access specifiers, listed below in decreasing accessibility:
  • Public- public classes, methods, and fields can be accessed from everywhere.
  • Protected- protected methods and fields can only be accessed within the same class to which the methods and fields belong, within its subclasses, and within classes of the same package.
  • Default(no specifier)- If you do not set access to specific level, then such a class, method, or field will be accessible from inside the same package to which the class, method, or field belongs, but not from outside this package.
  • Private- private methods and fields can only be accessed within the same class to which the methods and fields belong. private methods and fields are not visible within subclasses and are not inherited by subclasses.
     Situation 
     public 
     protected 
     default 
     private 
     Accessible to class
     from same package? 
    yes
    yes
    yes
    no
     Accessible to class
     from different package? 
    yes
     no, unless it is a subclass 
    no
    no


    40.What is final modifier?
    The final modifier keyword makes that the programmer cannot change the value anymore. The actual meaning depends on whether it is applied to a class, a variable, or a method.
    • final Classes- A final class cannot have subclasses.
    • final Variables- A final variable cannot be changed once it is initialized.
    • final Methods- A final method cannot be overridden by subclasses.

    41.What are the uses of final method?
    There are two reasons for marking a method as final:
    • Disallowing subclasses to change the meaning of the method.
    • Increasing efficiency by allowing the compiler to turn calls to the method into inline Java code.


    42.What is static block?
    Static block which exactly executed exactly once when the class is first loaded into JVM. Before going to the main method the static block will execute.

    43.What are static variables?
    Variables that have only one copy per class are known as static variables. They are not attached to a particular instance of a class but rather belong to a class as a whole. They are declared by using the static keyword as a modifier.

            static type  varIdentifier;

    where, the name of the variable is varIdentifier and its data type is specified by type.Note: Static variables that are not explicitly initialized in the code are automatically initialized with a default value. The default value depends on the data type of the variables.

    44.What is the difference between static and non-static variables?
    A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.
    45.What are static methods?
    Methods declared with the keyword static as modifier are called static methods or class methods. They are so called because they affect a class as a whole, not a particular instance of the class. Static methods are always invoked without reference to a particular instance of a class.Note:The use of a static method suffers from the following restrictions:
    • A static method can only call other static methods.
    • A static method must only access static data.
    • A static method cannot reference to the current object using keywords super or this.

    46.What is an Iterator ?
    • The Iterator interface is used to step through the elements of a Collection.
    • Iterators let you process each element of a Collection.
    • Iterators are a generic way to go through all the elements of a Collection no matter how it is organized.
    • Iterator is an Interface implemented a different way for every Collection.


    47.How do you traverse through a collection using its Iterator?
    To use an iterator to traverse through the contents of a collection, follow these steps:
    • Obtain an iterator to the start of the collection by calling the collection’s iterator() method.
    • Set up a loop that makes a call to hasNext(). Have the loop iterate as long as hasNext() returns true.
    • Within the loop, obtain each element by calling next().


    48.How do you remove elements during Iteration?
    Iterator also has a method remove() when remove is called, the current element in the iteration is deleted.

    49.What is the difference between Enumeration and Iterator?
    Enumeration
    Iterator
    Enumeration doesn't have a remove() methodIterator has a remove() method
    Enumeration acts as Read-only interface, because it has the methods only to traverse and fetch the objectsCan be abstract, final, native, static, or synchronized

    Note: So Enumeration is used whenever we want to make Collection objects as Read-only.
    50.How is ListIterator?
    ListIterator is just like Iterator, except it allows us to access the collection in either the forward or backward direction and lets us modify an element

    51.What is the List interface?
    • The List interface provides support for ordered collections of objects.
    • Lists may contain duplicate elements.


    52.What are the main implementations of the List interface ?
    The main implementations of the List interface are as follows :
    • ArrayList : Resizable-array implementation of the List interface. The best all-around implementation of the List interface.
    • Vector : Synchronized resizable-array implementation of the List interface with additional "legacy methods."
    • LinkedList : Doubly-linked list implementation of the List interface. May provide better performance than the ArrayList implementation if elements are frequently inserted or deleted within the list. Useful for queues and double-ended queues (deques).


    53.What are the advantages of ArrayList over arrays ?
    Some of the advantages ArrayList has over arrays are:
    • It can grow dynamically
    • It provides more powerful insertion and search mechanisms than arrays.



    54.Difference between ArrayList and Vector ?
    ArrayList
    Vector
    ArrayList is NOT synchronized by default.Vector List is synchronized by default.
    ArrayList can use only Iterator to access the elements.Vector list can use Iterator and Enumeration Interface to access the elements.
    The ArrayList increases its array size by 50 percent if it runs out of room.A Vector defaults to doubling the size of its array if it runs out of room
    ArrayList has no default size.While vector has a default size of 10.


    55.How to obtain Array from an ArrayList ?
    Array can be obtained from an ArrayList using toArray() method on ArrayList.

        List arrayList = new ArrayList();
         arrayList.add(…






        Object  a[] =
    arrayList.toArray();

     
    56.Why insertion and deletion in ArrayList is slow compared to LinkedList ?
    • ArrayList internally uses and array to store the elements, when that array gets filled by inserting elements a new array of roughly 1.5 times the size of the original array is created and all the data of old array is copied to new array.
    • During deletion, all elements present in the array after the deleted elements have to be moved one step back to fill the space created by deletion. In linked list data is stored in nodes that have reference to the previous node and the next node so adding element is simple as creating the node an updating the next pointer on the last node and the previous pointer on the new node. Deletion in linked list is fast because it involves only updating the next pointer in the node before the deleted node and updating the previous pointer in the node after the deleted node.

    57.Why are Iterators returned by ArrayList called Fail Fast ?
    Because, if list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

    58.How do you decide when to use ArrayList and When to use LinkedList?
    If you need to support random access, without inserting or removing elements from any place other than the end, then ArrayList offers the optimal collection. If, however, you need to frequently add and remove elements from the middle of the list and only access the list elements sequentially, then LinkedList offers the better implementation.
    59.What is the Set interface ?
    • The Set interface provides methods for accessing the elements of a finite mathematical set
    • Sets do not allow duplicate elements
    • Contains no methods other than those inherited from Collection
    • It adds the restriction that duplicate elements are prohibited
    • Two Set objects are equal if they contain the same elements

    60.What are the main Implementations of the Set interface ?
    The main implementations of the List interface are as follows:
    • HashSet
    • TreeSet
    • LinkedHashSet
    • EnumSet

    61.What is a HashSet ?
    • A HashSet is an unsorted, unordered Set.
    • It uses the hashcode of the object being inserted (so the more efficient your hashcode() implementation the better access performance you’ll get).
    • Use this class when you want a collection with no duplicates and you don’t care about order when you iterate through it.

    62.What is a TreeSet ?
    TreeSet is a Set implementation that keeps the elements in sorted order. The elements are sorted according to the natural order of elements or by the comparator provided at creation time.

    63.What is an EnumSet ?
    An EnumSet is a specialized set for use with enum types, all of the elements in the EnumSet type that is specified, explicitly or implicitly, when the set is created.

    64.Difference between HashSet and TreeSet ?
    HashSet
    TreeSet
    HashSet is under set interface i.e. it  does not guarantee for either sorted order or sequence order.TreeSet is under set i.e. it provides elements in a sorted  order (acceding order).
    We can add any type of elements to hash set.We can add only similar types 
    of elements to tree set.


    65.What is a Map ?
    • A map is an object that stores associations between keys and values (key/value pairs).
    • Given a key, you can find its value. Both keys  and  values are objects.
    • The keys must be unique, but the values may be duplicated.
    • Some maps can accept a null key and null values, others cannot.


    66.What are the main Implementations of the Map interface ?
    The main implementations of the List interface are as follows:
    • HashMap
    • HashTable
    • TreeMap
    • EnumMap


    67.What is a TreeMap ?
    TreeMap actually implements the SortedMap interface which extends the Map interface. In a TreeMap the data will be sorted in ascending order of keys according to the natural order for the key's class, or by the comparator provided at creation time. TreeMap is based on the Red-Black tree data structure.

    68.How do you decide when to use HashMap and when to use TreeMap ?
    For inserting, deleting, and locating elements in a Map, the HashMap offers the best alternative. If, however, you need to traverse the keys in a sorted order, then TreeMap is your better alternative. Depending upon the size of your collection, it may be faster to add elements to a HashMap, then convert the map to a TreeMap for sorted key traversal.

    69.Difference between HashMap and Hashtable ?
    HashMap
    Hashtable
    HashMap lets you have null values as well as one null key.HashTable  does not allows null values as key and value.
    The iterator in the HashMap is fail-safe (If you change the map while iterating, you’ll know).The enumerator for the Hashtable is not fail-safe.
    HashMap is unsynchronized.Hashtable is synchronized.

    Note: Only one NULL is allowed as a key in HashMap. HashMap does not allow multiple keys to be NULL. Nevertheless, it can have multiple NULL values.

    70.How does a Hashtable internally maintain the key-value pairs?
    TreeMap actually implements the SortedMap interface which extends the Map interface. In a TreeMap the data will be sorted in ascending order of keys according to the natural order for the key's class, or by the comparator provided at creation time. TreeMap is based on the Red-Black tree data structure.

    71.What Are the different Collection Views That Maps Provide?
    Maps Provide Three Collection Views.
    • Key Set - allow a map's contents to be viewed as a set of keys.
    • Values Collection - allow a map's contents to be viewed as a set of values.
    • Entry Set - allow a map's contents to be viewed as a set of key-value mappings.


    72.What is a KeySet View ?
    KeySet is a set returned by the keySet() method of the Map Interface, It is a set that contains all the keys present in the Map.

    73.What is a Values Collection View ?
    Values Collection View is a collection returned by the values() method of the Map Interface, It contains all the objects present as values in the map.

    74.What is an EntrySet View ?
    Entry Set view is a set that is returned by the entrySet() method in the map and contains Objects of type Map. Entry each of which has both Key and Value.

    75.How do you sort an ArrayList (or any list) of user-defined objects ?
    Create an implementation of the java.lang.Comparable interface that knows how to order your objects and pass it to java.util.Collections.sort(List, Comparator).

    76.What is the Comparable interface ?
    The Comparable interface is used to sort collections and arrays of objects using the Collections.sort() and java.utils.Arrays.sort() methods respectively. The objects of the class implementing the Comparable interface can be ordered.
    The Comparable interface in the generic form is written as follows:

        interface Comparable<T>

    where T is the name of the type parameter.

    All classes implementing the Comparable interface must implement the
    compareTo() method that has the return type as an integer. The signature of the compareTo() method is as follows:

         int i = object1.compareTo(object2)

    • If object1 < object2: The value of i returned will be negative.
    • If object1 > object2: The value of i returned will be positive.
    • If object1 = object2: The value of i returned will be zero.


    77.What are the differences between the Comparable and Comparator interfaces ?
    Comparable
    Comparato
    It uses the compareTo() method.
    int objectOne.compareTo(objectTwo).
    t uses the compare() method.
    int compare(ObjOne, ObjTwo)
    It is necessary to modify the class whose instance is going to be sorted.A separate class can be created in order to sort the instances.
    Only one sort sequence can be created.Many sort sequences can be created.
    It is frequently used by the API classes.It used by third-party classes to sort instances.




    Exceptions
    1.What is an exception?
    An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.
    2.What is error?
    An Error indicates that a non-recoverable condition has occurred that should not be caught. Error, a subclass of Throwable, is intended for drastic problems, such as OutOfMemoryError, which would be reported by the JVM itself.
    3.Which is superclass of Exception?
    "Throwable", the parent class of all exception related classes.
    4.What are the advantages of using exception handling?
    Exception handling provides the following advantages over "traditional" error management techniques:
    • Separating Error Handling Code from "Regular" Code.
    • Propagating Errors Up the Call Stack.
    • Grouping Error Types and Error Differentiation.

    5.What are the types of Exceptions in Java
    There are two types of exceptions in Java, unchecked exceptions and checked exceptions.
    • Checked exceptions: A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Each method must either handle all checked exceptions by supplying a catch clause or list each unhandled checked exception as a thrown exception.
    • Unchecked exceptions: All Exceptions that extend the RuntimeException class are unchecked exceptions. Class Error and its subclasses also are unchecked.

    6.Why Errors are Not Checked?
    A unchecked exception classes which are the error classes (Error and its subclasses) are exempted from compile-time checking because they can occur at many points in the program and recovery from them is difficult or impossible. A program declaring such exceptions would be pointlessly.

    7.Why Runtime Exceptions are Not Checked?
    The runtime exception classes (RuntimeException and its subclasses) are exempted from compile-time checking because, in the judgment of the designers of the Java programming language, having to declare such exceptions would not aid significantly in establishing the correctness of programs. Many of the operations and constructs of the Java programming language can result in runtime exceptions. The information available to a compiler, and the level of analysis the compiler performs, are usually not sufficient to establish that such run-time exceptions cannot occur, even though this may be obvious to the programmer. Requiring such exception classes to be declared would simply be an irritation to programmers.

    8.Explain the significance of try-catch blocks?
    Whenever the exception occurs in Java, we need a way to tell the JVM what code to execute. To do this, we use the try and catch keywords. The try is used to define a block of code in which exceptions may occur. One or more catch clauses match a specific exception to a block of code that handles it.

    9.What is the use of finally block?

The finally block encloses code that is always executed at some point after the try block, whether an exception was thrown or not. This is right place to close files, release your network sockets, connections, and perform any other cleanup your code requires.
Note: If the try block executes with no exceptions, the finally block is executed immediately after the try block completes. It there was an exception thrown, the finally block executes immediately after the proper catch block completes

10.What if there is a break or return statement in try block followed by finally block?
If there is a return statement in the try block, the finally block executes right after the return statement encountered, and before the return executes.
11.Can we have the try block without catch block?
Yes, we can have the try block without catch block, but finally block should follow the try block.Note: It is not valid to use a try clause without either a catch clause or a finally clause.

12.What is the difference throw and throws?
throws: Used in a method's signature if a method is capable of causing an exception that it does not handle, so that callers of the method can guard themselves against that exception. If a method is declared as throwing a particular class of exceptions, then any other method that calls it must either have a try-catch clause to handle that exception or must be declared to throw that exception (or its superclass) itself.

A method that does not handle an exception it throws has to announce this:

 public void myfunc(int arg) throws MyException {
        …
    }


throw: Used to trigger an exception. The exception will be caught by the nearest try-catch clause that can catch that type of exception. The flow of execution stops immediately after the throw statement; any subsequent statements are not executed.

To throw an user-defined exception within a block, we use the throw command:

 throw new MyException("I always wanted to throw an exception!");

13.How to create custom exceptions?
A. By extending the Exception class or one of its subclasses.
Example:

class MyException extends Exception {
 
public MyException() { super(); }
 
public MyException(String s) { super(s); }
  }


14.What are the different ways to handle exceptions?

There are two ways to handle exceptions:
  • Wrapping the desired code in a try block followed by a catch block to catch the exceptions.
  • List the desired exceptions in the throws clause of the method and let the caller of the method handle those exceptions.

JDBC
1.What is the JDBC?
Java Database Connectivity (JDBC) is a standard Java API to interact with relational databases form Java. JDBC has set of classes and interfaces which can use from Java application and talk to database without learning RDBMS details and using Database Specific JDBC Drivers.

2.What are the new features added to JDBC 4.0?
The major features added in JDBC 4.0 include :
  • Auto-loading of JDBC driver class
  • Connection management enhancements
  • Support for RowId SQL type
  • DataSet implementation of SQL using Annotations
  • SQL exception handling enhancements
  • SQL XML support

3.Explain Basic Steps in writing a Java program using JDBC?
JDBC makes the interaction with RDBMS simple and intuitive. When a Java application needs to access database :
  • Load the RDBMS specific JDBC driver because this driver actually communicates with the database (Incase of JDBC 4.0 this is automatically loaded).
  • Open the connection to database which is then used to send SQL statements and get results back.
  • Create JDBC Statement object. This object contains SQL query.
  • Execute statement which returns resultset(s). ResultSet contains the tuples of database table as a result of SQL query.
  • Process the result set.
  • Close the connection.


4.Exaplain the JDBC Architecture.
The JDBC Architecture consists of two layers:
  • The JDBC API, which provides the application-to-JDBC Manager connection.
  • The JDBC Driver API, which supports the JDBC Manager-to-Driver Connection.

The JDBC API uses a driver manager and database-specific drivers to provide transparent connectivity to heterogeneous databases. The JDBC driver manager ensures that the correct driver is used to access each data source. The driver manager is capable of supporting multiple concurrent drivers connected to multiple heterogeneous databases. The location of the driver manager with respect to the JDBC drivers and the Java application is shown in Figure 1.

Figure 1: JDBC Architecture


5.What are the main components of JDBC ?
The life cycle of a servlet consists of the following phases:
  • DriverManager: Manages a list of database drivers. Matches connection requests from the java application with the proper database driver using communication subprotocol. The first driver that recognizes a certain subprotocol under JDBC will be used to establish a database Connection.
  • Driver: The database communications link, handling all communication with the database. Normally, once the driver is loaded, the developer need not call it explicitly.
  • Connection : Interface with all methods for contacting a database.The connection object represents communication context, i.e., all communication with database is through connection object only.
  • Statement : Encapsulates an SQL statement which is passed to the database to be parsed, compiled, planned and executed.
  • ResultSet: The ResultSet represents set of rows retrieved due to query execution.


6.How the JDBC application works?
A JDBC application can be logically divided into two layers:
1. Driver layer
2. Application layer
  • Driver layer consists of DriverManager class and the available JDBC drivers.
  • The application begins with requesting the DriverManager for the connection.
  • An appropriate driver is choosen and is used for establishing the connection. This connection is given to the application which falls under the application layer.
  • The application uses this connection to create Statement kind of objects, through which SQL commands are sent to backend and obtain the results.


Figure 2: JDBC Application
7.How do I load a database driver with JDBC 4.0 / Java 6?
Provided the JAR file containing the driver is properly configured, just place the JAR file in the classpath. Java developers NO longer need to explicitly load JDBC drivers using code like Class.forName() to register a JDBC driver.The DriverManager class takes care of this by automatically locating a suitable driver when the DriverManager.getConnection() method is called. This feature is backward-compatible, so no changes are needed to the existing JDBC code.

8.What is JDBC Driver interface?
The JDBC Driver interface provides vendor-specific implementations of the abstract classes provided by the JDBC API. Each vendor driver must provide implementations of the java.sql.Connection,Statement,PreparedStatement, CallableStatement, ResultSet and Driver.

9.What does the connection object represents?
The connection object represents communication context, i.e., all communication with database is through connection object only.

10.What is Statement ?
Statement acts like a vehicle through which SQL commands can be sent. Through the connection object we create statement kind of objects.
Through the connection object we create statement kind of objects.


              Statement stmt  = conn.createStatement();

This method returns object which implements statement interface.


11.What is PreparedStatement?
A prepared statement is an SQL statement that is precompiled by the database. Through precompilation, prepared statements improve the performance of SQL commands that are executed multiple times (given that the database supports prepared statements). Once compiled, prepared statements can be customized prior to each execution by altering predefined SQL parameters.

    PreparedStatement pstmt = conn.prepareStatement("UPDATE EMPLOYEES SET SALARY = ? WHERE ID = ?");


        pstmt.setBigDecimal(1, 153833.00);



        pstmt.setInt(2, 110592);


Here: conn is an instance of the Connection class and "?" represents parameters.These parameters must be specified before execution.


12.What is the difference between a Statement and a PreparedStatement?
Statement
PreparedStatement
A standard Statement is used to create a Java representation of a literal SQL statement and execute it on the database.A PreparedStatement is a precompiled  statement. This means that when the PreparedStatement is executed, the RDBMS can just run the PreparedStatement SQL statement without having to compile it first.
Statement has to verify its metadata against the database every time.While a prepared statement has to verify its metadata against the database only once.
If you want to execute the SQL statement once go for STATEMENTIf you want to execute a single SQL statement multiple number of times, then go for PREPAREDSTATEMENT. PreparedStatement objects can be reused with passing different values to the queries


13.What are callable statements ?
Callable statements are used from JDBC application to invoke stored procedures and functions.

14.How to call a stored procedure from JDBC ?
PL/SQL stored procedures are called from within JDBC programs by means of the prepareCall() method of the Connection object created. A call to this method takes variable bind parameters as input parameters as well as output variables and creates an object instance of the CallableStatement class.
The following line of code illustrates this:

            CallableStatement stproc_stmt = conn.prepareCall("{call procname(?,?,?)}");

Here conn is an instance of the Connection class.

15.What are types of JDBC drivers?
There are four types of drivers defined by JDBC as follows:
  • Type 1: JDBC/ODBC—These require an ODBC (Open Database Connectivity) driver for the database to be installed. This type of driver works by translating the submitted queries into equivalent ODBC queries and forwards them via native API calls directly to the ODBC driver. It provides no host redirection capability.
  • Type2: Native API (partly-Java driver)—This type of driver uses a vendor-specific driver or database API to interact with the database. An example of such an API is Oracle OCI (Oracle Call Interface). It also provides no host redirection.
  • Type 3: Open Protocol-Net—This is not vendor specific and works by forwarding database requests to a remote database source using a net server component. How the net server component accesses the database is transparent to the client. The client driver communicates with the net server using a database-independent protocol and the net server translates this protocol into database calls. This type of driver can access any database.
  • Type 4: Proprietary Protocol-Net(pure Java driver)—This has a same configuration as a type 3 driver but uses a wire protocol specific to a particular vendor and hence can access only that vendor's database. Again this is all transparent to the client.

Note: Type 4 JDBC driver is most preferred kind of approach in JDBC

16.Which type of JDBC driver is the fastest one?
JDBC Net pure Java driver(Type IV) is the fastest driver because it converts the JDBC calls into vendor specific protocol calls and it directly interacts with the database.

17.Does the JDBC-ODBC Bridge support multiple concurrent open statements per connection?
No. You can open only one Statement object per connection when you are using the JDBC-ODBC Bridge.

18.Which is the right type of driver to use and when?
  • Type I driver is handy for prototyping
  • Type III driver adds security, caching, and connection control
  • Type III and Type IV drivers need no pre-installation

19.What are the standard isolation levels defined by JDBC?
The values are defined in the class java.sql.Connection and are:
  • TRANSACTION_NONE
  • TRANSACTION_READ_COMMITTED
  • TRANSACTION_READ_UNCOMMITTED
  • TRANSACTION_REPEATABLE_READ
  • TRANSACTION_SERIALIZABLE

Any given database may not support all of these levels.

20.What is resultset ?
The ResultSet represents set of rows retrieved due to query execution.

        ResultSet rs = stmt.executeQuery(sqlQuery);

21.What are the types of resultsets?
The values are defined in the class java.sql.Connection and are:
  • TYPE_FORWARD_ONLY specifies that a resultset is not scrollable, that is, rows within it can be advanced only in the forward direction.
  • TYPE_SCROLL_INSENSITIVE specifies that a resultset is scrollable in either direction but is insensitive to changes committed by other transactions or other statements in the same transaction.
  • TYPE_SCROLL_SENSITIVE specifies that a resultset is scrollable in either direction and is affected by changes committed by other transactions or statements within the same transaction.

Note: A TYPE_FORWARD_ONLY resultset is always insensitive.




22.What’s the difference between TYPE_SCROLL_INSENSITIVE and TYPE_SCROLL_SENSITIVE?
TYPE_SCROLL_INSENSITIVE
TYPE_SCROLL_SENSITIVE
An insensitive  resultset is like the snapshot of the data in the database when query was executed.A sensitive resultset does NOT represent a snapshot of data, rather it contains points to those rows which satisfy the query condition.
After we get the resultset the changes made to data are not visible through the resultset, and hence they are known as insensitive.After we obtain the resultset if the data is modified then such modifications are visible through resultset.
Performance not effected with insensitive.Since a trip is made for every ‘get’ operation, the performance drastically get affected.


22.What is rowset?
A RowSet is an object that encapsulates a set of rows from either Java Database Connectivity (JDBC) result sets or tabular data sources like a file or spreadsheet. RowSets support component-based development models like JavaBeans, with a standard set of properties and an event notification mechanism.
24.What are the different types  of RowSet ?
There are two types of RowSet are there. They are:
  • Connected - A connected RowSet object connects to the database once and remains connected until the application terminates.
  • Disconnected - A disconnected RowSet object connects to the database, executes a query to retrieve the data from the database and then closes the connection. A program may change the data in a disconnected RowSet while it is disconnected. Modified data can be updated in the database after a disconnected RowSet reestablishes the connection with the database.

25.What is the need of BatchUpdates?
The BatchUpdates feature allows us to group SQL statements together and send to database server in one single trip.
26.What is a DataSource?
A DataSource object is the representation of a data source in the Java programming language. In basic terms,
A DataSource is a facility for storing data.
DataSource can be referenced by JNDI.
Data Source may point to RDBMS, file System , any DBMS etc..
27.What are the advantages of DataSource?
The few advantages of data source are :
An application does not need to hardcode driver information, as it does with the DriverManager.
  • The DataSource implementations can easily change the properties of data sources. For example: There is no need to modify the application code when making changes to the database details.
  • The DataSource facility allows developers to implement a DataSource class to take advantage of features like connection pooling and distributed transactions.


28.What is connection pooling? what is the main advantage of using connection pooling?
A connection pool is a mechanism to reuse connections created. Connection pooling can increase performance dramatically by reusing connections rather than creating a new physical connection each time a connection is requested..