Java Interview Questions

What is JVM JRE JDK?

JVM:

JVM Java Virtual Machine It is responsible to converting Byte code to the machine specific code. JVM is also platform dependent and provides core java functions like memory management, garbage collection, security etc.

JRE:

JRE is the implementation of JVM, it provides platform to execute java programs. JRE consists of JVM and java binaries and other classes to execute any program successfully. JRE doesn’t contain any development tools like java compiler, debugger etc. If you want to execute any java program, you should have JRE installed but we don’t need JDK for running any java program.

JDK:

Java Development Kit is the core component of Java Environment and provides all the tools, executables and binaries required to compile, debug and execute a Java Program. JDK is platform specific software and that’s why we have separate installers for Windows, Mac and UNIX systems. We can say that JDK is superset of JRE since it contains JRE with Java compiler, debugger and core classes. Current version of JDK is 1.7 also known as Java

What are the OPPs Concepts?

There are four main Opps concepts are there in java

1. Inheritance

2. Abstraction

3. Encapsulation

4. Polymorphism

1. Inheritance:

When one object acquires all the properties and behaviors of parent object i.e. known as inheritance. It provides code reusability. It is used to achieve runtime polymorphism.

2. Abstraction

Hiding the internal details and just highlighting the set of services what they offering is called Abstraction

3. Encapsulation

Encapsulating data and corresponding methods into a single module is called Encapsulation

Encapsulation=DataHiding+Abstraction

4. Polymorphism

 Def: The process of representing one form into multiple forms is known as polymorphism.

A form is nothing but existence of a method in a particular class and it can perform various operations.

In object oriented programming we have two types of polymorphisms.

Static/ Compile time polymorphism: A static polymorphism is one in which methods are binding with an object at compile time.

Dynamic polymorphism: Dynamic polymorphism is one in which methods are binding with an object at run time.

What is method overloading?

If a class has multiple methods by same name but different parameters, it is known as Method Overloading.

What is method overriding?

If a subclass provides a specific implementation of a method that is already provided by its parent class, then it is known as Method Overriding. It is used for runtime polymorphism and it provides the specific implementation of the method.

What is difference between method overloading and method overriding?

Method Overloading	Method Overriding
If a class has multiple methods by same name with different types of parameters, it is known as Method Overloading	Method overriding is used to provide the specific                 implementation of the method That is already provided by its    super class.
2) Method overloading is performed with in a class.	Method overriding occurs in two classes that have IS-A relationship.
3) In case of method overloading parameter must be different.	In case of method overriding      parameter must be same.

What is Garbage Collector?

A Garbage collector is one of the inbuilt system background java program which will run along our regular java program for collecting unused memory space for improving the performance of the java applications

 What is Abstract class in java?

Abstract Classes: Abstract classes are those which containing some defined methods and some undefined methods. Undefined methods of a class are known as unimplemented/abstract methods.

What is Interface in java?

Interface means the contract between client and service provider is called Interface

The interface is a mechanism to achieve fully abstraction and multiple inheritances in java. There can be only abstract methods in the interface.

Interface also represents IS-A relationship.

It cannot be instantiated just like abstract class.

Why use Interface?

There are mainly three reasons to use interface. They are given below.

It is used to achieve fully abstraction.

By interface, we can support the functionality of multiple inheritances.

It can be used to achieve loose coupling.

What is difference between Abstract class and Interface?

Abstract Class	Interface
1.Multiple inheritance is not allowed through class	1.Multiple inheritance is possible through interface
2.Abstract class can contain concrete method and abstract  method	2. Interface contain only abstract method
3. Abstract class can contain constructor	3.Interface can’t contain constructor
4. Abstract class contain instance block and static block	4. Interface can’t contain both   instance and static block
5. Abstract class can contain instance and static variables	5.Interface can contain only static variables

What is cloning?

 The object cloning is a way to create exact copy of an object. For this purpose, clone() method of Object class is used to clone an object.

The java.lang.Cloneable interface must be implemented by the class whose object clone we want to create. If we don't implement Cloneable interface, clone () method generates CloneNotSupportedException.

The clone() method is defined in the Object class. Syntax of the clone() method is as follows:

What is Static in Java?

The static keyword is used in java mainly for memory management. We may apply static keyword with variables, methods, blocks and nested class. The static keyword belongs to the class than instance of the class.

Static variable If you declare any variable as static, it is known static variable.

The static variable can be used to refer the common property of all objects (that is not unique for each object) e.g. company name of employees, college name of students etc.

The static variable gets memory only once in class area at the time of class loading.

Advantage of static variable

 It makes your program memory efficient (i.e it saves memory).

A static method can be accessed without creating an instance of the class .

Static method If you apply static keyword with any method, it is known as static method.

Static block

It is used to initialize the static data member.

It is executed before main method at the time of class loading.

What is final Key word in java?

The final keyword in java is used to restrict the user. The final keyword can be used in many context. Final can be: variable method class

Final variable

If you make any variable as final, you cannot change the value of final variable(It will be constant).

Final method

If you make any method as final, you cannot override it.

Final class

If you make any class as final, you cannot extend it.

 What is super keyword in java?

Super keyword:

Super is a keyword which is used to differentiate base class features from derived class features. In java programming super keyword is play a very important role in the three places.

Super at variable level:

Whenever we inherit the data members of the super class into the derived class, there is a possibility that base class data members are similar to derived class data members. In this context JVM gets an ambiguity.

In order to differentiate base class data members from derived class data members in this context of base class members must be preceded by a keyword super.

Super at method level:

Whenever we inherit the base class methods into the derived class, there is a possibility that the base class methods are similar to derived class methods.

In this context JVM gets an ambiguity. In order to differentiate base class methods and derived class methods, in the context of derived class the base class methods must be preceded by super keyword.

 What is this keyword in java?

This is the one of the implicit keyword created by JVM and for supplied to every java program for two purposes they are

1. It always pointing to current class object.

2. Whenever we write a java program there is possibility that formal parameters and instance variable of a class are similar. In this context JVM gets ambiguity (not having clarity between formal parameters and data members).

3.In order to differentiate the formal parameters and data members of a class, data members of a class must be preceded by a keyword „this?.

4. SYN: this. Current class data members name.

What is Exception in java?

Exception: Exception means it is an event that disrupts the normal flow of the program is called Exception. It is an object which is thrown at runtime.

What is Exception Handling in java?

The process of converting the system error messages into the user friendly error messages is known as Exception Handling

 Handling runtime errors is called Exception handling. It is mainly used to handle checked exceptions.

What are Checked Exception and Unchecked Exception?

Types of exceptions in java

There are two types of Exceptions are there in java they are

Checked Exceptions and Un checked Exceptions

1. Checked exceptions

Checked exceptions means the exceptions which are checked by the compiler for smooth execution of the program at run time is called checked Exceptions

FileNotFoundException

IOException

2. Unchecked exceptions

Unchecked exceptions means the exceptions which are not checked by the compiler are called unchecked Exceptions such as Arithmetic Exception, NumberFormatException, and ArrayIndexOutOfBoundsExceptions

What is User Defined Exception in java?

 User defined exceptions means the Exceptions which are developed by the java programmer and supplied as a part of their project to deal with the common specific problems is nothing but a User defined Exceptions

What is Difference Between throw and throws in java?

throw	throws
1) Throw is used to explicitly throw an exception.	Throws is used to declare an exception.
2) Checked exceptions cannot be propagated with throw only.	Checked exception can be propagated with throws.
3) Throw is followed by an instance.	Throws is followed by class.
4) Throw is used within the method.	throws is used with the method Signature.
5)You cannot throw multiple exception	You can declare multiple exception e.g. public void method()throws IOException,SQLException.

What are the try catch and finally keywords in java?

Try: It is one of the predefined block here we can write the block of statements those will causes problems at runtime.

Catch: Catch is one of the predefined block which is used to write the block of statements which provides user friendly error messages by suppressing system error messages.

Finally Blocks:

It is one of the predefined block in which we write the block of statements which will terminates the resources of files, database connections which are obtained in the try block. In generally finally block contains resource relinquishing logic (terminating logic).

Finally block will executes compulsorily.

What is difference between Exception and Error?

An error is an irrecoverable condition occurring at runtime like out of memory error. These kind of jvm errors cannot be handled at runtime. Exceptions are because of condition failures, which can be handled easily at runtime.

What is ClassNotFoundException in java?

As the name suggests classNotFoundException in Java is a subclass of java.lang.Exception and Comes when Java Virtual Machine tries to load a particular class and doesn't found the requested class in class path. Another important point about this Exception is that, It is a checked Exception and you need to provide explicitly Exception handling while using methods which can possibly throw classnotfoundexception in java either by using try-catch block or by using throws clause
How to deal with the ClassNotFoundException

Verify that the name of the requested class is correct and that the appropriate .jar file exists in your classpath. If not, you must explicitly add it to your application’s classpath.

In case the specified .jar file exists in your classpath then, your application’s classpath is getting overriden and you must find the exact classpath used by your application.

In case the exception is caused by a third party class, you must identify the class that throws the exception and then, add the missing .jar files in your classpath.

What is Volatile Keyword?

Volatile is modifier applicable only for variables but not for methods and classes

If the value of the variable keeps on changing such type of variables we have to declare with volatile modifier

What is Memory Leak:

If an object having the reference then it is not eligible for Garbage collected even though we are not using that object in our program still it is not destroyed by the G.C such type of object is called Memory leak

 Types of Relationships in java?

Based on reusing the data members from one class to another class in JAVA we have three types of relationships. They are is-a relationship, has-a relationship and uses-a relationship.

Is-a relationship: Is-a relationship is one in which data members of one class is obtained into

Another class through the concept of inheritance.

Has-a relationship: Has-a relationship is one in which an object of one class is created as a data

Member in another class.

Uses-a relationship  : Uses-a relationship is one in which a method of one class is using an object

Of another class.

Inheritance is the technique which allows us to inherit the data members and methods from base class to derived class.

What is Composition?

 Composition means whenever Container object is destroyed all contained objects will be destroyed automatically that is without existing Container object there is no chance of existing contained objects

Holding the reference of the other class within some other class is known as composition.

 What is Aggregation?

Aggregation means whenever container object is destroyed there is no Guarantee of destruction of contained objects without existing container object there is chance of existing contained objects

What is difference between aggregation and composition?

Aggregation represents weak relationship whereas composition represents strong relationship. For example: bike has an indicator (aggregation) but bike has an engine (compostion).

What is String in java?

Generally, string is a sequence of characters. But in java, string is an object that represents a sequence of characters. The java.lang.String class is used to create string object.

How to create String object?

There are two ways to create String object:

1. by string literal

2. by new keyword

1).String Literal

Java String literal is created by using double quotes. For Example:

 1. String s="welcome";

 Each time you create a string literal, the JVM checks the string constant pool first. If the string already exists in the pool, a reference to the pooled instance is returned. If string doesn't exist in the pool, a new string instance is created and placed in the pool. For example:

1. String s1="Welcome";

2. String s2="Welcome"; //will not create new instance

 In the above example only one object will be created. Firstly JVM will not find any string object with the value "Welcome" in string constant pool, so it will create a new object. After that it will find the string with the value "Welcome" in the pool, it will not create new object but will return the reference to the same instance.

Note: String objects are stored in a special memory area known as string constant pool.

Why java uses concept of string literal?

To make Java more memory efficient (because no new objects are created if it exists already in string constant pool).

2) By new keyword

String s=new String("Welcome");//creates two objects and one reference va riable

In such case, JVM will create a new string object in normal(non pool) heap memory and the literal "Welcome" will be placed in the string constant pool. The variable s will refer to the object in heap(non pool).

Java String Example

public class StringExample{

 public static void main(String args[]){

String s1="java";//creating string by java string literal char ch[]={'s','t','r','i','n','g','s'};

String s2=new String(ch);//converting char array to string

String s3=new String("example");//creating java string by new keyword System.out.println(s1);

System.out.println(s2);

System.out.println(s3);

}}

outpot

Java

Strings

Example

What is immutable in strings

In java, string objects are immutable. Immutable simply means unmodifiable or unchangeable.

Once string object is created its data or state can't be changed but a new string object is created.

Let's try to understand the immutability concept by the example given below:

 class Testimmutablestring{

public static void main(String args[])

String s="Sachin";

s.concat(" Tendulkar");//concat() method appends the string at the end

System.out.println(s);//will print Sachin because strings are immutable ob jects

}}

What is mutable in strings?

Java String literal is created by using double quotes. For Example:

String s="welcome";

Each time you create a string literal, the JVM checks the string constant pool first. If the string already exists in the pool, a reference to the pooled instance is returned. If string doesn't exist in the pool, a new string instance is created and placed in the pool. For example:

String s1="Welcome";

String s2="Welcome";//will not create new instance

 In the above example only one object will be created. Firstly JVM will not find any string object with the value "Welcome" in string constant pool, so it will create a new object. After that it will find the string with the value "Welcome" in the pool, it will not create new object but will return the reference to the same instance.

 Why java uses concept of string literal?

 To make Java more memory efficient (because no new objects are created if it exists already in string constant pool).

What is difference between equals method and == operator in java

== -> is a reference comparison, i.e. both objects point to the same memory location

.equals() -> evaluates to the comparison of values in the objects

Main difference between == and equals in Java is that "==" is used to compare primitives while equals() method is recommended to check equality of objects. Another difference between them is that, If

both "==" and equals() is used to compare objects than == returns true only if both references points to same object while equals() can return true or false based on its  overridden implementation.One of the popular cases is comparing two String in Java in which case == and equals() method return different results.

Another scenario which creates confusion between == and equals method is when you compare two Objects. When you compare two references pointing to an object of type Object you should see the same result from both ==

 operator and equals method because default implementation of equals method just compare memory address of two objects and return true if two reference variable are pointing towards an exactly same object. Here is example of == vs equals method for comparing two objects:

What is toString and hash code in java?

We can use this method to find string representation of an object

Whenever we are trying to print any object reference internally to String() method will be executed

What is hashCode()?

For every object jvm will assign one unique id which is nothing but hashcode

Jvm uses hashcode will saving objects into hashtable or hashset or hashmap

Based on our requirement we can generate hashcode by overriding hashcode method in our class

If we are not overriding hashcode() method then Object class hashcode() method will be executed which generate hashcode based on address of the object but whenever we are overriding hashcode() method then hashcode is no longer related to Address of the object

What is difference between string and string buffer and string builder in java?

If the content will not be change frequently then we should go for string

StringBuffer is synchronized i.e. thread safe. It means two threads can't call the methods of StringBuffer simultaneously.

StringBuilder is non-synchronized i.e. not thread safe. It means two threads can call the methods of StringBuilder simultaneously.

How can we create immutable class in java ?

To create immutable class in java, you have to do following steps.

Declare the class as final so it can’t be extended.

Make all fields private so that direct access is not allowed.

Don’t provide setter methods for variables

Make all mutable fields final so that it’s value can be assigned only once.

Initialize all the fields via a constructor performing deep copy.

Perform cloning of objects in the getter methods to return a copy rather than returning the actual object reference.

To understand points 4 and 5, let’s run the sample Final class that works well and values doesn’t get altered after instantiation.

public final class Employee{ final String pancardNumber;

public Employee(String pancardNumber){ this.pancardNumber=pancardNumber;

}

public String getPancardNumber(){

return pancardNumber;

}}

Why string objects are immutable in java?

Because java uses the concept of string literal.Suppose there are 5 reference variables,all referes to one object "sachin".If one reference variable changes the value of the object, it will be affected to all the reference variables. That is why string objects are immutable in java.

What is difference between path and Class path

Path:We can use path variable to describes the location where the required Binary executable files are available If we are not setting path variable then java and javac commands wont work

Class path:We can use Class path to describes the location where required .class files are available

 If we are not setting the class path our program wont run

What is difference between enum, Enum and Enumeration?

enum: It is keyword which can be used to define a group of named constants

Enum: It is a class present in java.lang package which acts as a base class for all java enums

Enumeration: It is an Interface present in java.util package which can be used for retrieving objects from collection one by one

What is singleton class?

Singleton class means if any java class it is  allowed to create only one object such type of class is called Singleton class

The main advantage of single ton class is instead of creating a separate object for every requirement we can create a single object and reuse that object for every requirement

This approach improves memory utilization and performance of the system

Public class  ContactDAO

{

private static ContactDAO me = null;

 private ContactDAO()

  {

  }

  public static ContactDAO getInstance()

  {

    if (me == null) {

      me = new ContactDAO();

    }

    return me;

  }}

 What is marker interface?

If an interface wont contain any methods and by implementing that interface if our object will yet ability such type of interface are called marker interface or tag interface or ability interface

Ex: Serializable, Clonable, Randam Access, Single Thread modul

What is difference between final finally and finalize()

final: final is a keyword which can be used to variables, methods and classes

final variable is not changed

final methods are not override

final classes are not inherited

finally: finally is a block which is associated with try-catch to maintain clean-up code which should be executed always irrespective of whether exception raised or not raised and whether handled or not handled

finalize ():It is a method which should be executed by garbage collector before destroying any object to perform clean-up activities

What is deep cloning: The process of creating exactly duplicate independent object is called Deep cloning

 Test t=new Test();

Test t2=t1; //shallow cloning

Test t4=(Test)t.clone(); //deep cloning

What is shallow cloning?

The process of creating just duplicate object reference variable but not duplicate object is called shallow cloning

What is Serialization in java

Serialization in java is a mechanism of writing the state of an object into a byte stream.

It is mainly used in Hibernate, RMI, JPA, EJB and JMS technologies.

The reverse operation of serialization is called deserialization.

Advantage of Java Serialization

It is mainly used to travel object's state on the network (known as marshaling).

java.io.Serializable interface

Serializable is a marker interface (has no data member and method). It is used to "mark" java classes so that objects of these classes may get certain capability. The Cloneable and Remote are also marker interfaces.It must be implemented by the class whose object you want to persist.

The String class and all the wrapper classes implements java.io.Serializable interface by default.

Let's see the example given below:

import java.io.Serializable;

public class Student implements Serializable{

 int id;

String name;

public Student(int id, String name) {

this.id = id;

this.name = name;

}}

In the above example, Student class implements Serializable interface. Now its objects can be converted into stream.

Example of Java Serialization

In this example, we are going to serialize the object of Student class. The writeObject() method of ObjectOutputStream class provides the functionality to serialize the object. We are saving the state of the object in the file named f.txt.

import java.io.*;

class Persist{

public static void main(String args[])throws Exception{

Student s1 =new Student(211,"ravi");

 FileOutputStream fout=new FileOutputStream("f.txt");

ObjectOutputStream out=new ObjectOutputStream(fout);

out.writeObject(s1);

out.flush();  System.out.println("success"); } }

What is Deserialization in java?

Deserialization is the process of reconstructing the object from the serialized state.It is the reverse operation of serialization.

Example of Java Deserialization

import java.io.*;

class Depersist{

public static void main(String args[])throws Exception{

ObjectInputStream in=new ObjectInputStream(new FileInputStream("f.txt" ));

Student s=(Student)in.readObject();

 System.out.println(s.id+" "+s.name);

in.close();

}}

What is the transient keyword?

Java transient keyword is used in serialization. If you define any data member as transient, it will not be serialized.

Let's take an example, I have declared a class as Student, it has three data members id, name and age. If you serialize the object, all the values will be serialized but I don't want to serialize one value, e.g. age then we can declare the age data member as transient.

Example of Java Transient Keyword

In this example, we have created the two classes Student and PersistExample. The age data member of the Student class is declared as transient, its value will not be serialized.

If you deserialize the object, you will get the default value for transient variable.

Let's create a class with transient variable.

import java.io.Serializable;

public class Student implements Serializable{

int id;

String name;

transient int age;//Now it will not be serialized public Student(int id, String name,int age) {

this.id = id;

this.name = name;

this.age=age;

}}

What is Thread?

Thread: A flow control in java is known as “Thread”.

Multithreading:

The basic aim of the multi threading is to provide “concurrent execution”.

States/ Life cycle Thread:

New state: Once we create a thread object then is said to be in new state

Ready: if we call start () method then the thread will be enter into the ready state

Running State: if thread scheduler allocates CPU, then the thread will entered into runnable state

Waiting state: When you call wait method on running thread then thread will be moves to wait state

Halted State: A Halted State is one in which Thread has completed its execution

There are two ways to create a thread:

By extending Thread class

By implementing Runnable interface.

1)By extending Thread class:

class Multi extends Thread{

public void run(){

System.out.println("thread is running...");

}

public static void main(String args[]){

Multi t1=new Multi();         t1.start();    }   }

2)By implementing the Runnable interface:

class Multi3 implements Runnable{ public void run(){

System.out.println("thread is running...");

}

public static void main(String args[]){ Multi3 m1=new Multi3();

Thread t1 =new Thread(m1); t1.start();

}}

Best Approach to define a Thread

Among the two ways of defining a thread implements Runnable mechanism is recommended to use

 If the first approach our class always extending thread class and hence there is no chance of extending any other class

But in case of second approach we can extending some other class also while implementing Runnable interface hence second approach is recommended to use

15. What is use of yield () method?

When yield() method is called on running thread, that thread will give the chance to other threads which are in ready to run state with the highest priority than the running thread.

16. What is use of join () method in java thread?

When you call join() on the running thread then that thread will join at the end of other thread execution and waits until all other threads finishes the execution.

14. Public static final void sleep(long milliseconds) throws Interrupted Exception

This method is used for making the currently executing Thread to sleep for a period of time

If threads don’t want to perform any operation for a particular amount of time then we should go for sleep

public void wait(): This method is used for making the Thread to wait without specifying any time.

public void notify(): This method is used for transferring one Thread at a time from waiting state to Ready state.

public void notifyAll(): This method is used for transferring all the Threads at a time from waiting state to ready state

What is deadlock in thread?

Deadlock can occur in a situation when a thread is waiting for an object lock, that is acquired by another thread and second thread is waiting for an object lock that is acquired by first thread. Since, both threads are waiting for each other to release the lock, the condition is called deadlock.

What are the prevention techniques in thread?

What is synchronization?

Synchronization Techniques:

Synchronization is used in multithreading to eliminate inconsistent results

The concept of Synchronization must be always applied on common tasks for getting consistent results. That is if multiple Threads are performing common task then those Threads generates inconsistent result. To avoid this inconsistent result it is recommended to apply Synchronization concept.

DEF: The mechanism of allowing only one Thread among multiple Threads into the sharable area to perform read and write operations known as Synchronization.

What is synchronization block

Synchronized block can be used to perform synchronization on any specific resource of the method.

Suppose you have 50 lines of code in your method, but you want to synchronize only 5 lines, you can use synchronized block.

What is synchronization method?

Synchronized Methods: If any ordinary method is accessed by more than one Thread then the ordinary method generates an inconsistent result. To avoid this inconsistent result the definition of ordinary method must be preceded by a keyword synchronized for making the method as Synchronized. Based on Synchronized keyword we write before the methods, they are classified into two types they are

a) Synchronized instance methods.

b) Synchronized static methods.

What is difference between wait and sleep method in thread?

wait()	sleep()
1) The wait() method is defined in Object class.	The sleep() method is defined in              Thread class.
2) wait() method releases the lock.	The sleep() method doesn't releases the lock.

What is collection?

A group of individual objects as a single entity is called collection

What is collection Framework?

It defines several classes and interfaces which can be used to represent a group of objects as a single entity is called collection framework

 What is difference between Array List and Vector?

Array List	Vector
1).Array List is not synchronized.	1).Vector is synchronized.
2)Multiple threads can access Array List simultaneously hence Array List object is not thread safe	2).At any point only one thread is allowed to operate on vector object at a time hence vector object is thread safe
3) Array List increases its size by 50% of the array size.	3) Vector increases its size by doubling the array size.
4)threads are not required to wait and hence performance is high	4)It increases waiting time of a threads and hence performance is low

What is difference between arrays and collections in java?

Arrays	Collections
1. Arrays are fixed in size and hence once we created an array we are not allowed to increase or decrease the size based on our requirement.	Collections are grow-able in nature and hence based on our requirement we can increase or decrease the size.
2. Arrays can hold both primitives as well as objects.	Collections can hold only objects but not primitive.
3.Performance point of view arrays faster than collection	Performance point of view collections are slower than array
4. Arrays can hold only homogeneous elements.	Collections can hold both homogeneous and heterogeneous elements.
5. Memory point of view arrays are not recommended to use.	Memory point of view collections are recommended to use.
6. For any requirement, there is no ready method available.	For every requirement ready made method support is available.

What is difference between array List and Linked List?

ArrayList	LinkedList
1) ArrayList internally uses dynamic array to store the elements.	LinkedList internally uses doubly linked list to store the elements.
2) Manipulation with ArrayList is slow because it internally uses array. If any element is removed from the array, all the bits are shifted in memory.	Manipulation with LinkedList is faster than ArrayList because it uses doubly linked list so no bit shifting is required in memory.
3) ArrayList class can act as a list only because it implements List only.	LinkedList class can act as a list and queue both because it implements List and Deque interfaces.
4) ArrayList is better for storing and accessing data.	LinkedList is better for manipulating data.

What is difference between list and set?

1) List is an ordered collection it maintains the insertion order, which means upon displaying the list content it will display the elements in the same order in which they got inserted into the list.

Set is an unordered collection, it doesn’t maintain any order. There are few implementations of Set which maintains the order such as LinkedHashSet (It maintains the elements in insertion order).

2) List allows duplicates while Set doesn’t allow duplicate elements. All the elements of a Set should be unique if you try to insert the duplicate element in Set it would replace the existing value.

3) List implementations: ArrayList, LinkedList etc.

Set implementations: HashSet, LinkedHashSet, TreeSet etc.

4) List allows any number of null values. Set can have only a single null value at most.

What is difference between hash map and hash table?

HashMap	Hashtable
1. HashMap is non synchronized. It is not-thread safe and can’t be shared between many threads without proper ynchronization code	Hashtable is synchronized. It is thread-safe and can be shared with many threads.
2. HashMap allows one null key and multiple null values	Hashtable doesn’t allow any null key or value.
3) HashMap is a new class introduced in JDK 1.2.	Hashtable is a legacy class.
4) HashMap is fast.	4) HashMap is fast.
5) We can make the HashMap as synchronized by calling this code Map m = Collections.synchronizedMap(hashMap);	Hashtable is internally synchronized and can't be unsynchronized.
6) Iterator in HashMap is fail-fast.	Enumerator in Hashtable is not fail-fast.

What is hash set and hash map?

HashMap	Hash Set
HashMap  is an implementation of Map interface	HashSet is an implementation of Set Interface
HashMap Stores data in form of  key-value pair	HashSet Store only objects
Put method is used to add element in map	Add method is used to add element is Set
In hash map hashcode value is calculated using key object	Here member object is used for calculating hashcode value which can be same for two objects so equal () method is used to check for equality if it returns false that means two objects are different.
HashMap is faster than HashSet because unique key is used to access object	HashSet is slower than Hashmap

What is difference between hash map and tree map?

HashMap	TreeMap
implement basic map interface	implement navigable map interface
implemented by an array of buckets, each bucket is a LinkedList of entries	running time of basic operations: put(), get(), remove(), worst case O(lgn)
running time of basic operations: put(), average O(1), worst case O(n), happens when the table is resized; get(), remove(), average O(1)	running time of basic operations: put(), get(), remove(), worst case O(lgn)
not synchronized, to synchronize it: Map m = Collections.synchronizedMap(new HashMap(...));	not synchronized, to synchronize it: SortedMap m = Collections.synchronizedSortedMap(new TreeMap(...));
Iteration order of the map is unpredictable.	provide ordered iteration. higherKey(), lowerKey() can be used to get the successor and predecessor of a given key.

What is difference between hash set and linkedhashset?

Hash set	Linkedhashset
HashSet uses HashMap internally to store it’s elements.	LinkedHashSet uses  LinkedHashMap internally to store it’s elements.
HashSet doesn’t maintain any order of elements.	LinkedHashSet maintains insertion order of elements. i.e elements are placed as they are inserted.
HashSet gives better performance than the LinkedHashSet and TreeSet.	The performance of LinkedHashSet is between HashSet and TreeSet. It’s performance is almost similar to HashSet. But slightly in the slower side as it also maintains LinkedList internally to maintain the insertion order of elements.
HashSet gives performance of order O(1) for insertion, removal and retrieval operations.	LinkedHashSet also gives performance of order O(1) for insertion, removal and retrieval operations.
HashSet uses equals() and hashCode() methods to compare the elements and thus removing the possible duplicate elements.	LinkedHashSet also uses equals() and hashCode() methods to compare the elements.
HashSet allows maximum one null element.	LinkedHashSet also allows maximum one null element.

What is difference between hash map and linked hash map?

Hash map	Linked hash map
1) Hashmap follows hashtable.	1) LinkedHashmap follows hashtable and Linkedlist
2)Insertion order is not preserved	2)Insertion order is preserved

What is difference between hash set and tree set?

HashSet maintains no order whereas TreeSet maintains ascending order.

What is difference between iterate and List Iterator?

Iterator traverses the elements in forward direction only whereas ListIterator traverses the elements in forward and backward direction.

What is difference between Iterator and enumerator?

Iterator	Enumerator
1) Iterator can traverse legacy and non-legacy elements.	Enumeration can traverse only legacy elements.
2) Iterator is fail-fast.	Enumeration is not fail-fast.
3) Iterator is slower than Enumeration.	Iterator is slower than Enumeration.

What is difference between comparable and comparator?

comparable	comparator
1. We can use comparable to define default Natural sorting order	1.We can use Comparator to define customized sorting order
2. This interface present in java.lang Package	2.This interface present in java.util Package
3. Define only one method that is compareTo()	3.Define only two methods that is Compare() and Equals()
4. All wrapper classes and string class implements comparable interface	4.No predefined class implements comparator interface

What is difference between collection and collections?

Collection is an interface whereas Collections is a class. Collection interface provides normal functionality of data structure to List, Set and Queue. But, Collections class is to sort and synchronize collection elements.

What is difference between static import and normal import?

We can import normal import to import classes and interfaces of a package when ever we are using general import it is not required to use fyllyQualifiedName and we can use short names directly

We can use static import to import static variables and methods of a class .whenever we are using static import then it is not required to class name to access static member we can access directly

What are generics and what are the advantages?

Generics in Java

The Java Generics programming is introduced in J2SE 5 to deal with type-safe objects.

Before generics, we can store any type of objects in collection i.e. non-generic.

Now generics, forces the java programmer to store specific type of objects.

Advantage of Java Generics

There are mainly 3 advantages of generics. They are as follows:

1) Type-safety: We can hold only a single type of objects in generics. It doesn’t allow to store other objects.

2) Type casting is not required: There is no need to typecast the object.

Before Generics, we need to type cast.

List list = new ArrayList();  

list.add("hello");  

String s = (String) list.get(0);//typecasting  

After Generics, we don't need to typecast the object.

List<String> list = new ArrayList<String>();  

list.add("hello");  

String s = list.get(0);  

3) Compile-Time Checking: It is checked at compile time so problem will not occur at runtime. The good programming strategy says it is far better to handle the problem at compile time than runtime.

List<String> list = new ArrayList<String>();  

list.add("hello");  

list.add(32);//Compile Time Error  

Full Example of Generics in Java

Here, we are using the ArrayList class, but you can use any collection class such as ArrayList, LinkedList, HashSet, TreeSet, HashMap, Comparator etc.

import java.util.*;  

class TestGenerics1{  

public static void main(String args[]){  

ArrayList<String> list=new ArrayList<String>();  

list.add("rahul");  

list.add("jai");  

//list.add(32);//compile time error  

String s=list.get(1);//type casting is not required  

System.out.println("element is: "+s);   

Iterator<String> itr=list.iterator();  

while(itr.hasNext()){  

System.out.println(itr.next());  

}  }  }  

What is JDBC?

JDBC is a Java API that is used to connect and execute query to the database. JDBC API uses jdbc drivers to connects to the database

How to connect with database or Steps to connect with database?

There are 5 steps to connect any java application with the database in java using JDBC. They are as follows:

Register the driver class

Creating connection

Creating statement

Executing queries

Closing connection

1) Register the driver class

The forName() method of Class class is used to register the driver class. This method is used to dynamically load the driver class.

Syntax of forName() method

public static void forName(String className)throws ClassNotFoundExcept ion

Example to register the OracleDriver class

Class.forName("oracle.jdbc.driver.OracleDriver");

2) Create the connection object

The getConnection() method of DriverManager class is used to establish connection with the database.

Syntax of getConnection() method

1) public static Connection getConnection(String url)throws SQLException

 2) public static Connection getConnection(String url,String name,String password) throws SQLException

Example to establish connection with the Oracle database

Connection con=DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:xe","system","password");

3) Create the Statement object

The createStatement() method of Connection interface is used to create statement. The object of statement is responsible to execute queries with the database.

Syntax of createStatement() method

public Statement createStatement()throws SQLException

Example to create the statement object

Statement stmt=con.createStatement();

4) Execute the query

The executeQuery() method of Statement interface is used to execute queries to the database. This method returns the object of ResultSet that can be used to get all the records of a table.

Syntax of executeQuery() method

public ResultSet executeQuery(String sql)throws SQLException

Example to execute query

ResultSet rs=stmt.executeQuery("select * from emp");

while(rs.next()){

System.out.println(rs.getInt(1)+" "+rs.getString(2));

}

5) Close the connection object

By closing connection object statement and ResultSet will be closed automatically. The close() method of Connection interface is used to close the connection.

Syntax of close() method

public void close()throws SQLException

Example to close connection

con.close();

What is connection interface in jdbc?

A Connection is the session between java application and database. The Connection interface is a factory of Statement, PreparedStatement, and DatabaseMetaData i.e. object of Connection can be used to get the object of Statement and DatabaseMetaData. The Connection interface provide many methods for transaction management like commit(), rollback() etc.

What is statement interface in jdbc?

The Statement interface provides methods to execute queries with the database. The statement interface is a factory of ResultSet i.e. it provides factory method to get the object of ResultSet.

Commonly used methods of Statement interface:

The important methods of Statement interface are as follows:

1).public ResultSet executeQuery(String sql): is used to execute SELECT query. It returns the object of ResultSet.

2).public int executeUpdate(String sql): is used to execute specified query, it may be create, drop, insert, update, delete etc.

3).public boolean execute(String sql): is used to execute queries that may return multiple results.

4) public int[] executeBatch(): is used to execute batch of commands.

What is prepare statement in jdbc?

The PreparedStatement interface is a subinterface of Statement. It is used to execute parameterized query.

Let's see the example of parameterized query:

String sql="insert into emp values(?,?,?)";

As you can see, we are passing parameter (?) for the values. Its value will be set by calling the setter methods of PreparedStatement.

Why use Prepared Statement?

Improves performance: The performance of the application will be faster if you use PreparedStatement interface because query is compiled only once.

What is difference between statement and prepared statement in jdbc?

In case of Statement, query is compiled each time whereas in case of Prepared Statement, query is compiled only once. So performance of Prepared Statement is better than Statement

 What is result set in jdbc?

Result Set is one of the predefined interface which is used for holding the number of records after execution of select query in the Database software.

What is callable statement in jdbc?

Callable Statement Interface (Executing Stored Procedures orFunctions)

Callable Statement is used to call the stored procedures and functions,

CallableStatement stmt=con.prepareCall("{call insertR(?,?)}");

What is Batch Statement in jdbc?

Instead of executing a single query, we can execute a batch (group) of queries. It makes the performance fast.

The java.sql.Statement and java.sql.PreparedStatement interfaces provide methods for batch processing.

Advantage of Batch Processing

Fast Performance

Methods of Statement interface

The required methods for batch processing are given below:

                                Method                                                               Description                                                                                                                                                                                                                                                       

void addBatch(String query)                                       It adds query into batch.                                                                                              

int[] executeBatch()                                                     It executes the batch of queries.                                                                                                               

What is jdbc driver and how many types of drivers are there in jdbc?

JDBC Driver is a software component that enables for java application to interact with the database. There are 4 types of JDBC drivers:

JDBC-ODBC bridge driver

Native-API driver (partially java driver)

Network Protocol driver (fully java driver)

Thin driver (fully java driver)

What is servlet?

Servlet is a technology i.e. used to create web application.

Servlet is an API that provides many interfaces and classes including documentations.

Servlet is an interface that must be implemented for creating any servlet.

What is difference between get and post in servlets?

GET	POST
1) In case of Get request, only limited amount of data can be sent because data is sent in header.	In case of post request, large amount of data can be sent because data is sent in body.
2) Get request is not secured because data is exposed in URL bar.	Post request is secured because data is not exposed in URL bar.
3) Get request can be bookmarked.	Post request cannot be bookmarked.
4) Get request is idempotent . It means second request will be ignored until response of first request is delivered	Post request is non-idempotent.
5) Get request is more efficient and used more than Post.	Post request is less efficient and used less than get.

What is servlet collaboration?

The RequestDispatcher interface provides the facility of dispatching the request to another resource it may be html, servlet or jsp. This interface can also be used to include the content of another resource also. It is one of the way of servlet collaboration.

There are two methods defined in the RequestDispatcher interface.

Methods of RequestDispatcher interface

The RequestDispatcher interface provides two methods. They are:

public void forward(ServletRequest request,ServletResponse response)throws ServletException,java.io.IOException:Forwards a request from a servlet to another resource (servlet, JSP file, or HTML file) on the server.

public void include(ServletRequest request,ServletResponse response)throws ServletException,java.io.IOException:Includes the content of a resource (servlet, JSP page, or HTML file) in the response.

Can we call servlet destroy () from service ()?

As you know, destroy () is part of servlet life cycle methods, it is used to kill the servlet instance. Servlet Engine is used to call destroy (). In case, if you call destroy method from service(), it just execute the code written in the destory(), but it wont kill the servlet instance. destroy() will be called before killing the servlet instance by servlet engine.

Can we override static method?

We cannot override static methods. Static methods are belongs to class, not belongs to object. Inheritance will not be applicable for class members

 How to destroy the session in servlets?

By calling invalidate () method on session object, we can destroy the session.

What is transient variable?

Transient variables cannot be serialized. During serialization process, transient variable states will not be serialized. State of the value will be always defaulted after deserialization.

 In case, there is a return at the end of try block, will execute finally block?

Yes, the finally block will be executed even after writing return statement at the end fo try block. It returns after executing finally block.

Difference between WHERE and HAVING clause:

1. WHERE clause can be used with - Select, Insert, and Update statements, where as HAVING clause can only be used with the Select statement.

2. WHERE filters rows before aggregation (GROUPING), whereas, HAVING filters groups, after the aggregations are performed.

3. Aggregate functions cannot be used in the WHERE clause, unless it is in a sub query contained in a HAVING clause, whereas, aggregate functions can be used in Having clause

Difference between Generic Servlet and HttpServlet?

Generic Servlet	HttpServlet
GenericServlet class is direct subclass of Servlet interface.	HttpServlet class is the direct subclass of Generic Servlet.
Generic Servlet is protocol independent.It handles all types  of protocol  like http, smtp, ftp etc	HttpServlet is protocol dependent. It handles only http protocol
Generic Servlet only supports  service() method.It handles only simple request public void service(ServletRequest req,ServletResponse res ).	HttpServlet  supports public void service(ServletRequest req,ServletResponse res ) and protected void service(HttpServletRequest req,HttpServletResponse res).
Generic Servlet only supports  service() method.	HttpServlet supports also   doGet(),doPost(),doPut(),doDelete(),doHead(),doTrace(),doOptions()etc.

What is Difference between include directive and include action in JSP

Include directive	Include action
Resource included by including directive is loaded during jsp translation time	resource included by including action is loaded during request time.
Any change on included resource will not be visible include directive until jsp file compiles again	While in the case of include again any change in included resource will be visible in next request.
include directive is static import	while include action is dynamic import
include directive uses file attribute to specify resource to be included	while including action use page attribute for the same purpose.
Syntax: <%@ include file="loan.jsp" %>	Syntax: <jsp:include page="loan.jsp" %>

Difference between SendRedirect and Forward methods

Forward method	SendRedirect
When we use forward method request is transfer to other resource within the same server for further processing.	In case of sendRedirect request is transfer to another resource to different domain or different server for futher processing.
In case of forward Web container handle all process internally and client or browser is not involved.	When you use SendRedirect container transfers the request to client or browser so url given inside the sendRedirect method is visible as a new request to the client.
When forward is called on requestdispather object we pass request and response object so our old request object is present on new resource which is going to process our request	In case of SendRedirect call old request and response object is lost because it’s treated as new request by the browser.
Visually we are not able to see the forwarded address, its is transparent	In address bar we are able to see the new redirected address it’s not transparent.
In address bar we are able to see the new redirected address it’s not transparent.	SendRedirect is slower because one extra round trip is required beasue completely new request is created and old request object is lost.Two browser request requird.

What is the main difference between servlet context and servlet config?

Servlet context	Servlet config
1.ServletConfig object is one per servlet class	ServletContext object is global to entire web application
2.Object of ServletConfig will be created during initialization process of the servlet	Object of ServletContext will be created at the time of web application deployment
3. Destroyed once the servlet execution is completed.	destroyed once the application is removed from the server.
4.ServletConfig available in javax.servlet.*; package	ServletConfig available in javax.servlet.*; package

What is Servlet life cycle?

Servlet interface defines the life cycle methods of servlet such as init(), service() and destroy(). The Web container invokes the init(), service() and destroy() methods of a servlet during its life cycle.

The Web container loads the servlet class and creates instances of the servlet class.

The Web container invokes init() method of the servlet instance during initialization of the servlet. The init() method is invoked only once in the servlet life cycle.

The Web container invokes the service() method to allow a servlet to process a client request.

The service() method processes the request and returns the response back to the Web container.

The servlet then waits to receive and process subsequent requests

The Web container calls the destroy() method before removing the servlet instance from the service. The destroy() method is also invoked only once in a servlet life cycle.

How to call Stored Procedure in Hibernate?

CREATE PROCEDURE 'getCustomers'(status varchar(20))

 BEGIN

 SELECT * FROM customerInfo WHERE status = status;

 END $$

=======================================

Query query = session.createSQLQuery ("CALL getCustomers (: status)")

.add Entity (Customer. Class)

.setParameter ("status", "Active");

List result = query. List ();

for (int i=0; i<result.size(); i++){

 Customer customer = (Customer) result.get(i);

 System.out.println(customer.getCustomerName());

  }

How to call Stored Procedure in JDBC?

String getDBUSERByUserIdSql = "{call getDBUSERByUserId (?,?,?,?)}";

callableStatement = dbConnection.prepareCall(getDBUSERByUserIdSql);

callableStatement.setInt(1, 10);

callableStatement.registerOutParameter(2, java.sql.Types.VARCHAR);

callableStatement.registerOutParameter(3, java.sql.Types.VARCHAR);

callableStatement.registerOutParameter(4, java.sql.Types.DATE);

callableStatement.executeUpdate();

String userName = callableStatement.getString(2);

String createdBy = callableStatement.getString(3);

Date createdDate = callableStatement.getDate(4);

What is two-phase commit?

Two-phase commit is a transaction protocol designed for the complications that arise with distributed resource managers. With a two-phase commit protocol, the distributed transaction manager employs a coordinator to manage the individual resource managers. 

What is java.lang.OutOfMemoryError in Java

OutOfMemoryError in Java is a subclass of java.lang.VirtualMachineError and JVM throws java.lang.OutOfMemoryError when it ran out of memory in the heap. OutOfMemoryError in Java can come anytime in heap mostly while you try to create an object and there is not enough space on the heap to allocate that object
Types of OutOfMemoryError in Java

I have seen mainly two types of OutOfMemoryError in Java:

1) The java.lang.OutOfMemoryError: Java heap space
2) The java.lang.OutOfMemoryError: PermGen space

Though both of them occur because JVM ran out of memory they are quite different to each other and their solutions are independent of each other.

Since in most of JVM default size of Perm Space is around "64MB" you can easily run out of memory if you have too many classes or a huge number of Strings in your project.
An important point to remember is that it doesn't depend on –Xmx value so no matter how big your total heap size you can run OutOfMemory in perm space. The good thing is you can specify the size of permanent generation using JVM options "-XX: PermSize" and  "-XX: MaxPermSize" based on your project need.

How to solve java.lang.OutOfMemoryError: PermGen space

As explained in above paragraph this OutOfMemory error in java comes when Permanent generation of heap filled up. To fix this OutOfMemoryError in Java, you need to increase heap size of Perm space by using JVM option   "-XX: MaxPermSize". You can also specify initial size of Perm space by using    "-XX: PermSize" and keeping both initial and maximum Perm Space you can prevent some full garbage collection which may occur when Perm Space gets re-sized. Here is how you can specify initial and maximum Perm size in Java:

export JVM_ARGS="-XX:PermSize=64M -XX:MaxPermSize=256m"

Some time java.lang.OutOfMemoryError  in Java gets tricky and on those cases profiling remains ultimate solution.Though you have the freedom to increase heap size in java, it’s recommended that to follow memory management practices while coding and setting null to any unused references.
That’s all from me on OutOfMemoryError in Java I will try to write more about finding the memory leak in java and using profiler in some other post

What is Java Life Cycle?

A Java program is written using either a Text Editor like Textpad or an IDE like Eclipse and is saved as a .java file. (Program.java)

The .java file is then compiled using Java compiler and a .class file is obtained from it. (Program.class)

The .class file is now portable and can be used to run this Java program in any platform.

Class file (Program.class) is interpreted by the JVM installed on a particular platform. JVM is part of the JRE software.

The figure below explains the lifecycle of a Java Program. In words, the figure can be explained as:

What is ClassNotFoundException

ClassNotFoundException in Java is a subclass of java.lang.Exception and Comes when Java Virtual Machine tries to load a particular class and doesn't found the requested class in class path.

What is NoClassDefFoundError in Java
NoClassDefFoundError is an error that occurs when a particular class is present at compile time, but was missing at run time.

NoClassDefFoundError is an error that is thrown when the Java Runtime System tries to load the definition of a class, and that class definition is no longer available. The required class definition was present at compile time, but it was missing at runtime.

What is NullPointerException?

NullPointerException is a RuntimeException. In Java, a special null value can be assigned to an object reference. NullPointerException is thrown when an application attempts to use an object reference that has the null value. These include:

Calling an instance method on the object referred by a null reference.

Accessing or modifying an instance field of the object referred by a null reference.

If the reference type is an array type, taking the length of a null reference.

If the reference type is an array type, accessing or modifying the slots of a null reference.

If the reference type is a subtype of Throwable, throwing a null reference.

What is Index Out of Bound Exception

Index out of Bound Exception Occurs when a programmer tries to access an element of beyond the storage capacity of the index. 

Index Out of Bound Exception are further classified into two types-

1. Array Index Out of Bound Exception-These are the exception arises when a programmer tries to access an element beyond the capacity of index of the array. This exception is seen at run-time rather than the compile time. This causes the program to crash at run-time.Inorder to overcome this problem, you write the exceptional -handler that process the program. Now place the code in try block that cause exception in program. If exception arises in try block, you pass the control to the specific catch block preceding try block If there is no catch block, the program will terminated.

2. String Index Out of Bound Exception- The String Index Out of Bound Exception is a subclass of Index out of Bound Exception. This exception is thrown when  a String  object detects an index  out-of-range index. Usually, An string object occurs out-of-range , when the index is less than zero, or greater than or equal to the length of the string.

What is Different between Page and PageContext in Jsp?

Page and page context are implicit objects in jsp.These are created at JSP translated time. Page is used as a scope with in one jsp.pagecontext is used to initialize all implicit objects.(for example pagecontext.getServletConfig() etc.)

What are the Implicit Objects in Jsp?

Object	Type
out	JspWriter
request	HttpServletRequest
response	HttpServletResponse
config	ServletConfig
application	ServletContext
session	HttpSession
page Context	PageContext
page	Object
exception	Throwable

Can you call JavaScript method with one argument instead of two arguments in JavaScript?

Yes we can call the javascript method but second argument is undefined value will pass

jQuery html() Method

Definition and Usage

The html() method sets or returns the content (innerHTML) of the selected elements.

When this method is used to return content, it returns the content of the FIRST matched element.

When this method is used to set content, it overwrites the content of ALL matched elements.

Tip: To set or return only the text content of the selected elements, use the text() method.

jQuery text() Method

Definition and Usage

The text() method sets or returns the text content of the selected elements.

When this method is used to return content, it returns the text content of all matched elements (HTML markup will be removed).

When this method is used to set content, it overwrites the content of ALL matched elements.

Tip: To set or return the innerHTML (text + HTML markup) of the selected elements, use the html() method.

Example:

$("button").click(function(){
    $("p").text("Hello world!");
});

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