Top 10 Interview Questions

      1. What are the Data Types supported by Java? What is Autoboxing and Unboxing?

      One of the most common and fundamental Java interview questions.

     Ans :

     The eight Primitive Data types supported by Java are:

• Byte : 8-bit signed two’s complement integer. It has a minimum value of -128 and a maximum value of 127 (inclusive)
• Short : 16-bit signed two’s complement integer. It has a minimum value of -32,768 and a maximum value of 32,767 (inclusive).
• Int : 32-bit signed two’s complement integer. It has a minimum value of -2,147,483,648 and a maximum value of 2,147,483,647 (inclusive)
• Long : 64-bit signed two’s complement integer. It has a minimum value of -9,223,372,036,854,775,808 and a maximum value of 9,223,372,036,854,775,807 (inclusive)
• Float
• Double
  
  
  2. What is Function Over-Riding and Over-Loading in Java?
   
  This is a very important concept in OOPS and is a must know for every Java Programmer.
  Ans:
  Over-Riding: An override is a type of function which occurs in a class which inherits from another class. An override function “replaces” a function inherited from the base class, but does so in such a way that it is called even when an instance of its class is pretending to be a different type through polymorphism. That probably was a little over the top. The code snippet below should explain things better.
  
  
  Over-Loading: Overloading is the action of defining multiple methods with the same name, but with different parameters. It is unrelated to either overriding or polymorphism. Functions in Java could be overloaded by two mechanisms ideally:
  
• Varying the number of arguments.
• Varying the Data Type.
  
  3. What is the difference between a JDK and a JVM?
  
  

  Ans : 

  JDK is Java Development Kit which is for development purpose and it includes execution environment also. But JVM is purely a run time environment and hence you will not be able to compile your source files using a JVM.
  
  4. Difference between Abstract Class and Interface
  
  Ans:
  

  1. abstract keyword is used to create an abstract class and it can be used with methods also whereas interface keyword is used to create interface and it can’t be used with methods.
  2. Subclasses use extends keyword to extend an abstract class and they need to provide implementation of all the declared methods in the abstract class unless the subclass is also an abstract class whereas subclasses use implements keyword to implement interfaces and should provide implementation for all the methods declared in the interface.
  3. Abstract classes can have methods with implementation whereas interface provides absolute abstraction and can’t have any method implementations.
  4. Abstract classes can have constructors but interfaces can’t have constructors.
  5. Abstract class have all the features of a normal java class except that we can’t instantiate it. We can use abstract keyword to make a class abstract but interfaces are a completely different type and can have only public static final constants and method declarations.
  6. Abstract classes methods can have access modifiers as public, private, protected, static but interface methods are implicitly public and abstract, we can’t use any other access modifiers with interface methods.
  7. A subclass can extend only one abstract class but it can implement multiple interfaces.
  8. Abstract classes can extend other class and implement interfaces but interface can only extend other interfaces.
  9. We can run an abstract class if it has main() method but we can’t run an interface because they can’t have main method implementation.
  10. Interfaces are used to define contract for the subclasses whereas abstract class also define contract but it can provide other methods implementations for subclasses to use.

  
  
  5. What is String in Java? String is a data type?
  Ans :
  
  String is a Class in java and defined in java.lang package. It’s not a primitive data type like int and long. String class represents character Strings. String is used in almost all the Java applications and there are some interesting facts we should know about String. String in immutable and final in Java and JVM uses String Pool to store all the String objects.
  Some other interesting things about String is the way we can instantiate a String object using double quotes and overloading of “+” operator for concatenation.
  
  
  
  6. Difference between String, StringBuffer and StringBuilder?
  Ans :
   
  String is immutable and final in java, so whenever we do String manipulation, it creates a new String. String manipulations are resource consuming, so java provides two utility classes for String manipulations – StringBuffer and StringBuilder.
  StringBuffer and StringBuilder are mutable classes. StringBuffer operations are thread-safe and synchronized where StringBuilder operations are not thread-safe. So when multiple threads are working on same String, we should use StringBuffer but in single threaded environment we should use StringBuilder.
  StringBuilder performance is fast than StringBuffer because of no overhead of synchronization.
  
   7. What is the difference between GET and POST method?
  
  Ans:
  
  • GET is a safe method (idempotent) where POST is non-idempotent method.
  • We can send limited data with GET method and it’s sent in the header request URL whereas we can send large amount of data with POST because it’s part of the body.
  • GET method is not secure because data is exposed in the URL and we can easily bookmark it and send similar request again, POST is secure because data is sent in request body and we can’t bookmark it.
  • GET is the default HTTP method whereas we need to specify method as POST to send request with POST method.
  • Hyperlinks in a page uses GET method.  
    
    8. Do we need to override service() method?
    Ans: 
    When servlet container receives client request, it invokes the service() method which in turn invokes the doGet(), doPost() methods based on the HTTP method of request. I don’t see any use case where we would like to override service() method. The whole purpose of service() method is to forward to request to corresponding HTTP method implementations. If we have to do some pre-processing of request, we can always use servlet filters and listeners.
    9. Why HttpServlet class is declared abstract?
    
    Ans: 
    
    HttpServlet class provide HTTP protocol implementation of servlet but it’s left abstract because there is no implementation logic in service methods such as doGet() and doPost() and we should override at least one of the service methods. That’s why there is no point in having an instance of HttpServlet and is declared abstract class.
    
    10 .What are life cycle methods of a servlet?
    
     Ans :
    Servlet Life Cycle consists of three methods:
    
    1. 1. public void init(ServletConfig config) – This method is used by container to initialize the servlet, this method is invoked only once in the lifecycle of servlet.
       2. public void service(ServletRequest request, ServletResponse response) – This method is called once for every request, container can’t invoke service() method until unless init() method is executed.
       3. public void destroy() – This method is invoked once when servlet is unloaded from memory.
  
  
   

Basic Java Quetions

What Is an Object?

An object is a software bundle of related state and behavior. Software objects are often used to model the real-world objects that you find in everyday life. This lesson explains how state and behavior are represented within an object, introduces the concept of data encapsulation, and explains the benefits of designing your software in this manner.

What Is a Class?

A class is a blueprint or prototype from which objects are created. This section defines a class that models the state and behavior of a real-world object. It intentionally focuses on the basics, showing how even a simple class can cleanly model state and behavior.

What Is Inheritance?

Inheritance provides a powerful and natural mechanism for organizing and structuring your software. This section explains how classes inherit state and behavior from their superclasses, and explains how to derive one class from another using the simple syntax provided by the Java programming language.

What Is an Interface?

An interface is a contract between a class and the outside world. When a class implements an interface, it promises to provide the behavior published by that interface. This section defines a simple interface and explains the necessary changes for any class that implements it.

What Is a Package?

A package is a namespace for organizing classes and interfaces in a logical manner. Placing your code into packages makes large software projects easier to manage. This section explains why this is useful, and introduces you to the Application Programming Interface (API) provided by the Java platform.

Questions and Exercises: Object-Oriented Programming Concepts

Use the questions and exercises presented in this section to test your understanding of objects, classes, inheritance, interfaces, and packages.

What Is an Object?

Objects are key to understanding object-oriented technology. Look around right now and you'll find many examples of real-world objects: your dog, your desk, your television set, your bicycle.
Real-world objects share two characteristics: They all have state and behavior. Dogs have state (name, color, breed, hungry) and behavior (barking, fetching, wagging tail). Bicycles also have state (current gear, current pedal cadence, current speed) and behavior (changing gear, changing pedal cadence, applying brakes). Identifying the state and behavior for real-world objects is a great way to begin thinking in terms of object-oriented programming.
Take a minute right now to observe the real-world objects that are in your immediate area. For each object that you see, ask yourself two questions: "What possible states can this object be in?" and "What possible behavior can this object perform?". Make sure to write down your observations. As you do, you'll notice that real-world objects vary in complexity; your desktop lamp may have only two possible states (on and off) and two possible behaviors (turn on, turn off), but your desktop radio might have additional states (on, off, current volume, current station) and behavior (turn on, turn off, increase volume, decrease volume, seek, scan, and tune). You may also notice that some objects, in turn, will also contain other objects. These real-world observations all translate into the world of object-oriented programming.

A software object.

Software objects are conceptually similar to real-world objects: they too consist of state and related behavior. An object stores its state in fields (variables in some programming languages) and exposes its behavior through methods (functions in some programming languages). Methods operate on an object's internal state and serve as the primary mechanism for object-to-object communication. Hiding internal state and requiring all interaction to be performed through an object's methods is known as data encapsulation — a fundamental principle of object-oriented programming.
Consider a bicycle, for example:

A bicycle modeled as a software object.

By attributing state (current speed, current pedal cadence, and current gear) and providing methods for changing that state, the object remains in control of how the outside world is allowed to use it. For  example, if the bicycle only has 6 gears, a method to change gears could reject any value that is less than 1 or greater than 6.
Bundling code into individual software objects provides a number of benefits, including:

1. Modularity: The source code for an object can be written and maintained independently of the source code for other objects. Once created, an object can be easily passed around inside the system.
2. Information-hiding: By interacting only with an object's methods, the details of its internal implementation remain hidden from the outside world.
3. Code re-use: If an object already exists (perhaps written by another software developer), you can use that object in your program. This allows specialists to implement/test/debug complex, task-specific objects, which you can then trust to run in your own code.
4. Pluggability and debugging ease: If a particular object turns out to be problematic, you can simply remove it from your application and plug in a different object as its replacement. This is analogous to fixing mechanical problems in the real world. If a bolt breaks, you replace it, not the entire machine.

What Is a Class?

In the real world, you'll often find many individual objects all of the same kind. There may be thousands of other bicycles in existence, all of the same make and model. Each bicycle was built from the same set of blueprints and therefore contains the same components. In object-oriented terms, we say that your bicycle is an instance of the class of objects known as bicycles. A class is the blueprint from which individual objects are created.
The following Bicycle class is one possible implementation of a bicycle:

class Bicycle {

    int cadence = 0;
    int speed = 0;
    int gear = 1;

    void changeCadence(int newValue) {
         cadence = newValue;
    }

    void changeGear(int newValue) {
         gear = newValue;
    }

    void speedUp(int increment) {
         speed = speed + increment;   
    }

    void applyBrakes(int decrement) {
         speed = speed - decrement;
    }

    void printStates() {
         System.out.println("cadence:" +
             cadence + " speed:" + 
             speed + " gear:" + gear);
    }
}

The syntax of the Java programming language will look new to you, but the design of this class is based on the previous discussion of bicycle objects. The fields cadence, speed, and gear represent the object's state, and the methods (changeCadence, changeGear, speedUp etc.) define its interaction with the outside world.
You may have noticed that the Bicycle class does not contain a main method. That's because it's not a complete application; it's just the blueprint for bicycles that might be used in an application. The responsibility of creating and using new Bicycle objects belongs to some other class in your application.
Here's a BicycleDemo class that creates two separate Bicycle objects and invokes their methods:

class BicycleDemo {
    public static void main(String[] args) {

        // Create two different 
        // Bicycle objects
        Bicycle bike1 = new Bicycle();
        Bicycle bike2 = new Bicycle();

        // Invoke methods on 
        // those objects
        bike1.changeCadence(50);
        bike1.speedUp(10);
        bike1.changeGear(2);
        bike1.printStates();

        bike2.changeCadence(50);
        bike2.speedUp(10);
        bike2.changeGear(2);
        bike2.changeCadence(40);
        bike2.speedUp(10);
        bike2.changeGear(3);
        bike2.printStates();
    }
}

The output of this test prints the ending pedal cadence, speed, and gear for the two bicycles:

cadence:50 speed:10 gear:2
cadence:40 speed:20 gear:3

What Is Inheritance?

Different kinds of objects often have a certain amount in common with each other. Mountain bikes, road bikes, and tandem bikes, for example, all share the characteristics of bicycles (current speed, current pedal cadence, current gear). Yet each also defines additional features that make them different: tandem bicycles have two seats and two sets of handlebars; road bikes have drop handlebars; some mountain bikes have an additional chain ring, giving them a lower gear ratio.
Object-oriented programming allows classes to inherit commonly used state and behavior from other classes. In this example, Bicycle now becomes the superclass of MountainBike, RoadBike, and TandemBike. In the Java programming language, each class is allowed to have one direct superclass, and each superclass has the potential for an unlimited number of subclasses:

A hierarchy of bicycle classes.

The syntax for creating a subclass is simple. At the beginning of your class declaration, use the extends keyword, followed by the name of the class to inherit from:

class MountainBike extends Bicycle {

    // new fields and methods defining 
    // a mountain bike would go here

}

This gives MountainBike all the same fields and methods as Bicycle, yet allows its code to focus exclusively on the features that make it unique. This makes code for your subclasses easy to read. However, you must take care to properly document the state and behavior that each superclass defines, since that code will not appear in the source file of each subclass.

What Is an Interface?

As you've already learned, objects define their interaction with the outside world through the methods that they expose. Methods form the object's interface with the outside world; the buttons on the front of your television set, for example, are the interface between you and the electrical wiring on the other side of its plastic casing. You press the "power" button to turn the television on and off.
In its most common form, an interface is a group of related methods with empty bodies. A bicycle's behavior, if specified as an interface, might appear as follows:

interface Bicycle {

    //  wheel revolutions per minute
    void changeCadence(int newValue);

    void changeGear(int newValue);

    void speedUp(int increment);

    void applyBrakes(int decrement);
}

To implement this interface, the name of your class would change (to a particular brand of bicycle, for example, such as ACMEBicycle), and you'd use the implements keyword in the class declaration:

class ACMEBicycle implements Bicycle {

    int cadence = 0;
    int speed = 0;
    int gear = 1;

   // The compiler will now require that methods
   // changeCadence, changeGear, speedUp, and applyBrakes
   // all be implemented. Compilation will fail if those
   // methods are missing from this class.

    void changeCadence(int newValue) {
         cadence = newValue;
    }

    void changeGear(int newValue) {
         gear = newValue;
    }

    void speedUp(int increment) {
         speed = speed + increment;   
    }

    void applyBrakes(int decrement) {
         speed = speed - decrement;
    }

    void printStates() {
         System.out.println("cadence:" +
             cadence + " speed:" + 
             speed + " gear:" + gear);
    }
}

Implementing an interface allows a class to become more formal about the behavior it promises to provide. Interfaces form a contract between the class and the outside world, and this contract is enforced at build time by the compiler. If your class claims to implement an interface, all methods defined by that interface must appear in its source code before the class will successfully compile.

What Is a Package?

A package is a namespace that organizes a set of related classes and interfaces. Conceptually you can think of packages as being similar to different folders on your computer. You might keep HTML pages in one folder, images in another, and scripts or applications in yet another. Because software written in the Java programming language can be composed of hundreds or thousands of individual classes, it makes sense to keep things organized by placing related classes and interfaces into packages.
The Java platform provides an enormous class library (a set of packages) suitable for use in your own applications. This library is known as the "Application Programming Interface", or "API" for short. Its packages represent the tasks most commonly associated with general-purpose programming. For example, a String object contains state and behavior for character strings; a File object allows a programmer to easily create, delete, inspect, compare, or modify a file on the filesystem; a Socketobject allows for the creation and use of network sockets; various GUI objects control buttons and checkboxes and anything else related to graphical user interfaces. There are literally thousands of classes to choose from. This allows you, the programmer, to focus on the design of your particular application, rather than the infrastructure required to make it work.
The Java Platform API Specification contains the complete listing for all packages, interfaces, classes, fields, and methods supplied by the Java SE platform. Load the page in your browser and bookmark it. As a programmer, it will become your single most important piece of reference documentation.

Questions and Exercises: Object-Oriented Programming Concepts

Questions

1. Real-world objects contain ___ and ___.
2. A software object's state is stored in ___.
3. A software object's behavior is exposed through ___.
4. Hiding internal data from the outside world, and accessing it only through publicly exposed methods is known as data ___.
5. A blueprint for a software object is called a ___.
6. Common behavior can be defined in a ___ and inherited into a ___ using the ___ keyword.
7. A collection of methods with no implementation is called an ___.
8. A namespace that organizes classes and interfaces by functionality is called a ___.
9. The term API stands for ___?

Exercises

1. Create new classes for each real-world object that you observed at the beginning of this trail. Refer to the Bicycle class if you forget the required syntax.
2. For each new class that you've created above, create an interface that defines its behavior, then require your class to implement it. Omit one or two methods and try compiling. What does the error look like? 
 

 

                      Basics Of Java With Simple And Effective Notes

                             History of Java

• James Gosling and team developed the language
‘Oak’ around 1990.

• It was designed to control microprocessors for
electrical appliances like toaster, set top box, vcr, pda
etc.

• There aim was to make it platform independent.

• Latter language Oak renamed to Java.

• By 1994 Sun's HotJava browser appeared.

• Applets of java become very popular.

                                  What is Java?
 
• A high-level language
– The Java language is a high level one that at a glance

• Java byte code
– The Java language source code to byte code that
runs in the JVM.

• Java Virtual Machine
– The actual workhorse




                             Evolution of Java

• Development of Oak (1990)
• Internet Explosion (1993)
• Version 1.0 of the Java Development Kit (JDK) (1995)
• Version 1.1 (1997)
• Version 1.2 (1999)
• Version 1.3 (2000)
• Version 1.4 (2002)
• Version 5.0 (2005)




                                     Editions of Java

• Java 2 Standard Edition (J2SE)
– A core set of components to create simple
applications and support the advance applications

• Java 2 Enterprise Edition (J2EE)
– It provides a wide array of tools for building
middle ware software such as for database access
applications, online storefronts, and other
services.