Primitive Data Types

What in an Object (Just for Understanding)

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 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.

Visualization of an Object (Just for Understanding)

What is a Class. Explain with Example

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 speed = 0;
    int gear = 1;

    
    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(" 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 (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.

USING THE BICYCLE CLASS 

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.speedUp(10);
        bike1.changeGear(2);
        bike1.printStates();


        bike2.speedUp(10);
        bike2.changeGear(2);

        bike2.speedUp(10);
        bike2.changeGear(3);
        bike2.printStates();
    }
}

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

speed:10 gear:2
speed:20 gear:3

Operators in Java

The following quick reference summarizes the operators supported by the Java programming language.

Simple Assignment Operator

=       Simple assignment operator

Arithmetic Operators

+       Additive operator (also used
        for String concatenation)
-       Subtraction operator
*       Multiplication operator
/       Division operator
%       Remainder operator

Unary Operators

+       Unary plus operator; indicates
        positive value (numbers are 
        positive without this, however)
-       Unary minus operator; negates
        an expression
++      Increment operator; increments
        a value by 1
--      Decrement operator; decrements
        a value by 1
!       Logical complement operator;
        inverts the value of a boolean

Equality and Relational Operators

==      Equal to
!=      Not equal to
>       Greater than
>=      Greater than or equal to
<       Less than
<=      Less than or equal to

Conditional Operators

&&      Conditional-AND
||      Conditional-OR
?:      Ternary (shorthand for 
        if-then-else statement)

Type Comparison Operator

instanceof      Compares an object to 
                a specified type 

Bitwise and Bit Shift Operators

~       Unary bitwise complement
<<      Signed left shift
>>      Signed right shift
>>>     Unsigned right shift
&       Bitwise AND
^       Bitwise exclusive OR
|       Bitwise inclusive OR

Java Buzzwords

• Object Oriented: In Java, everything is an Object. Java can be easily extended since it is based on the Object model. It does not even allow any method outside a class. Even main method is inside a class compulsorily.
• Platform independent: Unlike many other programming languages including C and C++, when Java is compiled, it is not compiled into platform specific machine, rather into platform independent byte code. This byte code is distributed over the web and interpreted by virtual Machine (JVM) on whichever platform it is being run. 
   
• Simple:Java is designed to be easy to learn. If you understand the basic concept of OOP Java would be easy to master. All complex features of C/C++ are eliminated. And idea was to make the language as simple as possible.
  
• Secure:  With Java's secure feature it enables to develop virus-free, tamper-free systems.Java has an inbuilt Security Manager which is responsible for security of java applications at runtime. Java also provides a lot of security tools like key tools, policy tool and jar signer as part of jdk useful to develop secure applications
• Architectural-neutral :Java compiler generates an architecture-neutral object file format which makes the compiled code to be executable on many processors, with the presence of Java runtime system.
• Portable:Being architectural-neutral and having no implementation dependent aspects of the specification makes Java portable. Compiler in Java is written in ANSI C with a clean portability boundary which is a POSIX subset.
• Robust:Java makes an effort to eliminate error prone situations by emphasizing mainly on compile time error checking and runtime checking. Robustness is about having a code that runs for longer times without errors.
• Multithreaded: With Java's multithreaded feature it is possible to write programs that can do many tasks simultaneously. This design feature allows developers to construct smoothly running interactive applications. Java provides easy facility for developers to develop multi threaded applications which would be otherwise very difficult in other programing languages.
• Interpreted:Java byte code is translated on the fly to native machine instructions and is not stored anywhere. Compiler compiles java code. This compiled code is converted on the fly into native code. This process is called as interpretation. It makes java execution slow (marginally)
• High Performance: With the use of Just-In-Time compilers, Java enables high performance.
• Distributed:Java is designed for the distributed environment of the internet.
• Dynamic: Java is considered to be more dynamic than C or C++ since it is designed to adapt to an evolving environment. Java programs can carry extensive amount of run-time information that can be used to verify and resolve accesses to objects on run-time.