Introduction to java

Notes (JAVA)

C++ è Partially OOP

Java è Fully OOP

Principles of OOP:

1)      Object – Definitions:

(a)   Entity which has attributes and behavior

(b)   Entity which occupies space in memory

       For e.g. a marker is an object. Its attributes include – shape, color, size etc.

                                And its behavior is writing on board.

2)      Class – Definitions:

(a)    Class is a container which contains similar types of objects.

(b)   Class is a container which contains objects and methods.

3)      Encapsulation – Definition: Wrapping up of data into single unit that can be carried easily.

      Benefit: Carrying data becomes easy and data can be used wherever/whenever we want to.

For e.g. we can carry many things in a bag like laptop, charger, books etc.

      In case of programming, classes can be considered as an example of encapsulation.

4)      Data abstraction – Definition: Using the essential features without including the background details.

Benefit: Data hiding. No need to tell others how something works. They can simply use it without knowing the details.

5)      Polymorphism – Definition: Using a function in many different ways.

Benefit: Reduces time and increase the speed of compiler.

Types:

a)      Compile time – e.g. function overloading, operator overloading.

b)      Runtime - Virtual func. à Function with same name, return-type and parameters but diff. class. At this time, compiler can see only the child class’s functions but not the parent class. In order to use the functions of parent class, we use virtual functions.

6)      Inheritance – Definition: Using the properties/attributes of one class by another class.

Benefit: Reusability

Drawback: Time consuming. This problem was solved by Java using interfaces and through abstract classes.

7)      Dynamic binding – Execution of main function.

8)      Message communication

When objects communicate with each other with the help of functions, the process is known as message communication. For e.g. this concept is used in RMI (Remote Method Invocation), networking etc.

Language – Mode of communication

The first requirement for learning any language is – TOKEN.

Token – Smallest individual unit of a program.

Types:

1. Identifier – Name of data, method, class, interface etc.

2. Keyword – Reserved / Predefined words having specific meaning.

           These may be different for different languages.

3. Operator – Symbols that perform action on data.

    Types:

a)      Unary

b)      Binary

4. Separator – Symbols used to separate statements.

                         For e.g. semicolons, {}, (), [], comma etc.

5. Literal – Constant values.

                    For e.g. integers, float, strings etc.

Statements – Combination of tokens

Functions – Group of statements

Programs – Group of functions

Softwares – Group of programs

Machine code depends on the machine i.e. a machine code obtained after compilation on one machine will not necessary work on another. It is machine dependent.

In order to solve this problem, an intermediate code is used that can be shared among machines and further compiled to obtain the required machine code.

C++ à Machine dependent

Java à Platform independent

JAVA

Java is a pure OOP language created by Sun Microsystems. It was developed by James Gosling. It was developed in 1992.

Current Version of Java – 1.7

Java provides diff. kinds of features (explained ahead):

1.      Platform independent and portable

2.      Dynamic and Extensible

3.      Multithreaded application and Interactive

4.      Compiled and Interpreted

5.      Object Oriented

6.      High performance

7.      Distributed

Format of Java program

import java.io.*;                                                                      // importing package

class demo

{

            public static void main(String aa[])

{

            System.out.println(“Hello, How are you?”);

}

}

Package – Package is a container in which we have predefined classes, sub-classes and methods. We can use package in a program with the help of an import statement.

import java.io.*;

* indicates all classes and subclasses

System.out.println is a method of output stream class where:

·         System.out indicates “output stream class”

·         out – object

·         println – method

Note: print/println can be used in 19 ways. This is an example of polymorphism.

Installation of Java

Two files are essential for installation:

·         java

·         javac

These are kept in bin folder where java is installed i.e. java files are kept.

After installation of JDK, we need to give their path for compilation and execution of .java files by command prompt.

Giving path

·         Open System Properties by right clicking on my computer and choosing properties

·         Go to advanced system properties

·         Choose Environment variables

·         Click New to create new variable

·         Define Variable Name: PATH and Variable value: (add path of java and javac i.e. path of bin separated by semi-colon). E.g.

Variable Name: PATH

Variable value: C:\Program Files (x86)\Java\jdk1.5.0_06\bin; C:\Program Files (x86)\Java\jdk1.5.0_06\bin

Compiling and Executing

After writing java program in Notepad, save the file anywhere in the drive where java is installed with .java extension.

In command prompt:

Compilation:

Select the path where java file is stored.

Type: javac filename.java and enter

Note: In order to compile all the java files together à Type: javac *.java

Execution:

Type: java classname and enter

These type of applications created in command prompt are known as Console Applications or Console based or Character based applications.

In case, we want have saved different classes in different files:

For example, 3 classes are saved as 3 different files:

1. class demo1

            {

            public void show()

                        {

            System.out.println(“Show”);

}

}

2. class demo2

            {

            public void show()

                        {

            System.out.println(“Show”);

}

}

3. class maindemo

            {

                        public static void main(String aa[])

{

            demo1 d = new demo1();

            demo2 d1 = new demo2();

            d.show();

            d1.show();

}

}

Now if we want to compile the class maindemo, we need to compile both the above classes before that else the compiler will show an error.

Note: If we make any changes in a program, we need to compile it again.

Features of Java

1. Compiled and Interpreted

Computer language is either compiled or interpreted. Java combines both these approaches. Java has 2 stage system:

·         Java compiler translates source code into byte code instruction. Byte code is not machine code.

·         Then, Java interpreter generates machine code from byte code that can be directly run on machine.

2. Platform independent and portable

Java program can be easily moved from one computer system to another anywhere and anytime. Changes and updates in OS, processors and system resources will not force any changes in java program.

3. Object oriented

Java is a true object oriented programming language. Almost everything in java is object. All program codes and data reside in objects and classes.

e.g.

In this declaration:

                int a; à This can also be written as:

Integer a = new Integer();

That’s why, a can be considered as an object of Integer class.

4. Distributed

Java is designed as distributed language for creating applications on network. It has the ability to share both data and program. Java applications can access remote objects on the internet as well as they can be accessed on local system.

e.g. Java supports RMI (Remote Method Invocation).

5. Multithreaded and interactive

Multithreading refers to switching between processes.

Benefit: Multithreading means handling multiple tasks simultaneously.

Java supports multithreaded programs. This means that we need not to wait for the application to finish the task before beginning another.

6. High performance

High performance is impressive for an interpreted language. Java is faster than native languages like C, C++ etc. Its architecture is also designed to reduce overhead during the runtime.

7. Dynamic and extensible

Java programs support functions written in other languages such as C, C++ etc. These functions are known as native methods.

Java is also a dynamic language. It is capable of dynamically linking new class libraries and methods. This facility enables the programmer to use the efficient functions available in languages.

Applications in Java

Note: Applications are of two types:

·         CUI (Character User Interface) – Less user interactive (drawback)

·         GUI (Graphical User Interface)

We can create 2 types of applications in Java:

1.      Standalone applications

Standalone applications are those programs written in java to carry out certain tasks on a standalone local computer. We have to perform 2 steps to run these kinds of applications:

a.       Compiling source code into byte code using javac compiler

b.      Executing the java programs using interpreter

2.      Applets

Applets are small programs developed for internet applications. They use HTML, that’s why, they can run on internet. These are capable of running on web-browsers and applet viewer.