Java

Java Tutorial

Java is a high-level programming language originally developed by Sun Microsystems and released in 1995. Java runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX. This tutorial gives a complete understanding on Java.
This reference will take you through simple and practical approach while learning Java Programming language.

Audience

This reference has been prepared for the beginners to help them understand the basic to advanced concepts related to Java Programming languages.

Prerequisites

Before you start doing practice with various types of examples given in this reference, I'm making an assumption that you are already aware about what is a computer program and what is a computer programming language.

Compile/Execute Java Programs

We have provided you an option to execute available Java code online. Just click on Try it button avaiable at top-right corner of the code window to execute available code. There are certain examples involving multiple source files etc. which can not be executed using Try it option but you can try them them using compileonline.com, which is available on a high capacity dedicated server giving you real programming experience with a comfort of single click compilation and execution. Yes! it is absolutely free and its online.

Java - Overview

Java programming language was originally developed by Sun Microsystems, which was initiated by James Gosling and released in 1995 as core component of Sun Microsystems.s Java platform (Java 1.0 [J2SE]).
As of December 08 the latest release of the Java Standard Edition is 6 (J2SE). With the advancement of Java and its wide spread popularity, multiple configurations were built to suite various types of platforms. Ex: J2EE for Enterprise Applications, J2ME for Mobile Applications.
Sun Microsystems has renamed the new J2 versions as Java SE, Java EE and Java ME respectively. Java is guaranteed to be Write Once, Run Anywhere
Java is:
  • Object Oriented : In java everything is an Object. Java can be easily extended since it is based on the Object model.
  • 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.
  • Secure : With Java's secure feature it enables to develop virus-free, tamper-free systems. Authentication techniques are based on public-key encryption.
  • 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 Java runtime system.
  • Portable :being architectural neutral and having no implementation dependent aspects of the specification makes Java portable. Compiler and 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.
  • Multi-threaded : With Java's multi-threaded feature it is possible to write programs that can do many tasks simultaneously. This design feature allows developers to construct smoothly running interactive applications.
  • Interpreted :Java byte code is translated on the fly to native machine instructions and is not stored anywhere. The development process is more rapid and analytical since the linking is an incremental and light weight process.
  • 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.

History of Java:

James Gosling initiated the Java language project in June 1991 for use in one of his many set-top box projects. The language, initially called Oak after an oak tree that stood outside Gosling's office, also went by the name Green and ended up later renamed as Java, from a list of random words.
Sun released the first public implementation as Java 1.0 in 1995. It promised Write Once, Run Anywhere (WORA), providing no-cost run-times on popular platforms.
On 13 November 2006, Sun released much of Java as free and open source software under the terms of the GNU General Public License (GPL).
On 8 May 2007 Sun finished the process, making all of Java's core code free and open-source, aside from a small portion of code to which Sun did not hold the copyright.

Tools you will need:

For performing the examples discussed in this tutorial, you will need a Pentium 200-MHz computer with a minimum of 64 MB of RAM (128 MB of RAM recommended).
You also will need the following softwares:
  • Linux 7.1 or Windows 95/98/2000/XP operating system.
  • Java JDK 5
  • Microsoft Notepad or any other text editor
This tutorial will provide the necessary skills to create GUI, networking, and Web applications using Java.

Try It Option:

We have provided you an option to compile and execute available code online. Just click on Try it button avaiable at top-right corner of the code window to compile and execute available code. There are certain examples which can not be executed online, so we have skipped those examples.
public class MyFirstJavaProgram {

    public static void main(String []args) {
       System.out.println("Hello World");
    }
}
There may be a case that you do not see the result of the compiled/executed code, in such case you can re-try to compile and execute the code using execute button available in compliation pop-up window.

Java Environment Setup

Before we proceed further it is important that we set up the java environment correctly. This section guides you on how to download and set up Java on your machine. Please follow the following steps to set up the environment.
Java SE is freely available from the link Download Java. So you download a version based on your operating system.
Follow the instructions to download java and run the .exe to install Java on your machine. Once you installed Java on your machine, you would need to set environment variables to point to correct installation directories:

Setting up the path for windows 2000/XP:

Assuming you have installed Java in c:\Program Files\java\jdk directory:
  • Right-click on 'My Computer' and select 'Properties'.
  • Click on the 'Environment variables' button under the 'Advanced' tab.
  • Now alter the 'Path' variable so that it also contains the path to the Java executable. Example, if the path is currently set to 'C:\WINDOWS\SYSTEM32', then change your path to read 'C:\WINDOWS\SYSTEM32;c:\Program Files\java\jdk\bin'.

Setting up the path for windows 95/98/ME:

Assuming you have installed Java in c:\Program Files\java\jdk directory:
  • Edit the 'C:\autoexec.bat' file and add the following line at the end:
    'SET PATH=%PATH%;C:\Program Files\java\jdk\bin'

Setting up the path for Linux, UNIX, Solaris, FreeBSD:

Environment variable PATH should be set to point to where the java binaries have been installed. Refer to your shell documentation if you have trouble doing this.
Example, if you use bash as your shell, then you would add the following line to the end of your '.bashrc: export PATH=/path/to/java:$PATH'

Popular Java Editors:

To write your java programs you will need a text editor. There are even more sophisticated IDE available in the market. But for now, you can consider one of the following:
  • Notepad : On Windows machine you can use any simple text editor like Notepad (Recommended for this tutorial), TextPad.
  • Netbeans :is a Java IDE that is open source and free which can be downloaded from http://www.netbeans.org/index.html.
  • Eclipse : is also a java IDE developed by the eclipse open source community and can be downloaded from http://www.eclipse.org/.

Java Basic Syntax

When we consider a Java program it can be defined as a collection of objects that communicate via invoking each others methods. Let us now briefly look into what do class, object, methods and instance variables mean.
  • Object - Objects have states and behaviors. Example: A dog has states-color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.
  • Class - A class can be defined as a template/ blue print that describe the behaviors/states that object of its type support.
  • Methods - A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.
  • Instance Variables - Each object has its unique set of instance variables. An object's state is created by the values assigned to these instance variables.

First Java Program:

Let us look at a simple code that would print the words Hello World.
public class MyFirstJavaProgram {

   /* This is my first java program.  
    * This will print 'Hello World' as the output
    */

    public static void main(String []args) {
       System.out.println("Hello World"); // prints Hello World
    }
}
Lets look at how to save the file, compile and run the program. Please follow the steps given below:
  • Open notepad and add the code as above.
  • Save the file as : MyFirstJavaProgram.java.
  • Open a command prompt window and go o the directory where you saved the class. Assume its C:\.
  • Type ' javac MyFirstJavaProgram.java ' and press enter to compile your code. If there are no errors in your code the command prompt will take you to the next line.( Assumption : The path variable is set).
  • Now type ' java MyFirstJavaProgram ' to run your program.
  • You will be able to see ' Hello World ' printed on the window.
C : > javac MyFirstJavaProgram.java
C : > java MyFirstJavaProgram 
Hello World

Basic Syntax:

About Java programs, it is very important to keep in mind the following points.
  • Case Sensitivity - Java is case sensitive which means identifier Hello and hello would have different meaning in Java.
  • Class Names - For all class names the first letter should be in Upper Case.

    If several words are used to form a name of the class each inner words first letter should be in Upper Case.

    Example class MyFirstJavaClass
  • Method Names - All method names should start with a Lower Case letter.

    If several words are used to form the name of the method, then each inner word's first letter should be in Upper Case.

    Example public void myMethodName()
  • Program File Name - Name of the program file should exactly match the class name.

    When saving the file you should save it using the class name (Remember java is case sensitive) and append '.java' to the end of the name. (if the file name and the class name do not match your program will not compile).

    Example : Assume 'MyFirstJavaProgram' is the class name. Then the file should be saved as 'MyFirstJavaProgram.java'
  • public static void main(String args[]) - java program processing starts from the main() method which is a mandatory part of every java program..

Java Identifiers:

All java components require names. Names used for classes, variables and methods are called identifiers.
In java there are several points to remember about identifiers. They are as follows:
  • All identifiers should begin with a letter (A to Z or a to z ), currency character ($) or an underscore (_).
  • After the first character identifiers can have any combination of characters.
  • A key word cannot be used as an identifier.
  • Most importantly identifiers are case sensitive.
  • Examples of legal identifiers:age, $salary, _value, __1_value
  • Examples of illegal identifiers : 123abc, -salary

Java Modifiers:

Like other languages it is possible to modify classes, methods etc by using modifiers. There are two categories of modifiers.
  • Access Modifiers : default, public , protected, private
  • Non-access Modifiers : final, abstract, strictfp
We will be looking into more details about modifiers in the next section.

Java Variables:

We would see following type of variables in Java:
  • Local Variables
  • Class Variables (Static Variables)
  • Instance Variables (Non static variables)

Java Arrays:

Arrays are objects that store multiple variables of the same type. However an Array itself is an object on the heap. We will look into how to declare, construct and initialize in the upcoming chapters.

Java Enums:

Enums were introduced in java 5.0. Enums restrict a variable to have one of only a few predefined values. The values in this enumerated list are called enums.
With the use of enums it is possible to reduce the number of bugs in your code.
For example if we consider an application for a fresh juice shop it would be possible to restrict the glass size to small, medium and Large. This would make sure that it would not allow anyone to order any size other than the small, medium or large.

Example:

class FreshJuice {

   enum FreshJuiceSize{ SMALL, MEDUIM, LARGE }
   FreshJuiceSize size;
}

public class FreshJuiceTest {

   public static void main(String args[]){
      FreshJuice juice = new FreshJuice();
      juice.size = FreshJuice. FreshJuiceSize.MEDUIM ;
      System.out.println("Size: " + juice.size);
   }
}
Above example will produce following result:
Size: MEDUIM
Note: enums can be declared as their own or inside a class. Methods, variables, constructors can be defined inside enums as well.

Java Keywords:

The following list shows the reserved words in Java. These reserved words may not be used as constant or variable or any other identifier names.
abstractassertbooleanbreak
bytecasecatchchar
classconstcontinuedefault
dodoubleelseenum
extendsfinalfinallyfloat
forgotoifimplements
importinstanceofintinterface
longnativenewpackage
privateprotectedpublicreturn
shortstaticstrictfpsuper
switchsynchronizedthisthrow
throwstransienttryvoid
volatilewhile

Comments in Java

Java supports single line and multi-line comments very similar to c and c++. All characters available inside any comment are ignored by Java compiler.
public class MyFirstJavaProgram{

   /* This is my first java program.
    * This will print 'Hello World' as the output
    * This is an example of multi-line comments.
    */

    public static void main(String []args){
       // This is an example of single line comment
       /* This is also an example of single line comment. */
       System.out.println("Hello World"); 
    }
}

Using Blank Lines:

A line containing only whitespace, possibly with a comment, is known as a blank line, and Java totally ignores it.

Inheritance:

In java classes can be derived from classes. Basically if you need to create a new class and here is already a class that has some of the code you require, then it is possible to derive your new class from the already existing code.
This concept allows you to reuse the fields and methods of the existing class with out having to rewrite the code in a new class. In this scenario the existing class is called the super class and the derived class is called the subclass.

Interfaces:

In Java language an interface can be defined as a contract between objects on how to communicate with each other. Interfaces play a vital role when it comes to the concept of inheritance.
An interface defines the methods, a deriving class(subclass) should use. But the implementation of the methods is totally up to the subclass.

Java - Objects and Classes

Java is an Object Oriented Language. As a language that has the Object Oriented feature Java supports the following fundamental concepts:
  • Polymorphism
  • Inheritance
  • Encapsulation
  • Abstraction
  • Classes
  • Objects
  • Instance
  • Method
  • Message Parsing
In this chapter we will look into the concepts Classes and Objects.
  • Object - Objects have states and behaviors. Example: A dog has states-color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.
  • Class - A class can be defined as a template/ blue print that describe the behaviors/states that object of its type support.

Objects in Java:

Let us now look deep into what are objects. If we consider the real-world we can find many objects around us, Cars, Dogs, Humans etc. All these objects have a state and behavior.
If we consider a dog then its state is - name, breed, color, and the behavior is - barking, wagging, running
If you compare the software object with a real world object, they have very similar characteristics.
Software objects also have a state and behavior. A software object's state is stored in fields and behavior is shown via methods.
So in software development methods operate on the internal state of an object and the object-to-object communication is done via methods.

Classes in Java:

A class is a blue print from which individual objects are created.
A sample of a class is given below:
public class Dog{
   String breed;
   int age;
   String color;

   void barking(){
   }
   
   void hungry(){
   }
   
   void sleeping(){
   }
}
A class can contain any of the following variable types.
  • Local variables . variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and the variable will be destroyed when the method has completed.
  • Instance variables . Instance variables are variables within a class but outside any method. These variables are instantiated when the class is loaded. Instance variables can be accessed from inside any method, constructor or blocks of that particular class.
  • Class variables . Class variables are variables declared with in a class, outside any method, with the static keyword.
A class can have any number of methods to access the value of various kind of methods. In the above example, barking(), hungry() and sleeping() are methods.
Below mentioned are some of the important topics that need to be discussed when looking into classes of the Java Language.

Constructors:

When discussing about classes one of the most important sub topic would be constructors. Every class has a constructor. If we do not explicitly write a constructor for a class the java compiler builds a default constructor for that class.
Each time a new object is created at least one constructor will be invoked. The main rule of constructors is that they should have the same name as the class. A class can have more than one constructor.
Example of a constructor is given below:
public class Puppy{
   public puppy(){
   }

   public puppy(String name){
      // This constructor has one parameter, name.
   }
}
Java also supports Singleton Classes where you would be able to create only one instance of a class.

Creating an Object:

As mentioned previously a class provides the blueprints for objects. So basically an object is created from a class. In java the new key word is used to create new objects.
There are three steps when creating an object from a class:
  • Declaration . A variable declaration with a variable name with an object type.
  • Instantiation . The 'new' key word is used to create the object.
  • Initialization . The 'new' keyword is followed by a call to a constructor. This call initializes the new object.
Example of creating an object is given below:
public class Puppy{

   public Puppy(String name){
      // This constructor has one parameter, name.
      System.out.println("Passed Name is :" + name ); 
   }
   public static void main(String []args){
      // Following statement would create an object myPuppy
      Puppy myPuppy = new Puppy( "tommy" );
   }
}
If we compile and run the above program then it would produce following result:
Passed Name is :tommy

Accessing Instance Variables and Methods:

Instance variables and methods are accessed via created objects. To access an instance variable the fully qualified path should be as follows:
/* First create an object */
ObjectReference = new Constructor();

/* Now call a variable as follows */
ObjectReference.variableName;

/* Now you can call a class method as follows */
ObjectReference.MethodName();

Example:

This example explains how to access instance variables and methods of a class:
public class Puppy{
   
   int puppyAge;

   public Puppy(String name){
      // This constructor has one parameter, name.
      System.out.println("Passed Name is :" + name ); 
   }
   public void setAge( int age ){
       puppyAge = age;
   }

   public int getAge( ){
       System.out.println("Puppy's age is :" + puppyAge ); 
       return puppyAge;
   }
   public static void main(String []args){
      /* Object creation */
      Puppy myPuppy = new Puppy( "tommy" );

      /* Call class method to set puppy's age */
      myPuppy.setAge( 2 );

      /* Call another class method to get puppy's age */
      myPuppy.getAge( );

      /* You can access instance variable as follows as well */
      System.out.println("Variable Value :" + myPuppy.puppyAge ); 
   }
}
If we compile and run the above program then it would produce following result:
Passed Name is :tommy
Puppy's age is :2
Variable Value :2

Source file declaration rules:

As the last part of this section lets us now look into the source file declaration rules. These rules are essential when declaring classes, import statements and package statements in a source file.
  • There can be only one public class per source file.
  • A source file can have multiple non public classes.
  • The public class name should be the name of the source file as well which should be appended by .java at the end. For example : The class name is . public class Employee{} Then the source file should be as Employee.java.
  • If the class is defined inside a package, then the package statement should be the first statement in the source file.
  • If import statements are present then they must be written between the package statement and the class declaration. If there are no package statements then the import statement should be the first line in the source file.
  • Import and package statements will imply to all the classes present in the source file. It is not possible to declare different import and/or package statements to different classes in the source file.
Classes have several access levels and there are different types of classes; abstract classes, final classes etc. I will be explaining about all these in the access modifiers chapter.
Apart from the above mentioned types of classes, Java also has some special classes called Inner classes and Anonymous classes.

Java Package:

In simple it is a way of categorizing the classes and interfaces. When developing applications in Java, hundreds of classes and interfaces will be written, therefore categorizing these classes is a must as well as makes life much easier.

Import statements:

In java if a fully qualified name, which includes the package and the class name, is given then the compiler can easily locate the source code or classes. Import statement is a way of giving the proper location for the compiler to find that particular class.
For example following line would ask compiler to load all the classes available in directory java_installation/java/io :
import java.io.*;

A Simple Case Study:

For our case study we will be creating two classes. They are Employee and EmployeeTest.
First open notepad and add the following code. Remember this is the Employee class and the class is a public class. Now save this source file with the name Employee.java.
The Employee class has four instance variables name, age, designation and salary. The class has one explicitly defined constructor which takes a parameter.
import java.io.*;
public class Employee{
   String name;
   int age;
   String designation;
   double salary;
	
   // This is the constructor of the class Employee
   public Employee(String name){
      this.name = name;
   }
   // Assign the age of the Employee  to the variable age.
   public void empAge(int empAge){
      age =  empAge;
   }
   /* Assign the designation to the variable designation.*/
   public void empDesignation(String empDesig){
      designation = empDesig;
   }
   /* Assign the salary to the variable	salary.*/
   public void empSalary(double empSalary){
      salary = empSalary;
   }
   /* Print the Employee details */
   public void printEmployee(){
      System.out.println("Name:"+ name );
      System.out.println("Age:" + age );
      System.out.println("Designation:" + designation );
      System.out.println("Salary:" + salary);
   }
}
As mentioned previously in this tutorial processing starts from the main method. Therefore in-order for us to run this Employee class there should be main method and objects should be created. We will be creating a separate class for these tasks.
Given below is the EmployeeTest class which creates two instances of the class Employee and invokes the methods for each object to assign values for each variable.
Save the following code in EmployeeTest.java file
import java.io.*;
public class EmployeeTest{

   public static void main(String args[]){
      /* Create two objects using constructor */
      Employee empOne = new Employee("James Smith");
      Employee empTwo = new Employee("Mary Anne");

      // Invoking methods for each object created
      empOne.empAge(26);
      empOne.empDesignation("Senior Software Engineer");
      empOne.empSalary(1000);
      empOne.printEmployee();

      empTwo.empAge(21);
      empTwo.empDesignation("Software Engineer");
      empTwo.empSalary(500);
      empTwo.printEmployee();
   }
}
Now compile both the classes and then run EmployeeTest to see the result as follows:
C :> javac Employee.java
C :> vi EmployeeTest.java
C :> javac  EmployeeTest.java
C :> java EmployeeTest
Name:James Smith
Age:26
Designation:Senior Software Engineer
Salary:1000.0
Name:Mary Anne
Age:21
Designation:Software Engineer
Salary:500.0



























































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