Java (programming language)

• Java is a general-purpose computer programming language that is concurrent, class-based, object-oriented, and specifically designed to have as few implementation dependencies as possible. It is intended to let application developers "write once, run anywhere" (WORA), meaning that code that runs on one platform does not need to be recompiled to run on another.
• Java applications are typically compiled to bytecode that can run on any Java virtual machine (JVM) regardless of computer architecture. Java is, as of 2015, one of the most popular programming languages in use, particularly for client-server web applications, with a reported 9 million developers.
• Java was originally developed by James Gosling at Sun Microsystems (which has since merged into Oracle Corporation) and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++, but it has fewer low-level facilities than either of them.

• The original and reference implementation Java compilers, virtual machines, and class libraries were originally released by Sun under proprietary licences. As of May 2007, in compliance with the specifications of the Java Community Process, Sun relicensed most of its Java technologies under the GNU General Public License. 
• Others have also developed alternative implementations of these Sun technologies, such as the GNU Compiler for Java (bytecode compiler), GNU Classpath (standard libraries), and IcedTea-Web (browser plugin for applets).

Operators in java

Operator in java is a symbol that is used to perform operations. There are many types of operators in java such as unary operator, arithmetic operator, relational operator, shift operator, bitwise operator, ternary operator and assignment operator.

Operators	Precedence
postfix	expr++ expr--
unary	++expr --expr +expr -expr ~ !
multiplicative	* / %
additive	+ -
shift	<< >> >>>
relational	< > <= >= instanceof
equality	== !=
bitwise AND	&
bitwise exclusive OR	^
bitwise inclusive OR	|
logical AND	&&
logical OR	||
ternary	? :
assignment	= += -= *= /= %= &= ^= |= <<= >>= >>>=

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Useful Programs:

There is given some useful programs such as factorial number, prime number, fibonacci series etc.

It is better for the freshers to skip this topic and come to it after OOPs concepts.

1) Program of factorial number.

class Operation{  
  
 static int fact(int number){  
  int f=1;  
  for(int i=1;i<=number;i++){  
  f=f*i;  
  }  
 return f;  
 }  
  
 public static void main(String args[]){  
  int result=fact(5);  
  System.out.println("Factorial of 5="+result);  
 }  
}  

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2) Program of fibonacci series.

class Fabnoci{  
   
  public static void main(String[] args)  
  {  
    int n=10,i,f0=1,f1=1,f2=0;  
    for(i=1;i<=n;i++)  
    {  
      f2=f0+f1;  
      f0=f1;  
      f1=f2;  
          f2=f0;  
          System.out.println(f2);  
    }  
      
   }  
 }  

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3) Program of armstrong number.

class ArmStrong{  
  public static void main(String...args)  
  {  
    int n=153,c=0,a,d;  
    d=n;  
    while(n>0)  
    {  
    a=n%10;  
    n=n/10;  
    c=c+(a*a*a);  
    }  
    if(d==c)  
    System.out.println("armstrong number");   
    else  
    System.out.println("it is not an armstrong number");   
      
   }  
}  

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4) Program of checking palindrome number.

class Palindrome  
{  
   public static void main( String...args)  
  {  
   int a=242;  
   int  n=a,b=a,rev=0;  
   while(n>0)  
   {  
     a=n%10;  
     rev=rev*10+a;  
     n=n/10;  
   }  
   if(rev==b)  
   System.out.println("it is Palindrome");  
   else  
   System.out.println("it is not palinedrome");  
    
  }  
}  

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5) Program of swapping two numbers without using third variable.

class SwapTwoNumbers{  
public static void main(String args[]){  
int a=40,b=5;  
a=a*b;  
b=a/b;  
a=a/b;  
  
System.out.println("a= "+a);  
System.out.println("b= "+b);  
  
}  
}  

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6) Program of factorial number by recursion

class FactRecursion{  
  
static int fact(int n){  
if(n==1)  
return 1;  
  
return n*=fact(n-1);  
}  
  
public static void main(String args[]){  
  
int f=fact(5);  
System.out.println(f);  
}  
}  

Unicode System

Unicode is a universal international standard character encoding that is capable of representing most of the world's written languages.

Why java uses Unicode System?

Before Unicode, there were many language standards:

ASCII (American Standard Code for Information Interchange) for the United States. ISO 8859-1 for Western European Language. KOI-8 for Russian. GB18030 and BIG-5 for chinese, and so on.

This caused two problems: A particular code value corresponds to different letters in the various language standards. The encodings for languages with large character sets have variable length.Some common characters are encoded as single bytes, other require two or more byte.

To solve these problems, a new language standard was developed i.e. Unicode System.

In unicode, character holds 2 byte, so java also uses 2 byte for characters.

lowest value:\u0000

highest value:\uFFFF

Variable and Datatype in Java

In this page, we will learn about the variable and java data types. Variable is a name of memory location. There are three types of variables: local, instance and static. There are two types of datatypes in java, primitive and non-primitive.

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Variable

Variable is name of reserved area allocated in memory.

1. int data=50;//Here data is variable  

Types of Variable

There are three types of variables in java local variable instance variable static variable

Local Variable

A variable that is declared inside the method is called local variable.

Instance Variable

A variable that is declared inside the class but outside the method is called instance variable . It is not declared as static.

Static variable

A variable that is declared as static is called static variable. It cannot be local.

We will have detailed learning of these variables in next chapters.

Example to understand the types of variables

class A{  
int data=50;//instance variable  
static int m=100;//static variable  
void method(){  
int n=90;//local variable  
}  
}//end of class  

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Data Types in Java

In java, there are two types of data types primitive data types non-primitive data types

Data Type	Default Value	Default size
boolean	false	1 bit
char	'\u0000'	2 byte
byte	0	1 byte
short	0	2 byte
int	0	4 byte
long	0L	8 byte
float	0.0f	4 byte
double	0.0d	8 byte

Why char uses 2 byte in java and what is \u0000 ?

because java uses unicode system rather than ASCII code system. \u0000 is the lowest range of unicode system.To get detail about Unicode see below.

JVM (Java Virtual Machine)

JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime environment in which java bytecode can be executed.

JVMs are available for many hardware and software platforms (i.e.JVM is plateform dependent).

What is JVM?

It is:

1. A specification where working of Java Virtual Machine is specified. But implementation provider is independent to choose the algorithm. Its implementation has been provided by Sun and other companies.
2. An implementation Its implementation is known as JRE (Java Runtime Environment).
3. Runtime Instance Whenever you write java command on the command prompt to run the java class, and instance of JVM is created.

What it does?

The JVM performs following operation:

• Loads code
• Verifies code
• Executes code
• Provides runtime environment

JVM provides definitions for the:

• Memory area
• Class file format
• Register set
• Garbage-collected heap
• Fatal error reporting etc.

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Internal Architecture of JVM

Let's understand the internal architecture of JVM. It contains classloader, memory area, execution engine etc.

 

1) Classloader:

Classloader is a subsystem of JVM that is used to load class files.

2) Class(Method) Area:

Class(Method) Area stores per-class structures such as the runtime constant pool, field and method data, the code for methods.

3) Heap:

It is the runtime data area in which objects are allocated.

4) Stack:

Java Stack stores frames.It holds local variables and partial results, and plays a part in method invocation and return.

Each thread has a private JVM stack, created at the same time as thread.

A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.

5) Program Counter Register:

PC (program counter) register. It contains the address of the Java virtual machine instruction currently being executed.

6) Native Method Stack:

It contains all the native methods used in the application.

7) Execution Engine:

It contains:

1) A virtual processor

2) Interpreter:Read bytecode stream then execute the instructions.

3) Just-In-Time(JIT) compiler:It is used to improve the performance.JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation.Here the term ?compiler? refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.

JDK, JRE and JVM

Understanding the difference between JDK, JRE and JVM is important in Java. We are having brief overview of JVM here. If you want to get the detailed knowledge of Java Virtural Machine, move to the next page. Firstly, let's see the basic differences between the JDK, JRE and JVM.

JVM JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime environment in which java bytecode can be executed. JVMs are available for many hardware and software platforms. JVM, JRE and JDK are platform dependent because configuration of each OS differs. But, Java is platform independent. The JVM performs following main tasks: Loads code Verifies code Executes code Provides runtime environment JRE JRE is an acronym for Java Runtime Environment.It is used to provide runtime environment.It is the implementation of JVM.It physically exists.It contains set of libraries + other files that JVM uses at runtime. Implementation of JVMs are also actively released by other companies besides Sun Micro Systems. JDK JDK is an acronym for Java Development Kit.It physically exists.It contains JRE + development tools.

How to set path in Java

The path is required to be set for using tools such as javac, java etc.

If you are saving the java source file inside the jdk/bin directory, path is not required to be set because all the tools will be available in the current directory.

But If you are having your java file outside the jdk/bin folder, it is necessary to set path of JDK.

There are 2 ways to set java path:

1. temporary
2. permanent

1) How to set Temporary Path of JDK in Windows

To set the temporary path of JDK, you need to follow following steps:

• Open command prompt
• copy the path of jdk/bin directory
• write in command prompt: set path=copied_path

For Example:

set path=C:\Program Files\Java\jdk1.6.0_23\bin

Let's see it in the figure given below:

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2) How to set Permanent Path of JDK in Windows

For setting the permanent path of JDK, you need to follow these steps:

• Go to MyComputer properties -> advanced tab -> environment variables -> new tab of user variable -> write path in variable name -> write path of bin folder in variable value -> ok -> ok -> ok

For Example:

1)Go to MyComputer properties

2)click on advanced tab

3)click on environment variables

4)click on new tab of user variables

5)write path in variable name

6)Copy the path of bin folder

7)paste path of bin folder in variable value

8)click on ok button

9)click on ok button

Now your permanent path is set.You can now execute any program of java from any drive.

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Setting Java Path in Linux OS

Setting the path in Linux OS is same as setting the path in the Windows OS. But here we use export tool rather than set. Let's see how to set path in Linux OS:

export PATH=$PATH:/home/jdk1.6.01/bin/

Here, we have installed the JDK in the home directory under Root (/home).

Internal Details of Hello Java Program

In the previous page, we have learned about the first program, how to compile and how to run the first java program. Here, we are going to learn, what happens while compiling and running the java program. Moreover, we will see some question based on the first program.

What happens at compile time?

At compile time, java file is compiled by Java Compiler (It does not interact with OS) and converts the java code into bytecode.

In the previous page, we have learned about the first program, how to compile and how to run the first java program. Here, we are going to learn, what happens while compiling and running the java program. Moreover, we will see some question based on the first program.

What happens at compile time?

At compile time, java file is compiled by Java Compiler (It does not interact with OS) and converts the java code into bytecode.

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What happens at runtime?

At runtime, following steps are performed:

Classloader: is the subsystem of JVM that is used to load class files.

Bytecode Verifier: checks the code fragments for illegal code that can violate access right to objects.

Interpreter: read bytecode stream then execute the instructions.

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Q)Can you save a java source file by other name than the class name?

Yes, if the class is not public. It is explained in the figure given below:

To compile:	javac Hard.java
To execute:	java Simple

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Q)Can you have multiple classes in a java source file?

Yes, like the figure given below illustrates:

Simple Program of Java

In this page, we will learn how to write the simple program of java. We can write a simple hello java program easily after installing the JDK.

To create a simple java program, you need to create a class that contains main method. Let's understand the requirement first.

Requirement for Hello Java Example

For executing any java program, you need to install the JDK if you don't have installed, install it. set path of the jdk/bin directory.  create the java program compile and run the java program

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Creating hello java example

Let's create the hello java program:

class Simple{  
    public static void main(String args[]){  
     System.out.println("Hello Java");  
    }  
}  

save this file as Simple.java

To compile:	javac Simple.java
To execute:	java Simple

Output:Hello Java

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Understanding first java program

Let's see what is the meaning of class, public, static, void, main, String[], System.out.println().

• class keyword is used to declare a class in java.
• public keyword is an access modifier which represents visibility, it means it is visible to all.
• static is a keyword, if we declare any method as static, it is known as static method. The core advantage of static method is that there is no need to create object to invoke the static method. The main method is executed by the JVM, so it doesn't require to create object to invoke the main method. So it saves memory.
• void is the return type of the method, it means it doesn't return any value.
• main represents startup of the program.
• String[] args is used for command line argument. We will learn it later.
• System.out.println() is used print statement. We will learn about the internal working of System.out.println statement later.

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To write the simple program, open notepad by start menu -> All Programs -> Accessories -> notepad and write simple program as displayed below:

 

As displayed in the above diagram, write the simple program of java in notepad and saved it as Simple.java. To compile and run this program, you need to open command prompt by start menu -> All Programs -> Accessories -> command prompt.

 

To compile and run the above program, go to your current directory first; my current directory is c:\new . Write here:

To compile:	javac Simple.java
To execute:	java Simple

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How many ways can we write a java program

There are many ways to write a java program. The modifications that can be done in a java program are given below:

1) By changing sequence of the modifiers, method prototype is not changed.

Let's see the simple code of main method.

static public void main(String args[])  

2) subscript notation in java array can be used after type, before variable or after variable.

Let's see the different codes to write the main method.

public static void main(String[] args)  
public static void main(String []args)  
public static void main(String args[])  

3) You can provide var-args support to main method by passing 3 ellipses (dots)

Let's see the simple code of using var-args in main method. We will learn about var-args later in Java New Features chapter.

public static void main(String... args)  

4) Having semicolon at the end of class in java is optional.

Let's see the simple code.

class A{  
static public void main(String... args){  
System.out.println("hello java4");  
}  
};  

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Valid java main method signature

public static void main(String[] args)  
public static void main(String []args)  
public static void main(String args[])  
public static void main(String... args)  
static public void main(String[] args)  
public static final void main(String[] args)  
final public static void main(String[] args)  
final strictfp public static void main(String[] args)  

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Invalid java main method signature

public void main(String[] args)  
static void main(String[] args)  
public void static main(String[] args)  
abstract public static void main(String[] args)  

Resolving an error "javac is not recognized as an internal or external command" ?

If there occurs a problem like displayed in the below figure, you need to set path. Since DOS doesn't know javac or java, we need to set path. Path is not required in such a case if you save your program inside the jdk/bin folder. But its good approach to set path.