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Showing posts with label history of java. Show all posts
Showing posts with label history of java. Show all posts

Friday, January 23, 2015

Operators in java


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.

OperatorsPrecedence
postfixexpr++ 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= += -= *= /= %= &= ^= |= <<= >>= >>>=

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);  
 }  
}  

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);  
    }  
      
   }  
 }  

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");   
      
   }  
}  

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");  
    
  }  
}  

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);  
  
}  
}  

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);  
}  
}  

Variable and Datatype in Java


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.

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 


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)

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.
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.

Thursday, January 22, 2015

Simple Program of Java


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

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

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.
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

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");  
}  
};  

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) 

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. Click here for How to set path in java.