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Saturday, December 28, 2013

KNOW ABOUT JAVA

What is Java?
.
Java is:
.
Object Oriented
Platform independent:
Simple
Secure
Architectural- neutral
Portable
Robust
Multi-threaded
Interpreted
High Performance
Distributed
Dynamic
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Java Environment Setup :
.
Java SE is freely available from the link Download Java( http://www.oracle.com/technetwork/java/archive-139210.html ). So you download a version based on your
operating system.
You can refer to installation guide for a complete detail.
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Java Basic Syntax :
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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.
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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.
Instant Variables - Each object has its unique set of instant variables. An object's state is created by
the values assigned to these instant variables.
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First Java Program :
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Let us look at a simple code that would print the words Hello World .
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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
}
}
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
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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()
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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
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Java Modifiers :
Like other languages, it is possible to modify classes, methods, etc., by using modifiers. There are two
categories of modifiers.
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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.
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Java Variables :
We would see following type of variables in Java:
Local Variables
Class Variables (Static Variables)
Instance Variables (Non static variables)
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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.
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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{ SIZE , MEDIUM , LARGE }
FreshJuiceSize size ;
}
public class FreshJuiceTest{
public static void main ( String args []){
FreshJuice juice = new FreshJuice();
juice . size = FreshJuice. FreshJuiceSize.MEDIUM ;
System. out .println ( "Size :" + juice . size);
}
}
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.
abstract ,assert ,boolean, break
byte ,case ,catch ,char
class ,const ,continue ,default
do, double ,else ,enum
extends ,final ,finally ,float
for ,goto ,if, implements
import ,instanceof ,int ,interface
long ,native ,new, package
private, protected, public, return
short, static ,strictfp ,super
switch ,synchronized ,this, throw
throws, transient, try, void
volatile, while
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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" );
}
}
Data Types in Java
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There are two data types available in Java:
Primitive Data Types
Reference/Object Data Types
.
Primitive Data Types :
There are eight primitive data types supported by Java. Primitive data types are predefined by the
language and named by a key word. Let us now look into detail about the eight primitive data types.
.
byte
short
int
long
float
double
boolean
char
.
Reference Data Types :
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Reference variables are created using defined constructors of the classes. They are used to access
objects. These variables are declared to be of a specific type that cannot be changed. For example,
Employee, Puppy etc.
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Class objects, and various type of array variables come under reference data type.
Default value of any reference variable is null.
A reference variable can be used to refer to any object of the declared type or any compatible type.
Example : Animal animal = new Animal("giraffe");
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Java Literals :
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A literal is a source code representation of a fixed value. They are represented directly in the code without
any computation.
Literals can be assigned to any primitive type variable. For example:
byte a = 68;
char a = 'A'
String literals in Java are specified like they are in most other languages by enclosing a sequence of
characters between a pair of double quotes. Examples of string literals are:
"Hello World"
"two\nlines"
"\"This is in quotes\""
Java language supports few special escape sequences for String and char literals as well. They are:
Notation.                         Character represented
\n---------------------------- Newline (0x0a)
\r---------------------------- Carriage return (0x0d)
\f---------------------------- Formfeed (0x0c)
\b--------------------------- Backspace (0x08)
\s--------------------------- Space (0x20)
\t---------------------------- tab
\" ----------------------------Double quote
\'---------------------------- Single quote
\\---------------------------- backslash
\ddd -------------------------Octal character (ddd)
\uxxxx----------------------- Hexadecimal UNICODE character (xxxx)
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Java Access Modifiers :
.
Java provides a number of access modifiers to set access levels for classes, variables, methods and
constructors. The four access levels are:
.
.Visible to the package. the default. No modifiers are needed.
.Visible to the class only (private).
.Visible to the world (public).
.Visible to the package and all subclasses (protected).
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Java Basic Operators :
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Java provides a rich set of operators to manipulate variables. We can divide all the Java operators into the
following groups:
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The Arithmetic Operators :
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Operator---------------Description--------------------------------------------------------------- Example
+                              ..Addition - Adds values on either side of the operator                                         .A + B will give 30
-                             ..Subtraction - Subtracts right hand operand from left hand operand                 .A - B will give-10
*                            ..Multiplication - Multiplies values on either side of the operator                          .A * B will give200
/                           ..Division - Divides left hand operand by right hand operand                                 .B / A will give 2
%                      ..Modulus - Divides left hand operand by right hand operand and returns remainder   B % A will give 0
++                       ..Increment - Increase the value of operand by 1                                                      .B++ gives 21
--                        ..Decrement - Decrease the value of operand by 1                                                   .B-- gives 19
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The Relational Operators :
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Operator------Description -----------------------------------------------------------------Example
==                       .Checks if the value of two operands are equal or not,
                            .if yes then condition becomes true.                                                                     .(A == B) is not true.
!=                       .Checks if the value of two operands are equal or not,
                          .if values are not equal then condition becomes true.                                            .(A != B) is true.
>                       .Checks if the value of left operand is greater than the value of right
                         .operand, if yes then condition becomes true.                                                         .(A > B) is not true.
<                       .Checks if the value of left operand is less than the value of right operand,
                          .if yes then condition becomes true.                                                                        .(A < B) is true.
>=                     .Checks if the value of left operand is greater than or
                         .equal to the value of right operand, if yes then condition
                          .becomes true.                                                                                                             .(A >= B) is not true.
<=                     .Checks if the value of left operand is less than or
                         .equal to the value of right operand, if yes then condition becomes true.             .(A <= B) is true.
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The Bitwise Operators :
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Operator------Description-------------------------------------------------------- Example
&                      ..Binary AND Operator copies a bit to the result if
                        ..it exists in both operands.                                                         ..(A & B) will give 12 which is 00001100
|                       ..Binary OR Operator copies a bit if it exists in eather
                        ..operand.                                                                                       ..(A | B) will give 61 which is 00111101
^                      ..Binary XOR Operator copies the bit if it is set in one
                       ..operand but not both.                                                                 ..(A ^ B) will give 49 which is 00110001
~                    ..Binary Ones Complement Operator is unary and has the
                      ..efect of 'flipping' bits.                                                                    ..(~A ) will give -61 which is 1100
                                                                                                                                 ..0011 in 2's complement form due to
                                                                                                                                 ..a signed binary number.
<<                 ..Binary Left Shift Operator. The left operands value is
                    ..moved left by the number of bits specified by the right
                    ..operand.                                                                                           ..A << 2 will give 240 which is 11110000
>>                ..Binary Right Shift Operator. The left operands value is
                    ..moved right by the number of bits specified by the right
                    ..operand.                                                                                          ..A >> 2 will give 15 which is 1111
>>>              ..Shift right zero fill operator. The left operands value is
                    ..moved right by the number of bits specified by the right
                    ..operand and shifted values are filled up with zeros.                 ..A >>>2 will give 15 which is 00001111
More information is in next post, please comment if you want more about java