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8/8/2019 1.Java Basics
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JAVA BASICS
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What is Java?
Java is ahigh-level,third generationprogramming language,like C, Fortranandcan be used to write computer applications
that crunchnumbers, process words, playgames, store data or do any ofthethousandsof other things computer software can do.
Javatechnology is botha programminglanguageand a platform.
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JAVA PLATFORM
Aplatform is thehardware or software
environment in whicha program runs.
The Java platformhas two components:TheJava Virtual Machine
TheJava Application Programming Interface (API)
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JAVA BUZZ WORDS
OBJECT ORIENTED
PLATFORM INDEPENDENT
SIMPLE
SECURE
SUITABLE FOR WEB APPLICATION
ROBUST
PORTABLE
EXCEPTION HANDLING
MULTI THREADING BUILT IN API
OPEN SOURCE
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OBJECT ORIENTED PROGRAMMING
In object-oriented programs data is represented by
objects. Objects havetwo sections
Fields (Instancevariables) tellyou whatan
object is.
Methods tellyou whatan object does.
These fields and methods are closelytied to the
object's real world characteristics and behavior.
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OOP Principles
ENCAPSULATION
Binding ofattributes and behaviour insidea class
INHERITANCE
Deriving the properties of one class bythe other
POLYMORPHISM
Using the samename for differenttypes of
actions
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PLATFORM INDEPENDENT
Java is compiled to an intermediate form
called byte-code,a specialnative program
called the Java interpreter reads the byte code
and executes the corresponding native
machine instructions.
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COMPILATION
.JAVA FILE JAVAC.EXE .CLASS FILE
EXECUTION
.CLASS FILE JAVA.EXE .EXE FILE
SOURCE CODE BYTE CODECOMPILER
MACINE CODEINTERPRETERBYTE CODE
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Portability
Three features makeJava programs portable:
1. The language. TheJava language is completely
specified; all data-type sizes and formats are definedas part ofthelanguage.
2. The library. TheJava class library is available witha
Java runtime system, becausea portable program isofno use ifyou cannot usethe same class library on
every platform.
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3. The byte code. TheJava runtime system does
not compileyour source codeirectly into
machinelanguage Instead,Java programs aretranslated into machine-independent byte
code. The byte code is easily interpreted and
therefore can beexecuted onany platform
having aJava runtime system.
Portability
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Security
Therearetwo mainlines of defense: Interpreter level:
No pointer arithmetic
Garbage collection
Array bounds checking No illegal data conversions
Browser level (applies to applets only):
No local file I/O
Sockets back to host only
No calls to nativemethods
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Robustness
TheJava language is robust. Ithas severalfeatures designed to avoid crashes
during programexecution, including:
No pointer arithmetic Garbage collection--no bad addresses
Arrayand string bounds checking
No jumping to bad method addresses
Interfaces and exceptions
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EXCEPTIONS HANDLING
Java is having a specialtype ofexceptionhandling
mechanism whichallows theexecution ofthe
programevenafter getting a runtime bug.
It is handled by using a set of classes & keywords like
try
catch
finally
throw
throws
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JAVA IS MULTITHREADED
A single Java program canhavemany
differentthreads executing independently
and continuously.
This makes Javavery responsiveto user input.
For this it uses a built in features like Thread class
Runnable interface
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BUILT IN API
Javahas a rich set of collection of built in APIs,
which facilitates the programmer withmuch
ofthe programming aspects. Like,
java.lang
java.io
java.util
java.netand so on.,
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Coding Hello World Application
class HelloWorld
{
public static void main (String args[]) {System.out.println("Hello World!"); }
}
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SAVING
We can savea javaapplication by
Taking the class name whichhas themain()
Taking the class or interface declared as public
Tip:
enclosethe filename in double quotes as Notepadadd athreeletter ".txt" extensionto allthe filesthey save withouttelling you. Thus you can
unexpectedlyend up witha file called"HelloWorld.java.txt."
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Compilation & Execution
Under Windows, it's similar. You just do this ina DOS
shell.
C:> javac HelloWorld.java
C:> java HelloWorld
Hello World
C:>
Noticethatyou usethe .javaextension whencompiling a file, butyou do not usethe .class
extension when running a file.
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JAVA BUILDING BLOCKS
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JAVA TOKENS
WHITE SPACE
COMMENTS
IDENTIFIERS
LITERALS SEPERATORS
OPERATORS
DATA TYPES
CONTROL STATEMENT
ARRAYS
CLASSES, OBJECTS, METHODS
KEYWORDS
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WHITE SPACE
Blank space introduced for proper functioning
ofa program
Java is a free formlanguage whichnever caresabout white spaces
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COMMENTS
Its anonexecutable statement provided to
geta brief ideaaboutthemodules of
applications
Types:
Singleline://comment
Multi line:/*......comment..*/
Documentation:/**comment*/
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IDENTIFIERS
Group of uppercase,lowercase characters,
numbers,and some special characters
Rules: No special characters other than _ and $
It should not start withnumbers
No space in between
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LITERALS
We can createaliteral ofa datatype by creating
the constant representation of it.
Char
a Numbers 100
Float 10.09f
Double 10.09 Boolean true, false
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SEPERATORS
{} Braces
() Parenthesis
[] Brackets ; Semi colon
, Comma
. Access operator
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OPERATORS
Arithmetic
Bitwise
Logical Relational
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DATA TYPESTYPE KEYWORD SIZE
CHARACTER char 16
NUMBERS byte
short
int
long
8
16
32
64
FLOATING Pt float
double
3264
BOOLEAN boolean 1
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CONTROL STATEMENTS
Selection
if, switch
Iteration while, do-while, for
Jump
break, continue, return