1. Language Fundamentals:-
·
Reserved
words
·
Literals
·
Arrays
·
Types
of variables
·
Var-arg
methods
·
Main
method
·
Command
line argument
·
Java
coding standards
Identifiers:
A name in java program is called
identifier, which can use for identification purpose. It can be class name, method
name, variable name or label name.
Example:-
class Student
{
public static void main(String
arg[])
{
int a=10;
}
}
In the above program we have
5 identifiers (Student, main, String, arg, a)
Rules for java identifiers:-
·
The only allowed characters in java identifiers are
A to Z, a to z, 0 to 9, $, _
By mistake if we use other character, we will get an error at compile
time
·
Identifier should not start with digit(0-9)
Example:- int name123=10; //Correct
int 123name=10; //wrong
·
Java identifiers
are case sensitive
Example:-
class Student
{
int number=10; //Correct
int Number=10; //Correct
int NUMBER=10; //Correct
int total=number+Number+NUMBER; // Correct
}
In the above program that
three variables are different
·
There is no length
limit for identifiers but it is not good programming
int a=10; //Correct
int aaaaaaaabbbbbbbbcccccccccc=10; //Correct but
not good programming
·
we can’t use a
java reserved word as an identifier. If we use java reserved keyword as an
identifier, we will get an error at compile time.
int a=10; //Correct
int while=10;
//wrong(while is reserved keyword)
“while” is java reserved
keyword.
·
All predefine
java classes or interfaces can be used as identifiers.
Example:-
class Text
{
public static void main(String arg[])
{
int String=10;
int Runnable=20;
System.out.println(String+” “+Runnable);
}
}
Output:- 10 20
In the above example program
String is class and Runnable is interface but we used as identifiers.
Examples of valid and invalid identifiers
student_name Correct
Name# wrong
student@name
number123name Correct
name$ Correct
_$name Correct
int wrong(int is a data type)
Int Correct
Integer Correct
Reserved Words:-
In java some words are reserved to represent some meaning or
functionalities, such type of words are called reserved words.
Keywords for data types (8):-
byte,
short, int, long, float, double, boolean, char
Keywords for flow control (11):-
if,
else, switch, case, default, for, while, do, break, continue, return
Keywords for modifiers (11):-
public,
private, protected, final, abstract, synchronized, native, static, strictfp
(jdk1.2 keyword), transient, volatile
Keywords for exception handling (6):-
try,
catch, finally, throw, thrown, assert(jdk1.4 keyword)
Class related keywords (6):-
class,
interface, package, import, extends, implements
Object related keywords (4):-
new,
super, this, instanceof
void return type keyword:-
In
java return type is mandatory, method should be declare with void return type
Note:- In “C” language
return type is optional and default return type is “int”.
Unused Keywords:-
goto,
const.
Usage
of goto keyword creates several problems in old languages and hence sun
engineers ban this keyword in java.
We
use “final” keyword instead of “const”.
Note:- goto and const
are unused keywords in java and we are not allowed to use these keywords in our
program. If we are trying to use then we will get compile time error.
Reserved Literals:-
True,false
--- these keywords values for Boolean type.
Null --- this keyword default value for any
object reference.
enum keyword (jdk1.5 keyword):-
If
we want to define a group of named constants then we should go for enum.
Note***:-
·
All 53 reserved
keywords in java contain only lower case of alphabets.
·
Newly added
keywords are strictfp(jdk1.2 version), assert(jdk1.4 version), enum(jdk1.5
version).
·
In java we have
“new” keyword for create object but we don’t have “delete” keyword for delete
useless object because distraction of useless object is the responsibility of
garbage collector.
Common
mistakes:-
Correct wrong
const constant
instanceof instanceOf
strictfp
strictFp
synchronized synchronize
extends extend
implements implement
import imports
