Tokens in language fundamentals in oracle dbms

1. Tokens: 

Any logical unit in java programming is called as Lexeme.

The collection of Lexeme come under a particular group called as "Tokens".

Lexeme and tokens

Q. Find the no of lexemes & tokens in following expression?
int a = b + c *d;

Ans: int,a,=,b,=,+,*,d,; --------> 9 lexeme
int ----------------> data type
a,b,c,d ------------> Variables
=,+,* --------------> operator
; ------------------> special symbol/ terminator

Tokens ---------------> 4 types of token.
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To prepare java application, java has provided the following tokens:
1. Identifiers
2. Literals
3. Keywords/reserved words
4. operators

1. Identifiers:

-------------------
Identifier is a name provided to the programming elements like variables, methods, classes, interfaces.....

To provide identifiers in java program, java has provided the following rules & regulations.

A. Identifiers must not be started with a number, Identifiers may start with an alphabet, _ symbol, $          symbol but subsequent symbols may be a number, an alphabet, _ symbol, $ symbol.

EX: String 9eid = "err";----------------> invalid
String eid = "err-1";---------------> valid
String emp9Id = "err891";---------------> valid
float $esal =   6932145f;---------------> valid
String _eaddr = "Hyd";---------------> valid
String emp_Name = "bali";---------------> valid
--------------

B. Identifiers are not allowing all operators & all special symbol except _ and $ symbol.
EX: String emp.Name = "bali" ----------> invalid
String emp-Name = "bali" ----------> invalid
String emp_Name = "bali" ----------> valid
String emp$Name = "bali" ----------> valid
String emp+Name = "bali" ----------> invalid
String emp@Name = "bali" ----------> invalid
String _$_emp_$_$_Name = "bali" ----------> valid
---------------------------

C. Identifiers are not allowing spaces in the middle.
EX: forName(--); ---------> Valid
for Name(--); ---------> invalid
getInputStream(-); ----> valid
get Input Stream (-); ------> Invalid
---------------------

D. Identifiers must not be duplicated within the same scope, identifiers may be duplicated in two             different scopes.
EX:
class A
{
int i = 10;
short i = 20;-------->error
boolean f = true;
void m1()
{
float f = 23.4f -----> no error
double f = 2850; -----> error
long i = 20; ---------> no error
System.out.println(i); ------> print local variable
System.out.println(this.i); -----> print class level variable
}
}