Java Language Fundamentals

1. Language Fundamentals:-

·        Identifiers

·        Reserved words

·        Datatypes

·        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