Imortant Revision Notes for Classes and Constructors in Java

Class Declaration--> Syntax of class declaration is as follows:

(class modifiers) class (class name)
               (extends clause) (implements clause)  // Class header

{ // Class body
    -- field declarations

     --method declarations

     --nested class declarations

     --nested interface declarations
     --constructor declarations
     --initializer blocks

}

Methods of a Class--> They are the class members and are also called operations. They define behaviour of the class. 

Syntax of their declaration is as follows:

(method modifiers) (return type) (method name) ((formal parameter list))
         (throws clause) // Method prototype

{ // Method body
    --local variable declarations

    --nested local class declarations
    --statements
}

Method Overloading--> This means several methods share same name but have different argument list.

Constructor--> It is a special method which is used to initialize state of an object when it is created using new operator. 

It has following syntax:

(accessibility modifier) (class name) ((formal parameter list))
            (throws clause) // Constructor header
  { // Constructor body
    --local variable declarations
    --nested local class declarations
    --statements
  }

However there are few constraints for a method to be constructor:
a) Their modifiers can be only accessibility modifiers.
b) They cannot have a return type
c) Their should be same as the class name.

Default Constructor--> It is the constructor with no argument. Its syntax is: classname()

Implicit Default Constructor-->If no constructor is defined in the class, then implicit default constructor is provided by java.

Its syntax is:

classname(){super();}

Important Points to Remember in Java

Method Parameters:

Types of Method Parameters in Java:

a) Formal Parameters-->They are the parameters defined in the method definition.

b) Actual Parameters--> They are the parameters defined in the method call.

c) Final parameter--> If a formal parameter is declared to be final, then its value cannot be changed during its lifetime in the method body.


Note: Widening, narrowing, type promotion rule applies to the parameter passing as well. They are equivalent to assignment conversions.


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2. Arrays: 


a) Array--> Its a collection of homogeneous data elements.

b) Array Declaration-->Syntax for array declaration is as follows:

    (datatype)[](arrayname) or (datatype) (arrayname)[]

c) Array Construction--> This means allocating space to the array in computer memory. Its syntax is:

  (arrayname)=new (datatype)[(arraysize)]

d) Value assignment--> This means assigning values to the array elements.

e) Array Initialization--> Java provides the means of declaring, constructing and explicitly initializing an array in one declaration. Its syntax is:

(datatype)[](arrayname)={(array initialization list)};







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3. Anonymous Array--> They are used to initialize the array objects which are already declared. like:
   int a[];
 a=new int[]{1,5,5,3,9,7,3}; we cannot use a={1,5,5,3,9,7,3}; to initialize an array already declared.




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Notes on Java Fundamentals

Basic Lexical elements----
--->keywords & other reserved words

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1. Access modifiers - private,public,protected
2.other modifiers---abstract,final,static,synchronized,transient, volatile
3 Name space keywords -- import,package

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byte,short,int,long-- signed two's complement integers
boolean -- 1 bit
char-- 16 bit unicode characters rather than ASCII set used in C

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Formula ------2^(n-1) to 2^(n-1)-1`

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Exceptional Floating point constants---

Float.NAN (NAN Stands for Not A Number)
Float.NEGATIVE_INFINITY
Float.POSITIVE_INFINITY
Double.NAN
Double.NEGATIVE_INFINITY
Double.POSITIVE_INFINITY

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char represented in single quotes -
'\u0000'--- followed by four hexadecimal

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Integral can be decimal, hexadecimal or octal
Hexadecimal starts with 0x
Octals start with 0

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Floating point e or E stands for exponential value
eg 7e3 =7000

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note--> double x=77; is valid declaration as doublex =77D;

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To convert boolean, characters,integers,floating point numbers to String use valueOf() method
String is not null terminated.

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Once initialized string objects are inmutable i.e cannot change their contents. so use stringBuffer if the contents have to be changed.

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String bounds checked at run time rather than compile time.

string abc ="sdefg";
is not same as
string abc =new("string");

if we use first one compiler scans current cache to check for another string object with same literal value.
If it is there it reuses same object.

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