2- basic data types

Basic data types 
integer types: byte , Short , int , Long 
Float type: a float , Double 
character type: char 
boolean: boolean 

computer memory storage unit is the smallest byte ( byte ) i.e. 8-bit binary
 byte         --->     . 1 
Short / char     --->      2 
int / a float         --->     . 4 
Long / Double     --->     . 8 


integers 
in the range: 
byte : - 128 ~ 127 
Short : - 32768~ 32767 
int : - 2147483648 ~ 2147483647 
Long : - 9223372036854775808 ~ 9223372036854775807 


floating-point 
number is a decimal floating-point type, since when expressed in scientific notation of decimal, the decimal point can be "floating". 
As can be represented 1234.5 12.345x10², it can be expressed as 1.2345x10³, so called float. 
For float type, we need to add the suffix f. 
The float type can represent the largest 3.4x10³⁸, and double type can represent the largest 1.79x10³⁰⁸ 

Boolean 
boolean type boolean only true and false two values. 
B1 Boolean = to true ; 
Boolean B2 = to false ; 
Boolean isGreater = . 5 > . 3 ; //Evaluates to true 
int Age = 12 ; 
boolean isAdult = Age> = 18 ; // evaluates to false 

the Java stored on boolean and regulations do not, because theoretically store boolean needs only 1 bit, but will generally JVM internal boolean It represents an integer of 4 bytes. 

Character type 
character type char represents a character. Java char type addition may represent a standard ASCII, it may also represent a Unicode character: 
public  class the Main {
     public  static  void main (String [] args) {
         char A = ' A ' ;
         char ZH = ' in ' ; 
        the System. OUT .println (A); 
        the System. OUT.println (ZH); 
    } 
} 


! Note: char type uses single quotes   '   , and only one character, and double quotation marks to "string type to distinguish. 

Constants 
defined variable, if coupled with the final modifier, this variable becomes a constant: 
final Double = PI 3.14 ; // PI is a constant 

constants can not be reassigned after initialization when defining assignment again causes a compilation error. 
according to custom, the constant name is usually all uppercase. 

var keyword 
in some cases, the type name is too long, write more trouble for example:. 
the StringBuilder STRB = new new the StringBuilder (); 

this time, if you want to omit the variable type, the keyword may be used var 
var STRB = new new the StringBuilder (); 

compiler will automatically recognize STRB as   new new the StringBuilder ().

 * scope variables
In Java, a multi-line statements enclosed by {}, constituting statement block, the compiler may identify the beginning and end of a block of statements. And the variable defined in the statement block, which has a scope that is defined from the beginning to the end of the block. Beyond the scope of these variables are referenced, the compiler will complain. And the definition of variables, to be followed to minimize the scope of the principle, try to variables defined in the scope as small as possible, and do not reuse variable names. 
{ 
    ... 
    int i = 0 ; // variable i defined starting here 
    ... 
    { 
        ... 
        int x = . 1 ; // variable x here defined starting 
        ... 
        { 
            ... 
            String S = " Hello " ; // variable s from here defines 
            ... 
        } // variables s scope to this end 
        ...
         @Note that this is a new variable s, and above it the same name as a variable,
         // but because of different scopes, they are two different variables: 
        String s = " Hi " ; 
        ... 
    } // variables x and s this concludes the scope 
    ... 
} // variable i scope of this junction
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