Java variables (identifier; data type; variable classification)

Java variables

A variable name - the identifier

Identifier numbers, letters, underscores _, dollar signs $ composition, but can not start with a number.
Identifier sensitive to letter case.
Identifier no length limit.
Can not keywords as identifiers.

/ * Valid identifier: * / 
int AMOUNT =. 17 ;
 int $ = Apple. 8 ;
 int the _index =. 3 ;
 int Height = 180 ;
 / * no valid identifier: * / 
int  a float = 3.14;   // can not use the key word 
int #Index = 2;     // not use # 
int . 3A =. 9;         // can begin with a number

Two data types, variable - The basic data types, reference types

Variable data types

Third, the classification of variables - local variables, member variables, static variables

(A) local variables: the method as defined in statement blocks, at the beginning of the life cycle of the method from the statements or statements at the end of the block.

★ before using local variables must be declared and initialized.

public  class TestVarible {
     public  static  void main (String args []) { 
        { 
            int Age;     // Age is a local variable, only part of this statement block 
            System.out.printf ( "D% = Age", Age); // compiled error, age not initialized 
        } 
        Age = 18 is;        // compiler error, age not defined 
    } 
}

(B) the member variables: the methods defined within an external, class variables, objects are always associated with the life cycle.

★ automatically initializes the member variable declaration, the default initial value in the table below:

Default initial value
type of data	The initial value
int	0
char	'\u0000'
double	0.0
boolean	false

public  class TestVarible {
     int testValue;   // member variable, initial value 0 
}

★ automatically initialized when you declare a static variable, the default initial value of the member variables of the same.

public  class TestVarible {
     static  Double Score;     // static variables, score belong to the class, the initial value of 0.0 
    Boolean In Flag;            // member variable, flag belong to the object, the initial value to false 
    public  static  void main (String args []) { 
        { 
            int Age;         // local variables, age belongs to the present block of statements 
            Age =. 17 ; 
        } 
        int Fighting = 88;   // local variables, fighting method belonging 
    } 
}

Fourth, variable, constant variable - declarations, initialization

The only difference ★ often variable and variable is: constant argument can no longer be changed after initialization.

Data type variable name;            // declare variables 
final variable name data type;      // constant variable declarations 
variable name = value;                // initialize variables

★ When initializing variables, the default integer type int, float Default type double.

Fifth, the basic data types

★ low range of variables can be assigned to a variable high range, need to add the conversion when out of range!

(A) integer: byte, short, int, long

1. Basic information

type of data	used internal memory	It represents the range	definition
byte	1 byte	-128~127	-
short	2 bytes	-32768~32767	-
int	4 bytes	About 2.1 billion	-
long	8 bytes	-2⁶³~2⁶³-1	Increasing the end 'L' or 'l'

2. Representation

10 hex:-
Hex: start with 0x or 0X
Octal: beginning with 0
Binary: begin with 0b or 0B

public  class TestVarible {
     public  static  void main (String args []) {
         / * proper declaration and initialization * / 
        int A = 320000 ;
         int B = 'on' ;
         Long C = A;
         Long D = 3200000000L;   // value exceeds 2.1 billion, the need to increase the conversion: adding at the end of "L" or "L" 

        / * illegal initialization * / 
        byte E = 128;            // beyond the scope of 
        unsigned int F = 200 is;    // unsigned absence 
    } 
}

(B) Float: float, double

1. Basic information

type of data	used internal memory	It represents the range	definition
float	4 bytes	-3.403E38 ~ 3.403E38	Increasing the end 'F' or 'f'
double	8 bytes	-1.798E308 ~ 1.798E308	Increase in 'D' or 'd' end

2. Representation

10 hex:-
Scientific notation: aEb or aeb (a is a real number, b is an integer)

public  class TestVarible {
     public  static  void main (String args []) {
         / * proper declaration and initialization * / 
        a float A = 3.14F ;
         a float B = ( int ) 6.89 ;
         a float C = 0x0123;            // 0x0123 = 291, in in the range of 2.1 billion 

        / * illicit initialization * / 
        a float D = 1E4;               // compile error, 1e4 double type 
        a float E = 3.03d;             // compile error, 3.03d double type 

        / * a float type and double-type best Do not compare * / 
        a float m = 0.1f;
         Double n-= 0.1 ; 
        System.out.println (m == n-);    // output to false 
    } 
}

1. Basic information

char type 2 bytes, used in the form of Unicode encoding, which indicates a range from 0 to 65,535.

2. Representation

Character constant: 'x'
Hex: '\ uxxxx' (x in the range 0 ~ f)
Escape character : '\ x' (x have different meanings)

public  class TestVarible {
     public  static  void main (String args []) {
         / * proper declaration and initialization * / 
        char A = 'step' ;
         char B = 24179 ;
         char C = '\ u51e1' ; 
        System.out.printf ( "% C% C% C" , a, B, C); 

        / * illicit initialization * / 
        char D = '\ uabcg';   // G exceeds the range of 0 ~ f 
        int E = 'a' ;
         char F = E ;          // E type int, char type higher 
    } 
}

(D) Boolean: boolean

[Reference]:

Variable data types Photo: Speed School