Java Note------(conversions for variables, base data types, data types)

A. Variable : The variable is the amount that the value is allowed to change while the program is running. (Of the data, the pronoun, the number of its existence)
1) Statement:
int A;//declare an integer variable named a
int b,c,d;//Declare three integer variables named B,c,d
2) Name:
2.1) can only contain letters, numbers, _, $ characters,and cannot start with a number
2.2) Strictly case-sensitive
2.3) cannot use keywords
2.4) can be named in Chinese, but not recommended
2.5) Suggestions: See the name of the idea, the hump named Law
3) Initialization: First time assignment
3.1) declaring simultaneous initialization
NT A = +;
3.2) First declaration after initialization
int A;
a = +;
4) Use:
4.1) The use of variables must match the data type
int a = 25.678;//Compilation Error
4.2) variables must be declared and initialized before they are used
System.out.println (a);//compilation error, a not declared
int A;
System.out.println (a);//Compilation error, a uninitialized
int a = +;
System.out.println (a);//Correct
4.3) The use of the variable is the use of the number it has stored
int a = 5;
A = a + 10;//Take out a value of 5, plus 10, assign to a
//On the basis of a itself

Two. Basic data types

1) int: integer, 4 byte,-21 million to 21 multi billion
1.1) Integer Direct quantity default int type, cannot exceed range
int a = 250;//250 default int type
int b = 10000000000;//compilation error, 10 billion default int type, but Hyper range
1.2) Divide two integers, the result is an integer, the decimal place is unconditionally discarded
System.out.println (5/2);//2
System.out.println (2/5);//0
1.3) When an integer operation is over-range, an overflow occurs, which needs to be avoided
2) Long: Long Integer, 8 bytes
2.1) long direct amount after the number plus L or l
long a = 250L;
long B = 10000000000;//compilation error, 10 billion int type
long C = 10000000000L;
2.2) Large number of operations may overflow, it is recommended to add L after the first number
2.3) Gets the number of milliseconds since 1970.1.1 0 o'clock to this moment
Long a = System.currenttimemills ();
3) Double: floating-point, 8-byte
3.1) Floating-point direct quantity defaults to double type
Double A = 25.6378;//25.6378 default double type
3.2) Rounding error, can not be used in precise operation situations
4) Char: Character type, 2 bytes
4.1) Unicode encoding format, each character has a corresponding code
Char is a representation, essentially an int
4.2) Have and have only one character
4.3) The direct amount of the character must be placed in a pair of single quotation marks
4.4) For characters that are not easy to output, you need \ to escape
5) Boolean: Boolean, 1-byte, only true and false

Attachments: Basic Data types

Three. Conversion of data types:
1) divided into two types: from small to large in turn byte,short,int,long,float,double
Char
1.1) automatic type conversion (implicit type transfer): From small type to large type
1.2) Forced type conversion: from large type to small type
(The data type to convert to) variable
There may be a loss of overflow or precision
2) Two-point rule:
2.1) Integer direct amount can be directly assigned to Byte,short,char, but not over range
2.2) When the BYTE,SHORT,CHAR variable participates in the operation, it is converted to int again.

Java Note------(conversions for variables, base data types, data types)