2. Data type

Catalog

1.1 Integer variables

 1.2 Long integer variables

 1.3 Double precision floating point variables

1.4 Single precision floating point variables

1.5 Character type variable

 1.6 Byte type variables

1.7 Short integer variables

1.8 Boolean type variable

1.9 String type variables

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Variable refers to the variable amount of program running . It is equivalent to opening up a memory space to save some data . Type is the classification of variables , Different types of variables have different characteristics . What we are talking about " Variable " It's mainly about our " Memory " Such hardware devices are closely related .

1.1 Integer variables

Grammar format :

int Variable name = Initial value ;

Code example ：

int num = 5;// Define an integer variable 
System.out.println(num);

Be careful :

1. int The type of the variable is an integer

2. The variable name is the identifier of the variable . Later, we use the name to use variables

3. Java in = Indicates assignment ( It's not like math ), It means to set an initial value to a variable .

4. Initialization is optional , However, it is recommended that variables be explicitly initialized when they are created .

5. Finally, don't forget the semicolon , Otherwise, it will fail to compile .

6. // Notation . Comments serve as the explanatory part of the code , Does not participate in compilation run

7.int stay JAVA Four bytes in , The value range is -2^31———2^31-1.

lnteger yes int The wrapper class , Let's look at java The value range of integer data in

  notes :‘+' The usage here is , Examples are as follows ;

 1.2 Long integer variables

  Grammar format ：

long Variable name = Initial value ;

  Code example ：

long num = 10;//  Define a long integer variable ,  Initial value writing  10l  It's fine too ( Lowercase  L,  Not numbers 1).
System.out.println(num);

long stay JAVA Middle occupancy 8 Bytes ,Long Is its package name , Next, let's look at the value range of long integers

reflection ： Excuse me, String[] args What does this parameter do ？ Runtime command line arguments ！！！

 1.3 Double precision floating point variables

Grammar format ：

double Variable name = Initial value

Code example ：

double num = 1.0;
System.out.println(num);

Code 1 ：

int n = 1;
int m = 2;
System.out.println(n/m);

The execution result is 0; Because of java in int Divide int The value of is still int( The fractional part is discarded ）;

If you want 0.5; be ：

double a = 1.0;
double b = 2.0;
System.out.println(a / b);
//  Execution results 
0.5

Code 2 ：

double num = 1.1;
System.out.println(num * num)
    
//  Execution results 
1.2100000000000002

Java Medium double Although it is also 8 Bytes , But the memory layout of floating-point numbers is very different from that of integers , You can't just use 2 ^ n The data range is represented in the form of .

double stay JAVA Middle occupancy 8 Bytes ,Double Is its package name , Now let's check the value range

1.4 Single precision floating point variables

Grammar format ：

float Variable name = Initial value ;

Code example ：

float num = 1.0f;    //  writing  1.0F  It's fine too 
System.out.println(num);

float Type in the Java Four bytes in , The same is true IEEE 754 standard . Because the precision range of the data is small , Generally, floating point numbers are used in engineering Give priority to double, It is not recommended to use float.

1.5 Character type variable

Grammar format ：

char Variable name = Initial value ;

Code example ：

char ch = "A";

Be careful :

1. Java Use in Single quotation marks + A single letter Represents the literal value of a character .

2. A character in a computer is essentially an integer . stay C Used in language ASCII According to the character , and Java Use in Unicode According to the character . So one Two characters take two bytes , More types of characters , Including Chinese

Code example ：

 1.6 Byte type variables

Grammar format ：

byte Variable name = Initial value ;

Code example :

byte value = 0; 
System.out.println(value);

matters needing attention :

1. Byte types also represent integers . It takes only one byte , The scope is small ( -128 -> +127 )

2. Byte type and character type are not related to each other .

1.7 Short integer variables

Grammar format :

short Variable name = Initial value ;

Code example :

short value = 0; 
System.out.println(value);

matters needing attention :

1. short Occupy 2 Bytes , The data range represented is -32768 -> +32767

2. The scope of this expression is relatively small , Generally not recommended .

1.8 Boolean type variable

Grammar format :

boolean Variable name = Initial value ;

Code example :

boolean value = true; 
System.out.println(value);

Be careful :

1. boolean There are only two values for variables of type , true Said really , false Said the false .

2. Java Of boolean The type and int They can't transform each other , non-existent 1 Express true, 0 Express false In this way

3. Java The bytes occupied are not specified .

summary ：

Number of bytes	1	2	2	4	8	4	8	There's no rule
data type	byte	char	short	int	long	float	double	boolean
Packaging	Byte	Character	Short	lnteger	Long	Float	Double	Boolean

1.9 String type variables

Grammar format ：

String Variable name = " Initial value ";

Code example :

String name = "zhangsan"; 
System.out.println(name);

Be careful :

1. Java Use Double quotes + Several characters Represents the literal value of a string .

2. Different from the type above , String Not a basic type , It is Reference type ( I'll focus on it later ).

3. Some specific characters in a string that are not easy to express directly need to be escaped .

Escape character ：

A string of + operation , Represents string splicing :

You can also splice strings and integers :

  The above code description , When one + When there is a string in an expression , Is to perform string splicing behavior .