Java variables and basic data types

1.Java variables

1.1 Concepts of variables

• A storage area in memory;
• The region has its own name (variable name) and type (data type);
• The data of the region can be changed continuously within the same type;

1.1.1 Why you need to define variables

• Used for constant storage of the same type of constant and can be reused;

1.1.2 Defining the format of a variable

• Variable name of data type = initialization value;

1.1.3 Variable Use range

• The scope of the variable (valid between a pair of {}, "nearest principle");

1.2 Use of variables

1) Disclaimer

int A; Declares an integer variable named A;

int b,c,d; Represents a declaration of three integer variables, named b,c,d;

2) naming

• Can only contain numbers, captions, _ (underscore), and $ (dollar sign), and cannot start with a number;
• Strictly case-sensitive
• Cannot use keywords
• Can be named in Chinese, but not recommended
• Naming rules: suggested "Hump naming", "See the name", such as defining a variable to indicate student score: Studentscore

3) Initialization

Simultaneous initialization of declarations:

Declare and Initialize first:

int a;a = 250;

4) Use

• The operation of a variable is to manipulate the number it has stored.

int a = 5; Declare the variable A and assign a value of 5;int B = a+10; Declare variable B, take out the value of a of 5, plus 10, and assign to BSystem.out.println (b); The value of output B is 15a = A+b;  Remove the value of a of 5, then remove the value of B 15, and then the two number and re-assigned to ASYSTEM.OUT.PRINTLN (a); The value of output A is 20

• The operation of the variable must match the data type

int a = 3.14;//error: Type mismatch, the type Mismatch:cannot convert from double to int

• Variables must be declared and initialized before they are used

System.out.println (m);  Compile error, m not declared int m; System.out.println (m);  Compilation error, m not initialized

　　

Java variables and basic data types