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Variables and constants

2022-07-07 22:25:00 【51CTO】

       
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Variables and constants _ identifier

1、 What is a variable

A variable is a reference to a specific value .

2、 The composition of variables

Variables are composed of three elements, which are data types , Identifier and value .

The data type of a variable determines the value range of its value , size , Type, etc .Java Is a language that focuses on data types , Therefore, when declaring any variable, we should determine the data type of the variable .

The identifier of the variable is the unique identifier of the variable , We can determine the variable itself through the identifier . Generally speaking, an identifier is the name of a variable . This name is the only identifier of the variable . The identifier appears mainly to facilitate our manipulation of this variable , Flexibly carry out the subsequent operation of variables .

The value of a variable is to determine the specific value that its reference points to . stay Java Any variable that is not assigned a value in cannot be operated directly . Before being assigned , It can only be called declaring a variable .

3、 Variable operation

① Statement

       
       int 
       
       number;
      
1.

Variables and constants _ integer _02

② assignment

Assignment of any variable , Will use “=” assignment .

Variables and constants _ data type _03

③ Other related operations of variables

Such as arithmetic operation , Value change, etc .

4、 data type

Because we know that one of the three elements of variables is data type , So what are the data types ？

stay Java Data types in are divided into , Basic data type and reference type （ object / Packaging ）.

There are four basic data types ： integer , floating-point , Boolean type , Character type .

integer ： Integer is the type used to represent the value of the integer part .

According to the value range, we can divide integers into short integers （short）, integer （int）, Long integer （long）.

       
       int 
       
       number
       
       =
       
       999995;
       
       short 
       
       number2
       
       =
       
       33;
       
       long 
       
       number3
       
       =
       
       1564131365;
      
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Variables and constants _ integer _04

Floating point type ： Used to describe the type of value with decimal point .

Floating point type can be divided into single precision according to the precision of decimal point （float） Double precision （double）.

stay Java Double precision is used by default , Double precision is also used more in development . To use single precision , You have to use float Strong go .

Variables and constants _ integer _05

Character types are used to represent letters , Symbol .

Variables and constants _ data type _06

Boolean type is used to indicate right or wrong . There are only two values , One for true, One for false.

       
       boolean 
       
       aboolean
       
       =
       
       true;
       
       boolean 
       
       aboolean
       
       =
       
       false;
      
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Variables and constants _ integer _07

Reference types are not covered here for the time being , After talking about classes and objects, we will mention them there .

5、 Variable naming rule ：

Upper and lower case letters and numbers form
Composed of multiple words , First word lowercase , The rest of the words are capitalized . Such as （strNumber）.

6、 Constant

Constants are conventionality values , Such as PI,e, Four parameters of database connection, etc .

Usually constants use static final modification .

Constants are usually used by multiple methods in a class and their values do not change because they are used . This solves the problem that the value of variables will change due to their use .

Variables and constants _ data type _08

Naming rules ： Full name capitalization .

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