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This article mainly explains "what are the knowledge points of JAVA variables and data types". The content of the explanation in the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn what are the knowledge points of JAVA variables and data types.

Understanding of variables

1 what is a variable

The unit / container used by Java to temporarily store data.

two。 Three elements of variables

a. The data type of the variable

* * divided by numeric and non-numeric values: * * numerical values: byte short int long float double non-numeric values: char boolean String** is divided by basic and reference data types: * * basic data types: byte short int long float double char boolean reference data types: arrays, classes (String), interfaces, enumerations

b. Data type conversion

Automatic type conversion in Java

Int can convert double types directly, which is called automatic type conversion, of course, automatic type conversion needs to meet certain conditions:

The target type is compatible with the source type, such as double type compatible with int type

The target type is larger than the source type, for example, the length of the double type is 8 bytes and the int type is 4 bytes, so the data of int type can be stored directly in the variables of double type, but not vice versa.

Forced type conversion in Java

Forced type conversion is easy to cause data loss, so we need to pay more attention to it.

The syntax for casting is (data type) numeric value

Double avg1=75.8

Int avg2= (int) avg1

c. Naming and specification of variable names:

c. Variable name

It consists of numbers, letters, underscores, and dollar signs, and numbers cannot begin.

English words and pinyin combinations cannot be used, either full pinyin or whole words.

Keywords and reserved words cannot be used.

Use hump nomenclature, the first letter is lowercase, if there are more than one word, the first letter of each word is capitalized.

Hyphen writing: user_name

See the name and know the meaning

3. Variable

Declare before assigning

Int num

Num = 10

Declare and assign a value

Int num = 10

The understanding of constant

The so-called constant can be understood as a special variable, after its value is set, it is not allowed to be changed during the running of the program.

Final constant name = value

Final double PI=3.14

Constant names generally use uppercase characters.

The use of constants in programs can improve the maintainability of the code. For example, when developing a project, we need to specify the user's gender. At this time, we can define a constant SEX, assign it to "male", and call this constant directly where you need to specify the user's gender, so as to avoid program errors caused by the user's irregular assignment.

Scanner keyboard input

The console is just for the tools we use in the early learning syntax phase, or for future testing in the development phase.

Import Scanner

The import java.util.Scanner; must be placed above the class declaration and below the package declaration.

Create a Scanner object

Scanner scan = new Scanner (System.in)

Use the method for input

NextInt (); enter an integer

NextDouble (); enter decimal

Next (); can be used to enter a string

Recognition of operator assignment operator

That is, the operator used to assign values to the program. For example, boolean type, integer type, floating point type, character type, etc., can be assigned by "=". The "=" operation is from right to left, that is, the value or variable on the right is assigned to the variable on the left.

Arithmetic operator

That is, the addition (+), subtraction (-), multiplication (*), division (/), and remainder (%) operators in our mathematics. Here we operate on the three operators of addition, subtraction and multiplication.

For the division (/) operator. If two integers are divided, and if the integer is not cast of type float, the result is only the integer part, and the decimal part will be rounded off.

The residual operation is not to take the quotient, but to get the remainder. It is different from the division, except for quotient. You need to pay attention to it in programming.

The operator also includes the increment and decrement (+, -) operators

Relational operator

Therefore, the name implies, that is, to judge the relationship between two variables. The relationship here includes the following operators: greater than equal (> =), less than or equal to (), and less than (

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