Use of identifiers

1. Identifier: all that can be named by themselves are called identifiers
For example: class name, variable name, method name, interface name, registration, etc.

2. Naming rules of identifiers (conventional rules):
① It consists of 26 English letters in case, 0 ~ 9_ (underlined) or $.
② Number cannot start
③ Keywords and reserved words cannot be used, but they can be included.
④ Java is strictly case sensitive and has no limit on length.
⑤ Identifiers cannot contain spaces.

3. Naming conventions in Java (common name rules):

Package name: in case of multiple words, all letters are lowercase: xxxyyyzzz z

package javastudypackage
//Java studypackage is the package name

Class name and interface name: when composed of multiple words, the first letter of all words is capitalized: xxyyyzz.

public class JavaStudyClass
//JavaStudyClass is the class name

Variable name and method name: when composed of multiple words, the first word is lowercase, and the first letter after the second word is capitalized: xxxyyyzz

public void javaStudyMethod(){
}
//javaStudyMethod is the method name
int javaStudyValue,
//javaStudyValue is the variable name

Constant name: all letters are capitalized. When there are multiple words, each word is linked with an underscore XXX_YYY_ZZZ

public static final JAVA_STUDY_CONSTANT
//JAVA_STUDY_CONSTANT is a constant name

4. When naming, you should see the meaning of the name.

5

public class Two {
    public static void main(String []args){
        int myNumber = 1001;
        System.out.println(myNumber);
        //int student number = 1002;
        //System. out. Println (student number); The identifier can also be in Chinese

    }
    
}`

Data types in Java

Classification of data types

Basic data type

integer

public class value {
        public static void main(String[] args){
        //1 integer: byte (1 byte = 8bit) \ short (2 bytes) \ int (4 bytes) \ long (8 bytes)
        //byte range: - 128 ~ 127
        byte b1 = -128;
        byte b2 =  127;
        //byte b3 = 128;  This compilation failed
        System.out.println(b1);
        System.out.println(b2);
        //① When declaring a long variable, it must end with 'l' or 'l'
        //② In general, int is used when defining integer variables.
        short s1 = 128;
        int i1 = 12135;
        long l1 = 4684865489l;//When compiling, "L" or "L" at the end will not compile.

float

public class floatdouble(){
	public static void main(String arg[]){
//2 floating point type: float (4 bytes) \ double (8 bytes)
        //① Floating point type that represents a numeric value with a decimal point
        //② float indicates that the range of values is larger than long
        double d1 = 123.3;
        System.out.println(d1);//Output 123.3
        //④ When defining a float class variable, the variable should end with "F" or "F"
        float f1 = 123.4F;
        System.out.println(f1);//Output result: 123.4
        //⑤ Usually, double is used when defining floating-point variables.
        }
}

Character type (char)

① To define a char type variable, a pair of '' (single quotation marks) is usually used, and only one character can be written inside

 char c1 = 'a';  System.out.println(c1);
 //char c2 = 'ab'
 //This compilation will not pass because there can only be one character inside
  char c6 ='\n';// \n is a newline character, which is equivalent to enter
  char c7 = '\t';// \t is equivalent to Tab and Tab, both of which occupy one character

② Unicode values can be used to represent character variables

char c8 = '\u0043';//In the Unicode table, 0043 = c
System.out.println(c8);//c

boolean

① Boolean can only have two values true \ false
② It is often used in condition judgment and loop structure.

	boolean bb1 = true;
        System.out.println(bb1);//true
        boolean isMarried = true;
        if(isMarried){
                System.out.println("？？？");
        }else{
                System.out.println("This is you!");
        }//The output result is???

Reference data type

concept

In Java, variables of reference type are very similar to C/C + + pointers. The reference type points to an object, and the variable pointing to the object is the reference variable. These variables are specified as a specific type when declared, such as Employee, purchase, etc. Once a variable is declared, the type cannot be changed.
String is also a reference data type
Write after reference data type. A little big

Use of variables

① java defines the format of variables: data type, variable name = variable value
② Description:
1. Variables must be declared before assignment
2. Variables are defined within their scope (usually a ` ` in braces {}). They are valid within their scope.
3. Two variables with the same name cannot be declared in the same scope

class value {
    public static void main(String[] args){
        int myAge = 18;//Declaration of variables
        System.out.println(myAge);
        /*Error case:
        ①System.out.println(myNumber);
         Compilation error: myNumber has not been defined before using myNumber
         ②int myNumber:
         System.out.println(myNumber);
          Compilation error: myNumber has not been assigned before using myNumber
        */
 	      int myNumber = 1220200949;
          System.out.println(myNumber);//1220200949
     }

     
}