Fundamentals of java syntax

elements of grammar

Note: it mainly records the features of Java but not C + +

1. Notes

Single line and multiple lines are no different from C + +

Document notes:

/**
*@(Note Title 1) specific content
*@(Note Title 2) specific content
*/

2. Keywords

java can name identifiers in Chinese

3. Data type

Like C + +, java is also a strongly typed language

3.1 basic types

Integer type	meaning
byte	Occupy 1 byte: - 128 ~ 127
short	Occupy 2 bytes: - 32768 ~ 32767
int	Occupy 4 bytes: - 2147483648 ~ 2147483647
long	Accounting for 8 bytes: - 9223372036854775808 ~ 9223372036854775807

No long long type

int n1=10;
long n2=10L;    //Add an L to calibrate that 10 is a long type, otherwise an error will be reported

int i1=10;   //decimal system
int i2=010;  //Octal 0
int i3=0x10; //Hex 0x

int x=10_00_00    //Underline is not output when printing x. This method is mainly to see a big number more clearly

Floating point type	meaning
float	4 bytes
double	8 bytes

long f1=1.1F;     //Add an F to calibrate that 1.1 is a float type, otherwise an error will be reported
double f2=1.1;

Character type	meaning
char	2 bytes, Unicode encoding (U0000~UFFFF)

Boolean type	meaning
boolean (not bool)	Take one place. There are only two values: true and false

3.2 reference type

Class, interface, array

4. Type conversion

(type)Variable name     //Cast high -- > low

long a=1000000000;
int b=10;
long x=a*b;   //x will output 10000000000, and B will automatically convert upward during calculation
//If int a is declared, a*b will not generate type conversion, the calculation result will overflow, and x will obtain the overflow value

Note: 1. boolean cannot be converted.

2. You cannot convert an object type to an unrelated type.

3. When converting a high-capacity type (such as long) to a low-capacity type (such as int), perform a cast. Conversely, automatic conversion is performed.

4. There may be memory overflow or accuracy problems during conversion.

5. Variables

If the instance variable is not initialized, the general default values are in the form of 0, 0.0, null and false

Class variable: decorated with static

public class Demo{
    static double salary=2500;
    double age=25;
    
    public static void main(String[] args){
        Demo demo=new Demo();
        System.out.print(demo.age); //Instance variables must be called through objects
        System.out.print(salary);   //Class variables are not called through objects
    }
}

constant

java constants are decorated with final

Variable naming convention

All variables, methods and class names: see the meaning of the name
Class member variables: lowercase first letter and hump principle: monthSalary except the first word, the first letter of the following word shall be capitalized monthCommonBonus
Local variables: initial lowercase and hump principle
Constants: uppercase letters and underscores: MAX_VALUE
Class name: initial capitalization and hump principle: Man, LinkList
Method name: initial lowercase and hump principle

6. Operator

java has one more instanceof, which is the same as C + +

Instance is used to test whether the object on its left is the instance of the class on its right, and return the boolean data type.

boolean result = object instanceof class

public static void main(String[] args) {
   Object testObject = new ArrayList();
      displayObjectClass(testObject);
   }
   public static void displayObjectClass(Object o) {
      if (o instanceof Vector)
      System.out.println("Object is java.util.Vector Class");
      else if (o instanceof ArrayList)
      System.out.println("Object is java.util.ArrayList Class");
      else
      System.out.println("Object is " + o.getClass() + " Class");
   }
}
//The output is "the object is an instance of the java.util.ArrayList class"

If the object is an object of a class (or direct subclass, indirect subclass, or the same inheritance tree), instanceof will return true.

null is false when compared with any type with instanceof.

//A special case
int a=10,b=10;
System.out.print(""+a+b);   //Output 20
System.out.print(a+b+"");   //Output 1010
//From left to right, judge whether the "+" sign is a string connection or an arithmetic addition

More mathematical operations can call the Math class

7. Package mechanism

package

Similar to C + + namespace. It can also be regarded as a folder. You can avoid naming conflicts

package  pkg1[.pkg2[.pkg3...]]

Class A is placed in the directory Mitchell.tools, so when writing class A, you must add a sentence at the beginning

package Mitchell.tools;

To edit the HelloWorld class, you must add a sentence at the beginning

package Mitchell;

import package

The import statement must be placed under the package statement

import package1[.package2].(classname | *);    //*Represents all classes under related packages

8.JavaDoc

Purpose: generate your own API documents

basic content

Available parameters	meaning
@author	Author name
@version	Version number
@since	Indicates the earliest jdk version required
@param	Parameter name
@return	Return value
@throws	Exception thrown