Comments, identifiers, keywords
Writing notes is a very good habit
- Single-Line Comments
- multiline comment
- Documentation Comments
//This is a single line comment /* This is a multiline comment */ /** *This is a document comment, which can be @ followed by a lot of content that can be recognized by the program *@Description HelloWorld *@Author Accelerator */
Identifiers are case sensitive
data type
-
Strongly typed language - all variables must be declared before use, unlike Python
-
Weakly typed language
One byte = 8 bits
1byte = 8 bit
The data types of Java are divided into two categories
-
Basic type
-
value type
- Integer type
- byte: 1 byte - 128 ~ 127
- short: 2 bytes - 32768 ~ 32767 30000
- int: 4 bytes up to 2 billion
- long: 8 bytes
- Floating point type
- float: 4 bytes
- double: 8 bytes
- Character type
- char
- String
- Integer type
-
boolean type: only one bit, true and false
-
-
reference type
- class
- Interface
- array
When declaring data, pay attention to whether there will be cutting in the process of declaration
float f = 10.1F; //Adding F here is to declare that this 10.1 is a float type, so it will not be cut when assigning a value to the float object long l = 10L; //Add L here to declare that this 10 is a long type float f = 0.1F; //0.1 double d = 1.0/10; //0.1 //f is not equal to d float d1 = 121212121212121212F; float d2 = d1 + 1; //d1 equals d2 //Equivalent to scientific notation, many decimal places are offset /* Escape character /t tab /n Line feed */
It's best to avoid using floating-point numbers entirely for comparison
Base system
-
Binary: 0b
-
Octal: 0
-
Hex: 0x
For example, 010 is 8 and 0x10 is 16
Type conversion
size
Low ------------------------------------------ > High
byte，short，char —> int —> long —> float —> double
Saving decimals uses more digits than saving integers
//Cast type from high to low (because you need to cut) int i = 128; byte b = (byte)i; //At this time, the value of b is - 128, and the memory overflows, because byte has only 1 byte, - 128 ~ 127 //Automatic type conversion from low to high int i = 128; double b = i; //boolean cannot be converted, char c = 'a'; int d = c + 1; System.out.println(d); //Output 98 automatic type conversion System.out.println((char)d); //Output b cast int money = 10_0000_0000; //1 billion int years = 20; //long total = money*years; Wrong! Overflow before conversion //Correct usage: long total = money * (long)years
Variable, constant
variable
- Class variable
- Instance variable
- local variable
constant
final Constant name = value; //Constant names generally use uppercase characters
Naming conventions
- Class member variables: first letter lowercase and hump principle: monthSalary except the first word, the following words are capitalized
- Local variables: initial lowercase and hump principle
- Constants: uppercase letters and underscores MAX_VALUE
- Method name: initial lowercase and hump principle
- Class name: initial capitalization and hump principle
package base;
public class Demo01 {
//Class variable static: it belongs to a class and can be output directly below
static double salary = 2500;
//Attributes: Variables
//Instance variable: it is subordinate to the object and does not initialize itself. The default value is 0.0 null
String name;
int age;
//main method
public static void main(String[] args) {
//Use class variable static: directly use
System.out.println(salary);
//Using instance variables
Demo01 MyDemo = new Demo01();
System.out.println(MyDemo.name);
//local variable
int i = 10;
}
//Other methods
public void add(){}
}
operator
- Arithmetic operators: +, -, *, /,%, + +, –
- Assignment operator:=
- Relational operators: >, <, > =, < =, = =,! =, instanceof
- Logical operators: & &, ||,!
- Bitwise operators: &, |, ^, ~, > >, <, > > >
- Conditional operator:?:
- Extended assignment operator: + =, - =, * =/=
//The difference between a + + and + + A
int a = 3;
int b = a++; //Operation before self increment
int c = ++a; //Self increment before operation
a+=b; //a=a+b
a-=b; //a=a-b
//Bit operation
/*
A = 0011 1100
B = 0000 1101
--------------------------------
A&B 0000 1100 Bitwise AND
A|B 0011 1101 Bitwise OR
A^B 0011 0001 Bitwise XOR (bitwise addition)
~B 1111 0010 Bitwise inversion
Move left:<<
Shift right: > >
---------------------------------
2*8 = 16 How to calculate quickly?
2<<3 that will do
//String connector + as long as there is a string and it is not at the end, it is converted to a string
int a = 10;
int b = 20;
System.out.println(a+b); //Output 30
System.out.println(a+b+""); //Output 30
System.out.println(a+""+b); //Output 1020
System.out.println(""+a+b); //Output 1020
//Ternary operator
x ? y : z //x If true, the result is y, otherwise z
*/
Package mechanism
The essence is similar to folders. Packages have been created in src before.
Generally, the company domain name is inverted as the package name: com yishif. www
When entering the package name, the point is used to define the hierarchy between packages
Package reference
javaDoc
Generate comment information into help document
Create a new Doc package
package com.base;
//Adding to a class is a class annotation
/**
* @author yishif
* @version 1.0
* @since 1.8
*/
public class Doc {
String name;
//The method annotation is added to the method
/**
*
* @param name
* @return
* @throws Exception
*/
public String test(String name) throws Exception{
return name;
}
}
Method 1:
Right click the package, select file path -- > show in files, then open the command line at the current location and enter
javadoc -encoding UTF-8 -charset UTF-8 Doc.java
The middle code is to display Chinese, and then enter the name of the file to be generated
In the generated file, click index Html is enough
Method 2:
Tools --> generate javaDoc
Select the file output directory, etc. in order to process Chinese characters, fill in the column of Other command line arguments
-encoding utf-8 -charset utf-8