<>JAVA Basic grammar
* notes , identifier , keyword
* data type
* Type conversion
* variable , constant
<> notes
* Usually we write code , When the amount of code is small , We can also read what we write , But when the project structure becomes complicated , We need to use annotations .
* Comments are not executed , It's for our code writers
* Writing notes is a very good habit
* We must pay attention to the standard when writing code .
<>java There are three types of annotations in ：
*
Single-Line Comments //
*
multiline comment /* */
*
Documentation Comments /** * */
<> identifier
<>* keyword
* Keywords cannot have names
<>JAVA All components need names , Class name , Variable names and method names are called identifiers
1. Class name ; 2. Method name ; 3. Variable name ;
<> Identifier considerations
* All identifiers should be alphabetic （A-Z perhaps a-z）, Dollar sign ($), Or underline （_） start ;" Case sensitive “
* The first letter can be followed by a letter （A-Z perhaps a-z）, Dollar sign ($), Or underline （_） Or any character combination of numbers ;“ Special symbols cannot be used ”
* You cannot use keywords as variable or method names .
* Examples of legal identifiers ：age, $salary, _value, __1_value
* Example of illegal identifier ：123abc, -salary, #abc
* It can be named in Chinese , However, it is generally not recommended to use it in this way , Pinyin is also not recommended , very Low
<> data type
<> Strongly typed languages
* It is required that the use of variables should strictly comply with the regulations , All variables must be fixed before they can be used ; High security , But it's slow .
<> Weakly typed language
<>Java The data types of are divided into two categories
*
Basic types （primitive type）
integer
byte 1 byte 8 position java Range 2（7）-1 reach -2（7）
short 2 byte Rarely used 2（15）-1 reach -2（15）
int 4 byte Most commonly used 2（31）-1 reach -2（31）
long 8 byte Long plastic surgery 2（63）-1 reach -2（63）
decimal
float 4 byte
double 8 byte
other
char 2 byte No sign 2（15）-1
boolean 1 byte On behalf of truth true or false false
*
reference type （reference type）
String character string
String s = " abccccd "; System.out.println(s.charAt(2));// Display page 2 Coordinate symbol System.out.
println(s.indexOf("a"));// display "a" Coordinate position System.out.println(s.length());// Display coordinate length
System.out.println(s.equals("ccdda"));// Is the comparison correct true or false System.out.println(s
.compareTo("ABCD"));// Compare size a<b, Returns a positive value , Otherwise, the opposite is true . Equal to 0. System.out.println(s.toLowerCase()
);// Change to lowercase "abcd" System.out.println(s.toUpperCase());// Capitalization "ABCD" System.out.
println(s.trim());// When spaces exist " abcd ", Remove Spaces . System.out.println(s.split("b"));
// Cut from the designated position
<> What is a byte
*
position （bit）: It's a computer internal data Minimum unit of storage ,11001100 Is an eight bit binary number .
*
byte （byte）： It's in the computer data processing The basic unit of , It's customary to capitalize B To express .
*
1B（byte, byte ） = 8bit（ position ）
*
character ： It refers to the letters used in the computer , number , Words and symbols
*
1bit express 1 position
*
1Byte Represents a byte 1B=8b
*
1024B=1KB
*
1024KB=1M
*
1024M=1G
<> Integer expansion ：
<> Base system ：
*
Binary 0b （Integer.toBinaryString） Binary code
1010 transformation 0* 2（0）+1* 2（1）+0* 2（2）+1* 2（3）=0+2+0+8===>10
*
decimal system
(1011)2 Power , transformation 1* 2(3)+0* 2（2）+1* 2（1）+1* 2（0）=8+0+2+1=11（11）10 Power
123 transformation 3* 10（0）+2* 10（1）+1* 10（2）=3+20+100
*
octal number system 0 (0-7)
067 transformation 7* 8（0）+6* 8（1）=48+7=（55）10 Power
*
hexadecimal 0x 0-9 A-F(10-16) （Integer.toHexString） Hexadecimal code
transformation 10* 16（0）+15* 16（1）+14* 16（2）+9* 16（3）=10+240+3584+36864
<> Base conversion
<>n Base conversion 10 Base system
formula ： first place n(0）+ Second place n（1）+…+ The third x position * n（x-1) Power **
Binary to decimal
Decimal to binary
decimal system Binary
1 0001
2 0010
3 0011
4 0100
5 0101
6 0110
7 0111
8 1000
9 1001
10 1010
11 1011
12 1100
13 1101
14 1110
15 1111
<>10 Base conversion 2 Base system
*
Short division ：
35：（35/2=17（ more than 1）/2=8（1）/2=4（0）/2=2（0）/2=1（0）/2=0（1） Reverse reading ）=100011
100011 1* 2（0）+1* 2（1）+0* 2（2）+1* 2（5）=1+2+32=35（ Positive ）
*
Find power ：
35：35-2（5）=3-2（1）=1-2（0）=100011（ To the power of several 1, There's no good remedy 0）
216：216-2（7）=88-2（6）=24-2（4）=8-2（3）=11011000
<> Negative conversion
java It adopts binary complement
0000 0
0001 1
0010 2
0011 3
0100 4
0101 5
0110 6
0111 7
hold 1 The beginning is considered negative
1000 -8
1001 -7
1010 -6
1011 -5
1100 -4
1101 -3
1110 -2
1111 -1
4 How many digits can a binary complement represent ? 2（4）=16
Maximum number ?2（4-1）-1 ====》7
Minimum decimal ?-2（4-1） ====》-8
Suppose that a language is written in English 8 The binary complement represents a number, then the range is ?
2（8-1）-1 reach -2（8-1）
The binary complement indicates that the data range is related to the number of bits, if the number of bits is n
So the scope is -2（n-1） reach 2（n-1）-1
java Middle integer （int） Yes, it is 32 Bit binary complement representation
java int What's the scope of this -2（31） reach 2（31）-1
summary 2： 4 A bit of a look
7 0111 Negation = = =》1000 +1 = = = 》1001
-7 1001 Negation = = = 》0110 +1 = = =》0111
negative = Negation of positive number +1
Positive number = Negative negation +1
<> Floating point expansion ：
* float; double Not accurate enough ; In practice, floating-point numbers are infinite and discrete , however float It's limited , He will automatically round the error , About close , But it's not equal to ;
* Accurate calculation tools such as banks are best used BigDecimal Mathematical tools
* It is best to avoid using floating point numbers for accurate reconciliation
* It is best to avoid using floating point numbers for accurate reconciliation
* It is best to avoid using floating point numbers for accurate reconciliation
<> Character expansion ：
The essence of all characters is still numbers
Java use Unicode Character set encoding , format ’\u0000’-’\uffff’
（Unicode It is a universal fixed length character set in the world , All characters are 16 position ,2 Bytes ）
For characters that are not convenient to input, escape characters are used
Escape character ：
as ：’\n’“ Line feed ”,‘\t’“ Space ”,‘\’,‘\b’,‘\r’,‘"’,‘\u4e2d’.
Digital coding ：‘0’：48,‘1’: 49…
English code ：‘A’：65,‘B’：66…
Small English ：‘a’ : 97,‘b’：98…
practice ：
public void zifu(String a){ char[] zi = a.toCharArray();
//(String)a Convert character array toCharArray, And assignment int mu = 0; // letter String m1 = ""; int s = 0;
// number String s1 = ""; int k = 0; // Space int f = 0; // other String f1 = ""; for (int i
= 0; i <zi.length ; i++) { int p = (int)zi[i]; // take zi Is converted to Int numerical value , Compare with numerical value if ((
65 <= p && p<=90)||(97<= p && p <= 122)) {//A-Z The letter is 65-90,a-z The letter is 97-122 mu++; m1 =
m1+zi[i]+" "; }else if (48<= p && p <= 57){ //0-9 by 48-57 s++; s1 = s1 + zi[i] +
" "; }else if (p==32){ // The space is 32 k ++; }else { f++; f1 = f1 + zi[i] +" "; } }
System.out.println(" Number of existing letters ：" + mu); System.out.println(" The existing letters are ：" + m1); System.
out.println(" Number of existing numbers ：" + s); System.out.println(" The existing number is ：" + s1); System.out.
println(" Number of existing characters ：" + f); System.out.println(" The existing characters are ：" + f1); System.out.println(
" The number of spaces that exist ：" + k); } public static void main(String[] args) { System.out.println(
" Please enter a string ："); Scanner scanner = new Scanner(System.in); String a = scanner.
nextLine(); ZiFuSuZhu ziFuSuZhu = new ZiFuSuZhu(); ziFuSuZhu.zifu(a); scanner.
close(); }
<> Type conversion ：
*
because Java It's a strongly typed language , All the time when some operations are needed , Type conversion is required .
low --------------------------------------------------------------> high
byte, short, char -> int -> long -> float -> double
（ Decimals take precedence over integers ）
*
In operation , Different types of data are converted to the same type first , And then we do the calculation .
*
Force type conversion high - - low
*
Automatic type conversion
Direct input from low to high , No cast required
*
be careful
* Boolean values cannot be converted
* Object types cannot be converted to unrelated types
* When converting high capacity to low capacity , Cast required
* There may be memory overflow during conversion , Or precision !
char turn int
Spillover problem
<> variable
*
What are the variables ： It's a variable quantity !
*
Java Is a strongly typed language , Each variable must declare its type .
*
Java Variable is the most basic storage unit in program , Its elements include variable names , Variable type and scope .
*
matters needing attention ：
* Each variable has a type , Types can be base types , It can also be a reference type .
* The variable name must be a legal identifier .
* Variable declaration is a complete statement , So every statement must end with a semicolon .
<> Variable scope
*
Class variable
In class , It can be used in the whole class , It must be added in front static;“static int a = 666;”
*
Instance variables
In class , Subordinate to object ;“int age;//0”
If it does not initialize itself , The default value for this type 0,0.0
Boolean value ： The default is false; Except for the basic types , All other default values are null;
*
local variable
In the method , Values must be declared and initialized when used ;“int cba = 666;"
<> constant
*
constant （Constant)： initialization （initialize） You can't change the value after ! A constant value .
*
Constant can be understood as a special variable , When his value is set , It is not allowed to be changed during program running .
final Constant name = value ; final double PI = 3.14;
*
Constant names generally use uppercase characters .
<> Variable naming conventions
*
All variables , method , Class name ： See the name and know the meaning
*
Class member variable ： Initial lowercase and hump principle ：monthSalary
*
Except for the first word , Capitalize the following words
*
local variable ： Initial lowercase and hump principle
*
constant ： Capitals and underscores MAX_VALUE
*
Class name ： Capitalizing and humping principles ：Man,GoodMan
*
Method name ： Initial lowercase and hump principle ：run(),runRun()
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