Concept of identifier
* Java For various variables , The character sequences used in naming methods and classes are called identifiers
* Any place where you can name yourself is called an identifier int num1 = 99;
Naming rules for identifiers （ Must be observed ）
* from 26 English letters case ,0-9,_ or $ form
* Number cannot begin .int 5abc = 1;// error
* Keywords and reserved words cannot be used , But it can contain keywords and reserved words
* Java Strictly case sensitive in , Unlimited length .
* Identifier cannot contain spaces .int a b = 99;// error
Identifier naming conventions [ More professional ]
（1） Package name ： All letters are lowercase when composed of multiple words ：aaa.bbb.ccc // such as com.xjs.com
（2） Class name , Interface name ： Multi word composition , Capitalize all words ：XxxYyyZzz [ Large hump ] such as ： TankShotGame
（3） Variable name , Method name ： Multi word composition , First word initial lowercase , Start with the second word and capitalize each word ：xxxYyyZzz [ Small hump , abbreviation Hump method ] such as ：
tankShotGam
（4） Constant name ： All letters are capitalized . When there are multiple words, each word is underlined ：XXX_YYY_ZZZ
keyword
definition ： cover Java Language gives special meaning , A string used for special purposes （ word ）
characteristic ： All letters in the keyword are lowercase .
Reserved word
Java Reserved word ： existing Java Version not used , But later versions may be used as keywords . Avoid these reservations when naming identifiers yourself word
byValue,cast,future, generic, inner, operator, outer, rest, var , goto ,const
Base ( Basic skills of programmers )
Hexadecimal introduction
For integers , There are four representations ：
* Binary ：0,1 , full 2 enter 1. with 0b or 0B start .
* decimal system ：0-9 , full 10 enter 1.
* octal number system ：0-7 , full 8 enter 1. In numbers 0 Opening indication .
* hexadecimal ：0-9 and A(10)-F(15), full 16 enter 1. with 0x or 0X Opening indication . Here A-F Case insensitive . // Demo Quad //
public class BinaryTest { // Write a main method public static void main(String[] args) {
//n1 Binary int n1 = 0b1010; //n2 10 Base int n2 = 1010; //n3 8 Base int n3 = 01010;
//n4 16 Base int n4 = 0X10101; System.out.println("n1=" + n1);
System.out.println("n2=" + n2); System.out.println("n3=" + n3);
System.out.println("n4=" + n4); System.out.println(0x23A); } }
Binary conversion ( basic skill )
Example of binary to decimal conversion ：
Octal to decimal example ：
Hexadecimal to decimal ：
rule ： From lowest order ( right ) start , Extract the number on each bit , multiply 16 of ( digit -1) Power , Then sum .
case ： Please 0x23A Number converted to decimal 0x23A = 10 * 16^0 + 3 * 16 ^ 1 + 2 * 16^2 = 10 + 48 + 512
= 570
Convert decimal to binary ：
rule ： Divide the number by 2, Until Shang Wei 0 until , Then reverse the remainder from each step , Is the corresponding binary . case ： Please 34 Convert to binary = 0B00100010
Convert decimal to octal ：
rule ： Divide the number by 8, Until Shang Wei 0 until , Then reverse the remainder from each step , Is the corresponding octal . case ： Please 131 Convert to octal => 0203
Convert decimal to hexadecimal ：
rule ： Divide the number by 16, Until Shang Wei 0 until , Then reverse the remainder from each step , Is the corresponding hexadecimal .
case ： Please 237 Convert to hex => 0xED
Binary to octal ：
rule ： Start low , Group binary numbers by three bits , Convert to the corresponding octal number .
case ： Please ob11010101 Convert to octal ob11(3)010(2)101(5) => 0325
Binary to hexadecimal ：
rule ： Start low , Group binary numbers every four bits , Convert to the corresponding hexadecimal number .
case ： Please ob11010101 Convert to hex ob1101(D)0101(5) = 0xD5
Octal to binary ：
rule ： Convert octal numbers every 1 position , Convert to the corresponding one 3 Binary number of bits .
case ： Please 0237 Convert to binary 02(010)3(011)7(111) = 0b10011111
Hex to binary ：
rule ： Convert hexadecimal numbers every 1 position , Convert to corresponding 4 A binary number of bits .
case ： Please 0x23B Convert to binary 0x2(0010)3(0011)B(1011) = 0b001000111011
Description of binary in operation
（1） Binary is every 2 Carry system of carry ,0,1 Is a basic operator .
（2） Modern computer technology is all binary , Because it only uses 0,1 Two numbers match , Very simple and convenient , Easy to implement electronically . Information processed inside the computer , Are represented by binary numbers . Binary （binary） Numerical use 0 and 1 Two numbers and their combinations represent any number . Carry rule is “ Meet 2 enter 1”,
number 1 Represent different values on different bits , In right to left order , This value is multiplied by two .
Original code , Inverse code , Complement ( a key difficulty )
Bitwise Operators
java There are 7 Bit operation (&,|, ^ ,~,>>,<< and >>>）
also 3 Bitwise operators >>,>> , Operation rules :
（1） Arithmetic shift right >>： Low overflow , Sign bit invariant , And fill the overflow high bit with the sign bit
（2） Arithmetic shift left <<: Sign bit invariant , Low complement 0
（3）>>> Logical shift right is also called unsigned shift right , The operation rule is : Low overflow , High complement 0
（4） Special notes ： No, <<< Symbol
Operation in this chapter
Technology