Java basic syntax
notes
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We usually write code. When the amount of code is relatively small, we can still understand what we write, but when the project structure is complex, we need to use comments.
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Comments are not executed. They are for our code writers. Writing comments is a very good habit
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There are three types of annotations in Java:
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Single-Line Comments
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multiline comment
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Documentation Comments
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1 public class HelloWorld { 2 public static void main(String[] args) { 3 //Single-Line Comments 4 //Output a Hello，World 5 6 System.out.println("Hello,World!"); 7 8 //multiline comment /* notes */ 9 10 /* 11 many 12 that 's ok 13 notes 14 Buddhism 15 */ 16 17 //JavaDoc:Documentation Comments /** */ 18 19 /** 20 * @Description HelloWorld 21 * @Author JSS 22 */ 23 } 24 }
identifier
keyword
Identifier considerations
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All identifiers should start with letters (A-Z or a-z), dollar sign ($), or underscore ()
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The first character can be followed by any combination of letters (A-Z or a-z), dollar sign ($), underscore () or numbers
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Keywords cannot be used as variable or method names.
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Identifiers are case sensitive
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Examples of legal identifiers: age, $salary_ value, 1 _value
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Examples of illegal identifiers: 123abc, - salary. #abc
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It can be named in Chinese, but it is generally not recommended to use it in this way, and Pinyin is not recommended. It is very Low
1 public class Demo01 { 2 public static void main(String[] args) { 3 4 String Glory of Kings = "Hundred star king"; 5 //String Glory of Kings = "Stubborn bronze"; 6 System.out.println(Glory of Kings); 7 8 /* 9 String Ahello = "JSS"; 10 String hello = "JSS"; 11 String $hello = "JSS"; 12 String _hello = "JSS"; 13 */ 14 //Illegal identifier 15 16 //Case sensitive 17 //String Man = "JSS"; 18 //String man = "JSS"; 19 } 20 }
data type
Strongly typed language
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It is required that the use of variables should strictly comply with the regulations, and all variables must be defined before they can be used
Weakly typed language
Java data types fall into two categories
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Primitive type
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value type
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Integer type
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byte 1B -128~127
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short 2B -32768~32767
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int 4B
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long 8B
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Floating point type
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float 4B
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double 8B
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Character type Char 2B
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boolean type 1 bit 0 or 1
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true
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false
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Reference type
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class
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Interface
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array
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What are bytes
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Bit: it is the smallest unit of data storage in the computer. 11001100 is an eight bit binary number.
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Byte: it is the basic unit of data processing in the computer. It is customarily expressed in capital B
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1B (byte) = 8bit (bit)
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Characters: letters, numbers, words and symbols used in computers
1bit represents 1 bit, 1Byte represents a byte, 1B=8b. 1024B=1KB , 1024KB= 1 M 1024M=1G.
variable
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What is the variable: it is the variable!
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Java is a strongly typed language. Every variable must declare its type.
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Java variable is the most basic storage unit in a program. Its elements include variable name, variable type and scope.
1 type varName [=value] [{, varName[=value]}] ; 2 //Data type variable name =value;You can declare multiple variables of the same type separated by commas.
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matters needing attention:
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Each variable has a type, which can be either a basic type or a reference type.
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Variable name must be a legal identifier.
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Variable declarations are - complete statements, so each declaration must end with a semicolon
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1 public class Demo03 { 2 public static void main(String[] args) { 3 //Integer extension: binary 0 b Hex 0 x Octal 0 4 5 int i = 10; 6 int i2 = 010; //Octal 0 7 int i3 = 0x10; //Hex 0 x 0~9 A~F 8 9 System.out.println(i); 10 System.out.println(i2); 11 System.out.println(i3); 12 System.out.println("=================================================="); 13 14 //==================================================== 15 //Floating point extensions? How to express banking business? money 16 //BigDecimal Mathematical tools 17 //==================================================== 18 //float The finite discrete rounding error is approximately close to but not equal to 19 //double 20 //It is best to use floating point numbers for comparison 21 float f = 0.1f; //0.1 22 double d = 1.0/10; //0.1 23 System.out.println(f==d); 24 System.out.println(f); 25 System.out.println(d); 26 27 float d1 = 21212121221212f; 28 float d2 = d1 + 1; 29 30 System.out.println(d1 == d2);//true 31 32 //==================================================== 33 //Character extension? 34 //==================================================== 35 char c1 = 'a'; 36 char c2 = 'in'; 37 38 39 System.out.println(c1); 40 System.out.println((int)c1); //Force conversion 41 System.out.println(c2); 42 System.out.println((int)c2); //Force conversion 43 44 //All characters are still numbers in nature 45 //code Unicode 2 65536 bytes table: (97) = a 65 = A)Excel 2 16 65536 46 47 //U0000 UFFFF 48 49 char c3 = '\u0061'; 50 System.out.println(c3); //a 51 52 //Escape character 53 // \t Tab 54 // \n Line feed 55 // .... 56 57 System.out.println("Hello\nWorld"); 58 59 // 60 System.out.println("============================"); 61 String sa = new String("Hello World"); 62 String sb = new String("Hello World"); 63 System.out.println(sa==sb); 64 65 String sc = "Hello World"; 66 String sd = "Hello World"; 67 System.out.println(sc==sd); 68 //Object from memory 69 70 //Boolean extension 71 boolean flag = true; 72 73 if (flag==true){ } //Novice 74 if (flag){ } //an old hand 75 76 //less is More! The code should be concise and easy to read 77 78 } 79 80 } 81