directory
- Learning materials
- annotation
- The keyword
- identifier
- The data type
Learning materials
B station crazy god said: www.bilibili.com/video/BV12J…
annotation
public class HelloWorld {
public static void main(String[] args) {
// Single-line comment
System.out.println("Hello,World");
/* Multi-line comments */
/** Documentation */
/* * Document comments, which can be generated using Javadoc * * * */}}Copy the code
The keyword
abstract | assert | boolean | break | byte |
---|---|---|---|---|
case | catch | char | class | const |
continue | default | do | double | else |
enum | extends | final | finally | float |
for | goto | if | implements | import |
instanceof | int | interface | long | native |
new | package | private | protected | public |
return | strictfp | short | static | super |
switch | synchronized | this | throw | throws |
transient | try | void | volatile | while |
identifier
- All identifiers should start with A letter (a-z or a-z), 💲, or underscore (_)
- Class name (noun) uppercase: Users, TestDemo
- Method name (verb)/ variable name (noun) lowercase: getUsername, age
- Constants in all uppercase, multiple words connected with _ : USER_NAME
- Package name all in lowercase: com. company name. Project name module name
The data type
Java is a strongly typed language and variables need to be defined before they can be used
Graph LR Java data type –> 8 basic data types –> Numeric type –> Integer type Integer type –> byte — 1 byte range :-128 to 127, default :0 Integer type –> Short Two bytes range :-32768 to 32767, default value :0 Integer type –> int Four bytes range :-2147483648 to 2147483647, default value :0 Integer type –> Long Long integer length 8 bytes Range :-9223372036854775808 to 9223372036854775807, default value :0 Numeric type floating point type floating point type float 4 bytes, default value: 0.0 Floating point type –>double 8 bytes, default value 0.0 Numeric type –> character type char 2 bytes, default value eight basic data types –> Boolean Boolean Boolean –> 1 byte, value range :true and false, default value false Java data type –> reference data type Reference data type –> class, default value is null reference data type –> interface, default value is null reference data type –> array, default value is NULL
public class DataType {
public static void main(String[] args) {
// Eight basic data types
/ / integer
// Define an integer of type int by default
int intValue = 10;
byte byteValue = 20;
short shortValue = 30;
long longValue = 5000000000000L;
/ / octal
int intValue8 = 010;
System.out.println(intValue8);
/ / decimal
int intValue10 = 0x10;
System.out.println(intValue10);
/ / floating point number
float floatValue = 5.25 F;
// Float numbers defined by default are of type double
double doubleValue = 5.235;
// The character, which is also a number in nature, can be strong with (int)
char charValue = 'a';
System.out.println((int)charValue);
char charValueUnicode = '\u0061'; // a
System.out.println(charValueUnicode);
/ / a Boolean value
boolean trueValue = true;
boolean falseValue = false; }}Copy the code