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9、 Practical class
2022-07-23 11:53:00 【Piglet vs Hengge】
Catalog
Example 1： Enumeration defining gender
2、 Use enumeration to output weekly program information
Example 2： Basic data types are not allowed to be stored in the collection
2、 Wrapper class construction method
3、 Common methods of packaging
Example 4： Common methods of packaging
Example 5： Packing and unpacking
Example 6：Math Class common methods
Example 7：Random Some common methods of class
Example 9： Membership registration （ practice 01）
3、 Common string extraction methods
Example 10： Common string extraction methods
4、 String splitting （ Method ）
Example 11： String splitting （ Use ）
practice 02： Find the number of times a particular character appears
07、StringBuffer Classes and StringBuilder class
1、 Use StringBuffer Class handles strings
2、 Use StringBuilder Class handles strings
3、String class 、StringBuffer Class and StringBuilder Class comparison
（2）StringBuffer： String variable
（3）StringBuilder： String variable
4、 frequently-used StringBuffer Class methods use
Example 14：toString() Method （ Temporarily none ）
Example 15： Get the current time
Example 16：Calendar Class common methods and character examples
Example 17 ： Processing date （ Calculation 2015 year 4 month 6 Day is the week of the year ）
（1） adopt API Inquire about SimpleDateFormat class ： Date and time pattern table
（2）SimpleDateFormat Class common methods
Example 18： Use SimpleDateFormat Class formatting time
Example 19： Processing date （ For example 17 Expand ）
01、Java API Introduce
02、 Cognitive enumeration
1、 Enumeration overview
（1） from Java SE 5.0 Start ,Java Programming languages introduce a new type ---> enumeration
（2） An enumeration is a type consisting of a fixed set of constants , Use keywords enum Definition .
（3） Define enumeration syntax format ： To be added
Example 1： Enumeration defining gender
Requirement specification ：
Define enumerations that represent gender , Two enumeration constants represent " male " and " Woman ".
Code ：
（1）Genders class
（2）Student class
（3）StudentTest class
enum Enumeration class ：
package cn.bdqn.demo09;
public enum Genders {
// Enumeration is a type consisting of a set of fixed constants
male , Woman
}
Use the defined enumeration type "Genders" Declare variables "gender"
package cn.bdqn.demo09;
public class Student {
public String name; // full name
public Genders gender; // Gender
}
Implementation class
package cn.bdqn.demo09;
public class StudentTest {
public static void main(String[] args) {
// Create objects using the parameterless construction method
Student stu = new Student();
// Assign a value to a property
stu.name = " Zhang San ";
stu.gender = Genders. male ;
System.out.println(stu.name+"--"+stu.gender);
}
}
Output results ：
2、 Use enumeration to output weekly program information
stay Java in , Enumerations are usually used to represent a finite set of values , Used to check the constraint of the input value .
To be added
03、 Packaging
1、 Overview of packaging
Wrapper classes convert basic data types into objects
Wrapper classes are not intended to replace basic data types , It is only used when basic data types need to be represented by objects .
The role of packaging ：
（1） A series of practical methods are provided
（2） Basic data type data is not allowed in the collection , When storing numbers , Use the packaging type
The table below for ： Corresponding table of package class and basic data type
| Basic data type | Packaging |
| byte | Byte |
| boolean | Boolean |
| short | Short |
| char | Character |
| int | Integer |
| long | Long |
| float | Float |
| double | Double |
There are two main uses of packaging ：
（1） The wrapper class exists as a class corresponding to the basic data type , Facilitate the operation of objects
（2） The wrapper class contains the relevant attributes of each basic data type , If the maximum 、 minimum value , And the related operation method .
Example 2： Basic data types are not allowed to be stored in the collection
package cn.bdqn.demo10;
import java.util.ArrayList;
public class Demo01 {
public static void main(String[] args) {
ArrayList al = new ArrayList();
// Basic data types are not allowed to be stored in the collection , there 100 and 45 First, it is automatically converted to the corresponding packaging type , Then the corresponding packaging type will be converted to Object type , Store in collection
al.add(100);
al.add(45);
for (Object object : al) {
System.out.println(object);
}
}
}
2、 Wrapper class construction method
All wrapper classes can take the corresponding basic data type as a parameter , To construct their instances .
Example 3： All wrapper classes can take the corresponding basic data type as a parameter , To construct their instances .
matters needing attention ：
（1）Character class ： except Character Out of class , Other wrapper classes can construct their instances with a string as a parameter
for example ：Character character2 = new Character("char"); It's wrong.
（2）Boolean Class constructor parameters are String Type , If the content of the string is true( Regardless of case ), Then Boolean Objects represent true, Otherwise, it means false
（3） about Number Object of type , The string needs to be “ String in numeric form ”, Can't pass a null value , Otherwise, the compilation fails , The runtime throws NumberFormatException abnormal .
package cn.bdqn.demo10;
public class Demo02 {
public static void main(String[] args) {
// All wrapper classes can take the corresponding basic data type as a parameter , To construct their instances
// except Character Out of class , Other wrapper classes can construct their instances with a string as a parameter ( about Number Object of type , The string needs to be “ String in numeric form ”, Can't pass a null value )
System.out.println("-------------------(1)Byte----------------------");
byte byt = 10;
//byte1 It's an object , This object stores data byt
Byte byte1 = new Byte(byt);
Byte byte2 = new Byte("123");
//Byte byte3 = new Byte("abc");// The output here will report an error ：NumberFormatException, The string needs to be “ String in numeric form ”, Can't pass a null value
System.out.println(byte1);
System.out.println(byte2);
System.out.println(Byte.MAX_VALUE); // A constant , preservation byte The maximum value of type , namely 27-1.
System.out.println(Byte.MIN_VALUE); // A constant , preservation byte The minimum value that the type can take , namely -27.
System.out.println("-------------------(2)Short----------------------");
short sho=100;
Short short1 = new Short(sho);
Short short2 = new Short("1234");
System.out.println(short1);
System.out.println(short2);
System.out.println(Short.MAX_VALUE);
System.out.println(Short.MIN_VALUE);
System.out.println("-------------------(3)Integer----------------------");
int num = 1000;
Integer integer1 = new Integer(num);
Integer integer2 = new Integer("1122");
System.out.println(integer1);
System.out.println(integer2);
System.out.println("-------------------(4)Long----------------------");
long lon = 10000L;
Long long1 = new Long(lon);
Long long2 = new Long("156");
System.out.println(long1);
System.out.println(long2);
System.out.println("-------------------(5)Float----------------------");
float flo = 100.5F;
Float float1 = new Float(flo);
Float float2 = new Float("12.5");
System.out.println(float1);
System.out.println(float2);
System.out.println("-------------------(6)Double----------------------");
double dou = 100.5;
Double double1 = new Double(dou);
Double double2 = new Double("12.55");
System.out.println(double1);
System.out.println(double2);
System.out.println("-------------------(7)Boolean----------------------");
boolean bool = true;
Boolean boolean1 = new Boolean(bool); //true
// The content in the string is true（ Case insensitive ） When , Stored in the object is true, All other cases are false
Boolean boolean3 = new Boolean("true"); //true
Boolean boolean4 = new Boolean("false");//false
Boolean boolean2 = new Boolean("xiaozhu");//false
Boolean boolean5 = new Boolean(true); //true
Boolean boolean6 = new Boolean(false); //false
System.out.println("-------------------(8)Charater----------------------");
char cha = ' good ';
Character char1 = new Character(cha);
System.out.println(char1); // good
}
}
Output results ：
3、 Common methods of packaging
（1）XXXValue(): Convert wrapper class to base type
（2）toString(): Returns the basic type data represented by the wrapper object as a string （ Basic types -> character string ）
（3）parseXXX()： Convert the string to the corresponding basic data type （Character With the exception of , And the contents of the string should be able to be converted into numbers , Otherwise, an exception will be reported ）（ character string -> Basic types ）
（5）valueOf():
1） All packaging classes have the following methods （ Basic types -> Packaging ）
public static Type valueOf(type value)
2） except Character Out of class , Other packaging classes have the following methods （ character string -> Packaging ）
public static Type valueOf(String s)
Example 4： Common methods of packaging
package cn.bdqn.demo10;
public class Demo03 {
public static void main(String[] args) {
// Common methods of packaging
System.out.println("------------------(1)XXXValue()-----------------");
//XXXValue(): The wrapper class is converted to the basic data type
Long lon1 = new Long(125L);
long num1 = lon1.longValue(); //num1=125
System.out.println("------------------(2)toString()-----------------");
// toString(): Returns the basic type data represented by the wrapper object as a string （ Basic types -> character string ）
String sex = Character.toString(' male '); // male
String str = Boolean.toString(true); //true
Integer inte = new Integer(100);
String str1 = Integer.toString(inte);//100
System.out.println("------------------(3)parseXXX()-----------------");
// parseXXX()： Convert the string to the corresponding basic data type （Character With the exception of , And the contents of the string should be able to be converted into numbers , Otherwise, an exception will be reported ）（ character string -> Basic types ）
byte num2 = Byte.parseByte("123");
System.out.println(num2);//123
Boolean result = Boolean.parseBoolean("True");
System.out.println(result);//true
System.out.println("------------------(4)valueOf()-----------------");
//valueOf(): All packaging classes have the following methods （ Basic types -> Packaging ） public static Type valueOf(type value)
byte byte2 = 10;
Byte byte3 = Byte.valueOf(byte2);
Character cha = Character.valueOf(' Woman ');
// except Character Out of class , Other packaging classes have the following methods （ character string -> Packaging ）public static Type valueOf(String s)
Byte byte4 = Byte.valueOf("123");
//Character.valueOf("a"); It's wrong.
}
}
Output results ：
4、 Packing and unpacking
（1） Packing ： The basic data type is converted to the object of the wrapper class
（2） Unpacking ： The wrapper object is converted to the primitive type
（3）JDK1.5 after , Allow mixed mathematical operations for basic data types and packaging types
Example 5： Packing and unpacking
package cn.bdqn.demo10;
public class Demo04 {
public static void main(String[] args) {
// Packing and unpacking
System.out.println("-------------------(1) Packing ---------------");
// Passed before ： Basic types --> Packaging
byte byte1 = 10;
Byte byte2 = new Byte(byte1); //（ Basic types -> Packaging ）
Byte byte3 = Byte.valueOf(byte1);//valueOf():（ Basic types -> Packaging ）
// Packing ： Directly assign the basic data type to the wrapper class object
Byte byte4 = byte1;
System.out.println(byte4);
System.out.println("-------------------(2) Unpacking ---------------");
// Passed before ： Packaging --> Basic types
Integer int1 = new Integer(20);
int int2 = int1.intValue();
// Unpacking ：： Assign a wrapper class object directly to a variable of basic data type
int int3 = int1;
System.out.println(int3);
System.out.println("-------------------(3) Allow mixed mathematical operations for basic data types and packaging types ---------------");
Integer num1 = new Integer(100);
int num2 = 1000;
int sum = num1 + num2;
System.out.println(sum);
Integer sum2 = num1+num2;
System.out.println(sum2);
}
}
Output results ：
04、Math class
java.lang.Math Class provides some methods of basic numeric operations and set operations . All methods in this class are static . This class is final class , So there are no subclasses .
Example 6：Math Class common methods
（1）Math.random(): Random access [0.0,1.0) Number between
Pick one at random [num1,num2） Integer between
（2）PI： PI
（3）abs(): Find the absolute value
（4）ceil(double a): Return to a ratio a Big distance a The nearest integer （ Rounding up ）
floor(double a)： Return to a ratio a Small distance a The nearest integer （ Rounding down ）
（5）round()： Return data according to the principle of four colors and five entries
（6）max()/min(): Maximum / minimum value
（7）pow(double a,double b): return a Of b Power result
package cn.bdqn.demo01;
public class MathDemo {
public static void main(String[] args) {
System.out.println("------------（1）--------------");
//Math.random(): Random access [0.0,1.0) Number between
double num = Math.random();
System.out.println(num);
// Pick one at random [num1,num2） Integer between
//int num = (int)(Math.random()*(num2-num1)+num1);
System.out.println("---------------(2)PI---------------");
double pi = Math.PI;
System.out.println(pi); //3.141592653589793
System.out.println("---------------(3)abs(): Find the absolute value ---------------");
//abs(): Find the absolute value
System.out.println(Math.abs(-35));//35
System.out.println("---------------(4)-1 ceil(double a): Rounding up ---------------");
//ceil(double a): Return to a ratio a Big distance a The nearest integer
System.out.println(Math.ceil(3.8)); //4.0
System.out.println(Math.ceil(-3.5)); //-3.0
System.out.println("---------------(4)-2 floor(double a)： Rounding down ---------------");
//floor(double a)： Return to a ratio a Small distance a The nearest integer
System.out.println(Math.floor(3.8));//3.0
System.out.println(Math.floor(-3.5));//-4.0
System.out.println("---------------(5)round()： Return data according to the principle of rounding ---------------");
//round()： Return data according to the principle of four colors and five entries
System.out.println(Math.round(3.5)); //4
System.out.println("---------------(6)max()/min(): Maximum / minimum value ---------------");
System.out.println(Math.max(3, 5));//5
System.out.println(Math.min(7, 9));//9
System.out.println("---------------(7)pow(double a,double b): return a Of b Power result ---------------");
//pow(double a,double b): return a Of b Power result
System.out.println(Math.pow(3, 4)); //81.0
}
}
Output results ：
05、Random class
Random Class is used to generate random numbers .
Random There are two overloaded ways to construct classes , Here's the picture ：
| Construction method | explain |
| Random() | Create a new random generator |
| Random(long seed) | Using a single long Seed creates a new random generator |
Be careful ： If you initialize two with the same seed value Random object , Then call the same method with each object , Then the random number obtained is the same
Example 7：Random Some common methods of class
(1) nextBoolean(): Randomly generate boolean values
(2) nextInt()： Randomly generate integers
(3) nextInt(int n)： Random generation [0,n) Integer between
package cn.bdqn.demo02;
import java.util.Random;
public class RandomDemo {
public static void main(String[] args) {
System.out.println("------------ Construction method ：Random()- Create a new random generator -------------");
//Random(): Create a new random generator
// establish Random Class object
Random random1 = new Random();
System.out.println(random1);// Generate an address value
System.out.println("------------(1) nextBoolean(): Randomly generate boolean values ------------");
//nextBoolean(): Returns the next pseudo-random number , The return value type is Boolean
boolean result1 = random1.nextBoolean();
System.out.println(result1);// Randomly generate boolean values ,true or false
System.out.println("------------(2) nextInt()： Randomly generate integers ------------");
//nextint(): Returns the next pseudo-random number , It is evenly distributed in the sequence of the random number generator int value .
int result2 = random1.nextInt();
System.out.println(result2);
System.out.println("------------(3) nextInt(int n)： Random generation [0,n) Integer between ------------");
//nextInt(int n): Returns a pseudo-random number , It is taken from the random number generator sequence 、 stay 0（ Include ） And the specified value n（ barring ） Evenly distributed between int value .
int result3 = random1.nextInt(8);
System.out.println(result3);
System.out.println("------------ Construction method ：Random(long seed)- Using a single long Seed creates a new random generator -----------");
//Random(long seed): Using a single long Seed creates a new random generator
// If you initialize two with the same seed value Random object , Then call the same method with each object , Then the random number obtained is the same
Random random2 = new Random(100L);
System.out.println(random2.nextInt());
Random random3 = new Random(100L);
System.out.println(random3.nextInt());
}
}
Output results ：
06、String class
1、String Class Overview
stay Java in , The string is called String Type object to handle .String Class is located java.lang In bag , By default , The package is automatically imported into all programs .
establish String The method of the object is as follows ：
（1）String s = "HelloWorld";
（2）String s = new String();
（3）String s = new String("HelloWorld");
2、String Class
（1）length(): Get string length
（2）equals(): Compare the contents of two strings （ English letters are case sensitive ）
（3）equalsIgnoreCase(): Compare the contents of two strings （ English letters are not case sensitive ）
（4）toLowerCase() Method : Convert uppercase English letters to lowercase
（5）toUpperCase() Method : Convert lowercase English letters to uppercase
（6） String connection ： Use + and concat
Example 8：String Class
package cn.bdqn.demo03;
public class StringDemo {
public static void main(String[] args) {
System.out.println("--------(1)length(): Get string length -----------");
String str = "abcdefgh";
//length(): Get string length
System.out.println(str.length()); //8
String str1 = "qwertyuiop";
if(str1.length()<6||str1.length()>18){
System.out.println(" The length of the password should be 6~18 Between , Please re-enter ");
}
System.out.println("--------(2)equals(): Compare the contents of two strings （ English letters are case sensitive ）-----------");
//equals(): Compare the contents of two strings （ English letters are case sensitive ）
String str2 = "abcdefg";
String str3 = "abcdefG";
System.out.println(str2.equals(str3)); //false
System.out.println("--------(3)equalsIgnoreCase(): Compare the contents of two strings （ English letters are not case sensitive ）-----------");
System.out.println(str2.equalsIgnoreCase(str3)); //true
System.out.println("--------(4)toLowerCase() Method : Convert uppercase English letters to lowercase -----------");
System.out.println("--------(5)toUpperCase() Method : Convert lowercase English letters to uppercase -----------");
String str4 = "ABCDqwert";
System.out.println(str4.toLowerCase()); //abcdqwert
System.out.println(str4.toUpperCase()); //ABCDQWERT
System.out.println("--------(6) String connection ： Use + and concat-----------");
String str5 = " The moon on the sea floor is the moon in the sky ";
String str6 = " Here was a sweetheart ";
System.out.println(str5+","+str6); // The moon on the sea floor is the moon in the sky , Here was a sweetheart
System.out.println(str5.concat(","+str6)); // The moon on the sea floor is the moon in the sky , Here was a sweetheart
}
}
Output results ：
Example 9： Membership registration （ practice 01）
Requirement specification ：
Realize member registration , requirement
（1） The user name length is not less than 3
（2） The password length is not less than 6
（3） The two passwords must be the same when registering
Code ：
package cn.bdqn.demo03;
import java.util.Scanner;
public class Demo02 {
public static void main(String[] args) {
/*
* Realize member registration ,
* Requirement specification :
* （1） The user name length is not less than 3
* （2） The password length is not less than 6
* （3） The two passwords must be the same when registering
*/
Scanner input = new Scanner(System.in);
System.out.println("************ Welcome to the registration system ************");
String userName; // user name
String pwd; // password
String repPwd; // Enter the password again
while (true) {
System.out.print(" Please enter a user name ：");
userName = input.next();
System.out.print(" Please input a password ：");
pwd = input.next();
System.out.print(" Please input the password again ：");
repPwd = input.next();
if (userName.length() < 3 || pwd.length() < 6) {
System.out.println(" User name length cannot be less than 3, Password length cannot be less than 6!");
continue;
} else {
if (!pwd.equals(repPwd)) {
System.out.println(" The two passwords are not the same ");
continue;
} else {
System.out.println(" Registered successfully , Please remember your user name and password .");
break;
}
}
}
}
}
3、 Common string extraction methods
| Method | explain |
| public int indexOf(int ch) | Search for the first character that appears ch（ Or a string value）, If not found , return -1 |
| public int indexOf(String value) | |
| public int lastIndexOf(int ch) | Search for the last character that appears ch（ Or a string value）, If not found , return -1 |
| public int lastIndexOf(String value) | |
| public String substring(int index) | Extract the string part starting from the location index |
| public String substring(int beginindex, int endindex) | extract beginindex and endindex The string part between , Include the character starting the index , Excluding the characters that end the index |
| public String trim() | Returns a copy of the call string without any spaces before and after |
Example 10： Common string extraction methods
package cn.bdqn.demo03;
public class StringDemo02 {
public static void main(String[] args) {
System.out.println("--------------(1)-1 indexOf() ----------------");
// public int indexOf(int ch): Search for the first character that appears ch（ Or a string value）, If not found , return -1
// public int indexOf(String value) Search for the first character that appears ch（ Or a string value）, If not found , return -1
// frequently-used ASCII The value of the code ：A:65 a:97 0:48
String str = "abcdefghijk01Amnabc";
int num = str.indexOf(98);// Search for the first occurrence ASCII The code value is 98 The characters of （ Or a string ）,ASCII Code value 98 For the b
System.out.println(num); // 1
int num1 = str.indexOf("c");// The first string to appear in the search is c, And returns the subscript value .
System.out.println(num1); // 2
int num2 = str.indexOf("de");// The first string to appear in the search is de, And returns the subscript value .
System.out.println(num2); // 3
System.out.println("--------------(1)-2 lastIndexOf()----------------");
// public int lastIndexOf(int ch): Search for the last character that appears ch（ Or a string value）, If not found , return -1
// public int lastIndexOf(String value): Search for the last character that appears ch（ Or a string value）, If not found , return -1
// Search for the last occurrence ASCII The code value is 98 The characters of （ Or a string ）,ASCII Code value 98 For the b
System.out.println(str.lastIndexOf(98)); //17
// The last string to appear in the search is m, And returns the subscript value .
System.out.println(str.lastIndexOf("m")); //14
System.out.println("--------------(2)-1 substring(int index)----------------");
//public String substring(int index): Extract the string part starting from the location index
String str1 = "abcdef1Amnabc";
String newStr = str1.substring(3);
System.out.println(newStr); //def1Amnabc
System.out.println("--------------(2)-2 substring(int beginindex, int endindex)----------------");
/* public String substring(int beginindex, int endindex):
* extract beginindex and endindex The string part between , Include the character starting the index , Excluding the characters that end the index
*/
String str2 = "abcdef1Amnabc";
String newStr1 = str2.substring(3, 6);
System.out.println(newStr1); //def
System.out.println("--------------(3) trim() ----------------");
//public String trim(): Returns a copy of the call string without any spaces before and after
String str3 = " abc qwert ";
String newStr2 = str3.trim();
System.out.println(str3); // abc qwert
System.out.println(newStr2); //abc qwert
}
}
Output results ：
4、 String splitting （ Method ）
Example 11： String splitting （ Use ）
package cn.bdqn.demo03;
public class StringDemo03 {
public static void main(String[] args) {
System.out.println("----------(1) String[] split(String regex)--------------");
// String[] split(String regex) : Split the string according to the splitting rules
String song = " Outside the long Pavilion , Beside the old road , The grass is green , The evening wind blows , The sound of willow flute is broken , The sunset is beyond the mountain ";
String[] str1 = song.split(","); // By comma “,” Split
for (String string : str1) {
System.out.println(string);
}
System.out.println("----------------------");
String love = " I love you, you don't love me, but I love you very much, but I just don't love you ";
String[] loves = love.split(" Love ");// according to " Love " Split
for (String string : loves) {
System.out.println(string);
}
System.out.println("----------(2) charAt(int index): Returns the char value --------------");
//charAt(int index): Returns the char value
String str2 = " The moon on the sea is the moon in the sky , Here was a sweetheart ";
char ch = str2.charAt(3);
System.out.println(ch); // yes
System.out.println("----------(3) endsWith(String suffix): Tests whether the string ends with the specified suffix --------------");
//boolean endsWith(String suffix): Tests whether the string ends with the specified suffix . Return value is Boolean
boolean result = str2.endsWith(" receive patrons ");
System.out.println(result);//true
System.out.println("----------(4) getBytes(): Use the platform's default character set to set this String Encoded as byte Sequence , And store the results in a new byte Array . --------------");
//getBytes(): Use the platform's default character set to set this String Encoded as byte Sequence , And store the results in a new byte Array .
String str3 = "abcdefg10086";
byte[] byte1 = str3.getBytes();
System.out.println(byte1[1]); //98: Back to str The index of this array is 1 The elements of the ASCII Code value
char result1 = (char)byte1[1];
System.out.println(result1); // Take the above obtained ASCII Code value 98 Convert to b
}
}
Output results ：
practice 02： Find the number of times a particular character appears
Requirement specification ：
Enter a string , Then enter the character you want to find , Judge the number of times the character appears in the string
07、StringBuffer Classes and StringBuilder class
1、 Use StringBuffer Class handles strings
Frequently modify the string （ Such as string concatenation ） when , Use StringBuffer Class can greatly improve the efficiency of program execution .
（1） How to use StringBuffer class ：StringBuffer Class is located java.util In bag , yes String Class enhancement class .StringBuffer Class provides many methods to use .
（2） frequently-used StringBuffer Class method ：insert() Method 、append() Method 、toString() Such method .
2、 Use StringBuilder Class handles strings
java.lang.StringBuilder yes JDK 5.0 New classes in version , It is a mutable class . This class provides a StringBuffer Compatible classes , Designed to be used as StringBuffer A simple alternative to , In most implementations , It is better than StringBuffer Execute quickly . Use StringBuilder Class handles strings in the same way StringBuffer Classes are basically the same .
3、String class 、StringBuffer Class and StringBuilder Class comparison
（1）String： String constant
String Is immutable , In every time right String Changing the type is actually equivalent to generating a new String object , Then point to the new String object , therefore It's better not to use a string that often changes its content String type , Because every time an object is generated, it has an impact on system performance .
（2）StringBuffer： String variable
StringBuffer It's a variable string , In every time right StringBuffer When the object changes , Would be right StringBuffer The object itself operates , Instead of generating new objects , Then change the object reference . therefore , stay It is recommended to use when string objects change frequently StringBuffer class .
for example ： String connection operation ,StringBuffer Class is more efficient than String Class height .
（3）StringBuilder： String variable
JDK 5.0 The version provides StringBuilder class , It and StringBuffer Class equivalence , The difference lies in StringBuffer Class is thread safe ,StringBuilder Classes are single threaded , No synchronization is provided , Theoretically more efficient .
4、 frequently-used StringBuffer Class methods use
（1）insert()： Insert the parameter into the specified position of the string and return . Parameters can include String Any type of （ for example boolean、char、double、float、int、long etc. ）
（2）append()： Connect the parameter to the string and return .
（3）toString()： take StringBuffer String of type 1 Convert to String Type and return
（ character string 1.toString）
Example 12：insert() Method
Requirement specification ：
Convert a numeric string into a comma separated numeric string , That is, every three numbers from the right are separated by commas
package cn.bdqn.demo01;
import java.util.Scanner;
public class StringBufferDemo01 {
public static void main(String[] args) {
/*
* Convert a numeric string into a comma separated numeric string ,
* That is, every three numbers from the right are separated by commas
* */
// First step ： To get the number entered by the keyboard
Scanner input = new Scanner(System.in);
System.out.print(" Please enter a string of numbers ：");
String num = input.next();
// The second step ： Use later StringBuffer Generic insert() Method , So first we have to num convert to StringBuffer
StringBuffer sb = new StringBuffer(num);
for (int i = sb.length()-3;i>0;i-=3) {
sb.insert(i, ",");
}
System.out.println(sb);
}
}
Output results ：
Example 13：append()
Use append()： Connect the parameter to the string and return .
package cn.bdqn.demo01;
public class Demo01 {
public static void main(String[] args) {
String num = "123";
// Use StringBuffer Generic append() Method , take num convert to StringBuffer
StringBuffer sb = new StringBuffer(num);
// Use append()： Connect the parameter to the string and return .
sb.append(" Hengge 520");
System.out.println(sb);
}
}
Output results ：
Example 14：toString() Method （ Temporarily none ）
toString()： take StringBuffer String of type 1 Convert to String Type and return
08、 Date operation class
1、Date class
java.util.Date class ：Date Class object is used to represent date and time , This class provides a series of methods for operating the components of date and time .Date The most used class is the class that gets time , Such as Date date = new Date(); This code uses the current time of the system to create a date object .
Example 15： Get the current time
package cn.bdqn.demo02;
import java.util.Date;
public class DateDemo01 {
public static void main(String[] args) {
// establish Date Class object
Date date = new Date();
System.out.println(date);// Display the year, month and day of the current time 、 Minutes and seconds ：Sat Jul 02 09:14:59 CST 2022
// Get the current number of years
int year = date.getYear()+1900;// Oblique shoulder indicates that this method is out of date
System.out.println(year);//2022
// Get the current number of weeks
int day = date.getDay();
switch(day){
case 0:
System.out.println(" Sunday "); // Sunday is 0, Monday is 1,...., Saturday is 6
case 6:
System.out.println(" Saturday ");
}
}
}
Output results ：
2、Calendar class
java.util.Calendar class ：Calendar Class objects are also classes used to represent dates and times , It can be seen as Date An enhanced version of class .Calendar Class provides a set of methods , Allows you to convert a time in milliseconds to years 、 month 、 Japan 、 Hours 、 branch 、 second . You can put Calendar Class as a perpetual calendar , The current time is displayed by default , Of course, you can also check other times .
（1）Calendar Class is an abstract class , You can do this in a static way getInstance() Methods to get Calendar Class object , In fact, the obtained object is an object of its subclass .
（2） Used to set and get dates / Specific parts of time data .
（3）Calendar Class provides methods and static fields to manipulate calendars , As shown in the following table ：
| Method to get properties | explain |
| int get(int field) | Return the value of the given calendar segment |
| YEAR | Indication year |
| MONTH | Indicates the month |
| DAY_OF_MONTH | Indicates a day of the month |
| HOUR | When instructed |
| MINUTE | Indicates the minutes of an hour |
| SECOND | Indicates the second in a minute |
| DAY_OF_YEAR | Today is the day of the year |
| DAY_OF_MONTH | Today is the day of the month |
| DAY_OF_WEEK | Today is the day of the week |
| WEEK_OF_YEAR | This week is the first few weeks of the year |
| WEEK_OF_MONTH | This week is the first few weeks of the month |
Example 16：Calendar Class common methods and character examples
package cn.bdqn.demo04;
import java.util.Calendar;
public class CalendarDemo01 {
public static void main(String[] args) {
// By inquiring API know Calendar Class is an abstract class , Cannot instantiate directly ,
// You can call Calendar Class getInstance() Methods to get getInstance() Methods to get Calendar Class reference
// getInstance(): Get a calendar using the default time zone and locale .
Calendar cal = Calendar.getInstance();
System.out.println(cal);// Output cal You can get a lot of data , For example, date, hour, minute, second, week, etc , But the form is not what we want , So we're going to transform
//int get(int field): Returns the value of the given calendar field , Inquire about API lookup Calendar Class field
// Year of acquisition ： Field (YEAR)
int year = cal.get(Calendar.YEAR);
System.out.println(" year ："+year);
// Get the month : Field (MONTH)
int month = cal.get(Calendar.MONTH);
System.out.println(" month ："+(month+1));// In calendar cal In the first month of 0 Express , For the second month 1 Express , And so on
// Acquisition date : Field (DAY_OF_MONTH)
int day = cal.get(Calendar.DAY_OF_MONTH);
System.out.println(" Japan ："+day);
// When getting / branch / second : Field (HOUR/MINUTE/SECOND)
int hour = cal.get(Calendar.HOUR);
int minute = cal.get(Calendar.MINUTE);
int second = cal.get(Calendar.SECOND);
System.out.println(" present time ："+hour+":"+minute+":"+second);
// Get how many days today is this year : Field (DAY_OF_YEAR)
int dayYear = cal.get(Calendar.DAY_OF_YEAR);
System.out.println(" Today is the first day of the year "+dayYear+" God ");
// Get week : Field ()
int dayWeek = cal.get(Calendar.DAY_OF_WEEK);
System.out.println(" Today is a week "+(dayWeek-1));
}
}
Output results ：
Example 17 ： Processing date （ Calculation 2015 year 4 month 6 Day is the week of the year ）
package cn.bdqn.demo04;
import java.util.Calendar;
public class CalendarDemo02 {
public static void main(String[] args) {
Calendar cal = Calendar.getInstance();
cal.set(2015, 3, 6);// Notice that the month starts from 0 Start ,4 Monthly use 3 Express
System.out.println(cal);
int woy = cal.get(Calendar.WEEK_OF_YEAR);
System.out.println(woy);
}
}
Output results ：
3、SimpleDateFormat class
java.text.SimpleDateFormat class :SimpleDateFormat Class is DateFormat Subclasses of classes ,SimpleDateFormat Class is DateFormat Subclasses that are used more in class , It's a concrete class that formats and parses dates in a locale related way , Such as "yyyy-MM-dd HH:mm:ss" Is a specified date and time format .
（1） adopt API Inquire about SimpleDateFormat class ： Date and time pattern table
（2）SimpleDateFormat Class common methods
（1）format(): Will be given Date Format as date / Time string , And add the result to the given StringBuffer.
Example 18： Use SimpleDateFormat Class formatting time
package cn.bdqn.demo03;
import java.text.SimpleDateFormat;
import java.util.Date;
public class SimpleDateFormaDemo {
public static void main(String[] args) {
/*java.text.SimpleDateFormat class :
* SimpleDateFormat It's a concrete class that formats and parses dates in a locale related way .
* It allows formatting （ date -> Text ）、 analysis （ Text -> date ） And standardization .
* */
// establish Date Class object
Date date = new Date();
System.out.println(date);// Output the above cal You can get a lot of data , For example, date, hour, minute, second, week, etc , But the form is not what we want , So we're going to transform
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss E D w");// Inquire about API You know ：y: year ---M: Month of the year ---d: Days in a month ---H: The number of hours in a day （0-23）---m: The number of minutes in an hour ---s: Number of milliseconds
String str = sdf.format(date);//format(): Will be given Date Format as date / Time string , And add the result to the given StringBuffer.
System.out.println(str);
}
}
Output results ：
Example 19： Processing date （ For example 17 Expand ）
Requirement specification
（1） Get the current time usage SimpleDateFormat With “ year - month - Japan ” Way to show
（2） Calculation 2015 year 4 month 6 Day is the week of the year
package cn.bdqn.demo04;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
public class CalendarDemo00 {
public static void main(String[] args) {
/*
* Requirement specification ：
* （1） Get the current time , Use SimpleDateFormat With “ year - month - Japan ” Way to show
* （2） Calculation 2015 year 4 month 6 Day is the week of the year
* */
//（1） Get the current time , Use SimpleDateFormat With “ year - month - Japan ” Way to show
// establish Date Class object
Date date = new Date();
SimpleDateFormat cal = new SimpleDateFormat("yyyy-MM-dd");
//format(): Will be given Date Format as date / Time string , And add the result to the given StringBuffer.
String str = cal.format(date);
System.out.println(" The current time is ："+str);
//（2） Calculation 2015 year 4 month 6 Day is the week of the year
Calendar cal1 = Calendar.getInstance();
cal1.set(2015, 3, 6);
int weekYear = cal1.get(Calendar.WEEK_OF_YEAR);
System.out.println("2015 year 4 month 6 Day is the... Of the year "+weekYear+" A few weeks ");
}
}
Output results ：
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