Day-14 common APIs

1.String
1.1 summary
java.lang.String： It's a string class , The bottom floor is a final Embellished char Array , therefore String Many features are the properties of arrays
such as Once the length is determined , Can't change
① Once the string is created , This string object cannot be changed
② To improve string access and storage efficiency ,java The virtual machine uses a caching mechanism , Save all strings in the string constant pool
③ During program execution , If you want to use a string a, String s1=‘a’; First, search the string constant pool , If not, create one If there is String s2=‘a’; No more creation , Take the existing one a return
So lead to String s1=‘a’ String s2=‘a’ here Use s1==s2 He is also true, Because they point to the same string object , Namely a
1.2 Basic use

1.3 Do not splice frequently
Because the string cannot be changed after it is created , If you splice frequently , Efficiency is very low , And garbage collection may also have problems
1.4 Construction method
① Literal String s1=“djalskdjl”;
②String s2= new String(“xxx”);
③ Byte array
byte[] bytes={97,98,99,100,101,102};
String s3=new String(bytes);
System.out.println(s3) //abcdef
④ Byte array , Just cut it off ,4 Indicates the starting position （ contain ）,2 Denotes number
String s4 = new String(bytes,4,2);
System.out.println(s4);
⑤ A character array
char[] chars={‘a’,‘b’,‘c’};
String s5= new String(chars);
System.out.println(s5);
⑥ A character array , Just cut it off ,1 Indicates the starting subscript ,2 Denotes number
String s6=new String(chars,1,2);
System.out.println(s6);
1.5 Common methods
Study API 1. What is the function 2. What are input and output parameters 3. How to use it?

//1 char charAt(int index): Returns... In the string , The character in the specified position 
	  String s1="qwehihasd";
	  char c1=s1.charAt(2);
	  //e
	  System.out.println(c1);
	//boolean endsWith(String suffix): Determines whether the string ends with the specified string 
	    //boolean startsWith(String prefix): ditto , Judgment begins 
	 System.out.println("Hello.java".endsWith(".java"));
	 // Note blank space , If there is a space, it won't match 
	 System.out.println("Hello.java".endsWith(".java"));
	 //3 boolean equalslgnoreCase(String str): Case insensitive comparison of two strings for equality 
	   System.out.println("abc".equalsIgnoreCase("aBc"));
	 //4 byte[] getBytes(): Converts a string to a byte array and returns 
	   byte[] bytes="abc".getBytes();
	   for(int i=0;i<bytes.length;i++){
    
		   System.out.println(bytes[i]);
	   }
	   //6 int indexOf(String str): Gets the starting index of the specified string in the string , No return found -1
	      System.out.println("dahdadladl".indexOf("da"));//0
	      System.out.println("ashdaljdl".indexOf("asd"));//-1
	    //7 int indexOf(String str,int index):
	      // Search from the specified location （ contain ）, Gets the starting index of the specified string in the string , No return found -1
	      System.out.println("asyhdklaisudo".indexOf("m", 5));//-1
	    //8 index lastIndexOf(String str):  ditto , The last index to appear   No return found -1
	      System.out.println("sahdalid".lastIndexOf("a", 1));//1
	    //9 int length(): Returns the length of the string 
	      System.out.println("dahdkah".length());
	    //10 String replaceAll(String regex.String replacement);
	      // Replace specified characters , regular expression 
	      //String replace(String str.String replacement); Regular expressions are not supported 
	      // use 1 hold a Replaced and returned a new string 
	      // regular expression , But there is no regular expression , There is no difference 
	      System.out.println("hucgasdqweasd".replaceAll("a", "1"));
	      System.out.println("hucgasdqweasd".replace("a", "1"));
			//  because  .  In regular expressions , Arbitrary character 
			System.out.println("qwe.rty.yui.uio".replaceAll(".", ","));
			//  have access to  \  escape 
			System.out.println("qwe.rty.yui.uio".replaceAll("\\.", ","));
			System.out.println("qwe.rty.yui.uio".replace(".", ","));

			// 11 String[] split(String regex) :  Split string , Return string array , regular expression , Be careful  .....
			//  spot , Need to escape 
			String s2 = "2022.1.14";
			String[] arr = s2.split("\\.");
			for (int i = 0; i < arr.length; i++) {
    
				System.out.println(arr[i]);
			}

			// 12 String substring(int begin); In the string , Substring starting with a subscript ( contain )
			System.out.println("abcdef".substring(2));// cdef

			// 13 String substring(int begin, int end) :
			//  In the string , Start with a subscript ( contain ) Substring ending at a subscript ( It doesn't contain )
			System.out.println("abcdef".substring(2, 4));// cd

			// 14 String trim() :  Remove the spaces on both sides of the string 
			System.out
					.println(" a da sadasd adafga ");
			System.out
					.println(" a dada d dafafg sadasd "
							.trim());

			// 15 String toUpperCase() :  Turn capitalization 
			// String toLowerCase() :  Turn lowercase 
			System.out.println("asd".toUpperCase());

			// 16 static String valueOf(Object obj) :
			//  Calling the toString Method , If null, No more calls toString Instead, it returns a string null
			String_05 s = null;
			//  When printing a reference type , Automatically called String Of valueOf  therefore   Automatically called toString Method 
			System.out.println(s); Insert a code chip here 

2 StringBuffer and StringBuilder
2.1 summary

StringBuffer and StringBuilder :  Are all string buffers , It can be used for splicing \
 * 
 * StringBuffer and StringBuilder and String difference 
 * 
 * 	String :  The bottom is char Array , Fixed length , Once created , Non modifiable , Not suitable for string splicing 
 * 
 * StringBuffer and StringBuilder :  The bottom is char Array , Lengthening , Apply for a piece of space in memory in advance , Used to save many characters 
 * 		 If the reserved space is insufficient , Automatic capacity expansion 
 * 		 Default capacity is  16, Expand capacity   ( Current capacity +1)*2   16->34->70
 * 
 * StringBuffer and StringBuilder  The difference between  
 * 		StringBuffer  Thread safety , In multithreaded environment , No problem 
 * 		StringBuilder  Non-thread safety , In multithreaded environment , There may be problems 

2.2 Basic use

//  Create an object 
		StringBuilder sb = new  StringBuilder();
		String[] arr = {
    "a","b","c"};
		for (int i = 0; i < arr.length; i++) {
    
			// append  Is to add data , Do the splicing operation 
			//  You can chain call 
			sb.append(arr[i]).append(",");
		}

2.3 Common methods 

//  reverse 
		sb.reverse();
		//  Get the number of existing elements 
		System.out.println(sb.length());
		//  Current capacity ( The length of the array )
		System.out.println(sb.capacity());
		// toString You can put StringBuilder Convert to String type 
		String str = sb.toString();
		
		System.out.println(str);

3. Packaging
3.1 summary
【 ask 】 Want to do more with basic type data , What do I do ？
【 answer 】 The most convenient way is to encapsulate it as an object . Because more properties and behaviors can be defined in the object description to operate on the basic data type . We don't need to encapsulate basic types by ourselves ,JDK It has been encapsulated for us .
【 Concept 】

1. Boxing is the automatic conversion of basic data types to wrapper types
2. Unpacking is automatically converting the wrapper type to the basic data type
3. 
   3.2 Use

public static void main(String[] args) {
    
		//  Basic types 
		byte b1 = 12;
		//  Encapsulate as wrapper class 
		Byte b2 = new Byte(b1);
		m1(b2);
		long l1 = 123L;
		Long l2 = new Long(l1);
	}

3.3Integer
3.3.1 Basic use

public static void main(String[] args) {
    
		//  Maximum 
		System.out.println(Integer.MAX_VALUE);
		System.out.println(Integer.MIN_VALUE);
		System.out.println(Long.MAX_VALUE);
		System.out.println(Double.MAX_VALUE);
		
		//  Create objects 
		Integer i1 = new Integer(123);
		//  Passing strings can also , But it must be pure numbers 
		Integer i2 = new Integer("1231");
		//  Report errors  java.lang.NumberFormatException: For input string: "1231a"
		Integer i3 = new Integer("1231a");
	}

3.3.2 Reciprocal transformation

// int ---> Integer
		Integer i1 = new Integer(123);
		i1 = Integer.valueOf(123);
		
		// Integer --> int
		int i2 = i1.intValue();
		
		// String --> integer
		Integer i3 = new Integer("123");
		i3 = Integer.valueOf("123");
		
		// Integer --> String 
		String string = i3.toString();
		
		// int --> String
		String string1 = 123+"";
		
		// String --> int 
		//  important  static int parseInt(String str) :  Convert a pure numeric string to int type 
		int i4 = Integer.parseInt("123");
		double d1 = Double.parseDouble("2.4");
		
		//  Put numbers in binary string form , And back to 
		String s1 = Integer.toBinaryString(10);
		//  ditto , octal 
		s1 = Integer.toOctalString(10);
		//  ditto , Hexadecimal 
		s1 = Integer.toHexString(10);
		System.out.println(s1);

3.3.3 Common methods

3.3.4 Automatic packing and unpacking

java1.5 New characteristics   Automatic packing and unpacking 
 * 		 Packing  :  Basic types   To   Packaging type  
 * 		 Unpacking  :  Packaging type   To   Basic types 
 * 
 *  Eight kinds of packaging   It's all overwritten  toString  and  equals() Method 

	// 1.5  That's what I wrote before 
		Integer i1 = Integer.valueOf(123);
		int i2 = i1.intValue();
		
		// 1.5  Start 
		Integer i3 = 123; //  After the compilation   Is equal to  Integer i3 = Integer.valueOf(123);
		int i4 = i3; //  After the compilation   Is equal to  int i4 = i3.intValue(); 
		m1(i3);
		// 1234 by int type , The automatic packing will be carried out first  Integer type , Then polymorphic transformation occurs to Object type 
		m1(1234);
	}

3.3.5 Deep into the integer constant pool

public static void main(String[] args) {
    
		int i = -128;
		System.out.println(-i);
		Integer i1 = new Integer(10);
		Integer i2 = new Integer(10);
		// false
		System.out.println(i1 == i2);
		// true
		System.out.println(i1.equals(i2));
		Integer i3 = Integer.valueOf(-126);
		Integer i4 = Integer.valueOf(-126);
		// true
		System.out.println(i3 == i4);

		Integer i5 = 12; //  After the compilation  Integer.valueOf(12);
		Integer i6 = 12;
		// true
		System.out.println(i5 == i6);

		Integer i7 = 222; //  Is equal to  new Integer(222)
		Integer i8 = 222;
		// false ,  Because the range of integer constant pool is  -128 ~ 127 Between 
		System.out.println(i7 == i8);
	}

3.3.6 summary
summary :

•   When we   adopt  new Integer(xx)  When creating objects ,  No matter how much it's worth ,. ==  Forever false
  

•   When we use  Integer i1 = xxx;  In this way , After the compilation   Will be converted to Integer.valueOf() ;  This method will pass through the integer constant pool 
  

•   If   xxx This value    stay  -128  To 127 Between   be   Unwanted new object , Instead, use the objects already created in the constant pool 
  

•   otherwise   No longer in scope ,  Is still equal to  new Integer(xxx)  This way, 
  

•   therefore  
  

•  	Integer i1 = xxx;
  

•  	Integer i2 = xxx;   If  xxx  stay -128 ~ 127  Between   be  i1 == i2  yes  true
  

•   however   Be careful  Integer Comparison between types   Still need to use  equals  Comparison is the best 
  

4 System
4.1 summary
System Class provides the public static long currentTimeMillis() Used to return the current time Between and 1970 year 1 month 1 Japan 0 when 0 branch 0 The time difference in milliseconds between seconds .
This method is suitable for calculating time difference .
System Class represents system , Many system level properties and control methods are placed inside the class . This class is located in java.lang package .
Because the constructor of this class is private Of , So we can't create objects of this class , That is, it can't be realized Instantiate this class . Its internal member variables and member methods are static Of , So it can also be very convenient To call .
Member variables
System Class contains in、out and err Three member variables , They represent the standard input stream
( Keyboard entry ), Standard output stream ( Monitor ) And standard error output stream ( Monitor ).
Member method
native long currentTimeMillis()：
The function of this method is to return the current computer time , Time is expressed in the current computer
Between and GMT Time ( Greenwich mean time )1970 year 1 month 1 Number 0 when 0 branch 0 The number of milliseconds in seconds .
void exit(int status)：
The function of this method is to exit the program . among status The value of is 0 It means normal exit , Non zero means
Abnormal exit . Using this method, the exit function of program can be realized in GUI programming .
4.2 Common methods

5 Date
5.1 summary
Get time and time operations
5.2 Construction method

//  Get current system time 
		Date d1 = new Date();
		//  Incoming milliseconds , Get the world from the time origin to the specified number of milliseconds 
		Date d2 = new Date(1000);
		// Fri Jan 14 16:15:57 CST 2022
		System.out.println(d1);
		// Thu Jan 01 08:00:01 CST 1970
		System.out.println(d2);

5.3 Common methods

// 1642148327849  Gets the number of milliseconds for the specified time 
		System.out.println(d1.getTime());
		//  Convert time into   String type 
		System.out.println(d1.toString());

5.4 format

/** *  year  y *  month  M *  Japan  d *  when  H *  branch  m *  second  s *  millisecond  S */
		//  Create a time format object 
		SimpleDateFormat sdf = new SimpleDateFormat("yyyy year MM month dd Japan  HH:mm:ss SSS");
		//  format , Return string 
		String string = sdf.format(d1);
		System.out.println(string);
		
		//  String in time format   Convert to Date object 
		String strDate = "2022/01/14 16:21:36";
		//  Format the time format of the object   And the format of the string   Agreement 
		sdf = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
		//  transformation   return Date object 
		Date date = sdf.parse(strDate);
		System.out.println(date);

5.5 Calendar

//  Get the current calendar 
		Calendar c = Calendar.getInstance();
		//  Get the current day of the week  ,  Sunday is the first day 
		System.out.println(c.get(Calendar.DAY_OF_WEEK));
		//  year 
		System.out.println(c.get(Calendar.YEAR));
		//  month  , 0  yes 1 month  , 11 yes 12 month ,  therefore   result  +1  That's right 
		System.out.println(c.get(Calendar.MONTH)+1);
		//  Japan   Get the day of the month today 
		System.out.println(c.get(Calendar.DAY_OF_MONTH));
		//  when  12 hourly 
		System.out.println(c.get(Calendar.HOUR));
		// 24 hourly 
		System.out.println(c.get(Calendar.HOUR_OF_DAY));
		//  branch 
		System.out.println(c.get(Calendar.MINUTE));
		//  second 
		System.out.println(c.get(Calendar.SECOND));

6.Random
6.1 summary
Random number from 0 Start
6.2 Use

//  Create random number generator 
		Random r = new Random();
		//  stay int Within the scope of   I'm going to randomly generate a number 
		int i = r.nextInt();
		System.out.println(i);
		
		//  Pass in 10  The explanation should be in  0~9 Between generation 
		i = r.nextInt(10);
		System.out.println(i);
		
		//  Generate  10~22 
		// nextInt( Maximum  -  minimum value  + 1) +  minimum value 
		int result = r.nextInt(100 - 20 + 1) + 20;
		System.out.println(result);