Use of JDK Chinese API documents

JDK11 Chinese API document download link
Before learning common classes, I will first introduce the use of the following JDK API documents. If you don't have one, you can search online documents or download resources, as shown above
Using API documents, we can quickly query the functions and usage methods of the methods contained in the classes we need.
Generally, we first open the API document and then use the index to search the class name, as shown in the figure below

String

A string is a class. A string variable is a reference data type
The content of "" is a string. As long as it is a string, you can call the method in the string

	System.out.println("zhangsan".equals("ddd"));

The common methods in the string are as follows
Declare the following objects first

	String strA = "zhangsan";
	String strB =  "LISI";
	String strC =   " Wang Wu";

Length returns the string length length(). Note that the length at this time is after the method rather than the variable length

	System.out.println("Length of returned string:"+strA.length());

String concat enation or+

	System.out.println("String splicing:"+strA.concat("Wang Wu"));
	System.out.println("String splicing:"+strB.concat("Li Si"));

String judgment
Equality judgment

	System.out.println("Compare the contents of two strings and return true Otherwise return false: "+strA.equals("zhangsana"));

Judge whether it is empty

	System.out.println("Judge whether the string is empty and return true Otherwise return salse: "+strA.isEmpty());

Equality judgment ignores case

	System.out.println("Comparing the contents of two strings, ignoring equal case, returns true Otherwise return false: "+strA.equalsIgnoreCase("ZHANGSAN"));

The integer 0 returned by dictionary order comparison represents the same row. It is a negative number before it, otherwise it is a positive number

	System.out.println("The integer 0 returned by dictionary order comparison represents the same row. It is a negative number before it, otherwise it is a positive number: "+strA.compareTo(strB));

Compare in dictionary order, ignoring case

	System.out.println("Compare in dictionary order, ignoring case:"+strA.compareToIgnoreCase("ZHANGSAN"));

Judge whether to start with a specific character, return true, otherwise return false

	System.out.println("Determines whether to start with a specific string:"+strA.startsWith("zh"));

Determines whether to end with a specific string

System.out.println("Determines whether to end with a specific string: "+strA.endsWith("a"));

String conversion
Convert all strings to uppercase

System.out.println("Convert all strings to uppercase:"+strA.toUpperCase());

Convert all strings to lowercase

System.out.println("Convert all strings to lowercase:"+strB.toLowerCase());

Convert string to byte array

System.out.println("Convert string to byte array:"+strA.getBytes());

Converts the basic data type to a string

System.out.println("Converts the basic data type to a string:"+strA.valueOf(123));

Remove the front and back blanks

System.out.println("Remove the previous blank:"+strC.trim());

Returns the index (subscript) of a string

	System.out.println("Returns a string according to the index. The contents of the index:"+strA.charAt(3));
	System.out.println("Returns its index value based on a string:"+strA.indexOf("s"));

String interception

	System.out.println("Intercept from index value to end:"+strA.substring(2));
	System.out.println("From the beginning of the index value to the end of the index value, it is closed on the left and opened on the right"+strA.substring(2, 4));

Character string segmentation is based on specific characters, and the split string is received as a character array

	String strD =  "The shade of green leaves is thick, and the pools, pavilions and water pavilions are all over. It's much cooler";
	String[] strE = new  String[3]; 
	
	strE= strD.split("，");
	
	for(String str:strE) {
		System.out.println(str);
	}

Complete code

public class StringDemo {
	
	
	public static void main(String[] args) { 
	
	//A string is a class. A string variable is a reference data type
	//The content of "" is a string. As long as it is a string, you can call the method in the string
	System.out.println("zhangsan".equals("ddd"));
	//The common methods in the string are as follows
	String strA = "zhangsan";
	String strB =  "LISI";
	String strC =   " Wang Wu";
	
	//Length returns the string length length(). Note that the length at this time is after the method rather than the variable length
	System.out.println("Length of returned string:"+strA.length());
	
	//String concat enation or+
	System.out.println("String splicing:"+strA.concat("Wang Wu"));
	System.out.println("String splicing:"+strB.concat("Li Si"));
	
	//String judgment 
	//Equality judgment 
	System.out.println("Compare the contents of two strings and return true Otherwise return false: "+strA.equals("zhangsana"));
	//Judge whether it is empty
	System.out.println("Judge whether the string is empty and return true Otherwise return salse: "+strA.isEmpty());
	//Equality judgment ignores case
	System.out.println("Comparing the contents of two strings, ignoring equal case, returns true Otherwise return false: "+strA.equalsIgnoreCase("ZHANGSAN"));
	//The integer 0 returned by dictionary order comparison represents the same row. It is a negative number before it, otherwise it is a positive number 
	System.out.println("The integer 0 returned by dictionary order comparison represents the same row. It is a negative number before it, otherwise it is a positive number: "+strA.compareTo(strB));
	//Compare in dictionary order, ignoring case
	System.out.println("Compare in dictionary order, ignoring case:"+strA.compareToIgnoreCase("ZHANGSAN"));
	//Judge whether to start with a specific character, return true, otherwise return false
	System.out.println("Determines whether to start with a specific string:"+strA.startsWith("zh"));
	//Determines whether to end with a specific string
	System.out.println("Determines whether to end with a specific string: "+strA.endsWith("a"));
	
	System.out.println("");	
	System.out.println("");
	//String conversion
	//Convert all strings to uppercase
	System.out.println("Convert all strings to uppercase:"+strA.toUpperCase());
	//Convert all strings to lowercase
	System.out.println("Convert all strings to lowercase:"+strB.toLowerCase());
	//Convert string to byte array
	System.out.println("Convert string to byte array:"+strA.getBytes());
	//Converts the basic data type to a string
	System.out.println("Converts the basic data type to a string:"+strA.valueOf(123));
	//Remove the front and back blanks
	System.out.println("Remove the previous blank:"+strC.trim());
	
	
	System.out.println("");	
	System.out.println("");
	//Returns the index (subscript) of a string
	System.out.println("Returns a string according to the index. The contents of the index:"+strA.charAt(3));
	System.out.println("Returns its index value based on a string:"+strA.indexOf("s"));
	
	
	System.out.println("");	
	System.out.println("");
	//String interception 
	System.out.println("Intercept from index value to end:"+strA.substring(2));
	System.out.println("From the beginning of the index value to the end of the index value, it is closed on the left and opened on the right"+strA.substring(2, 4));
	
	//Character string segmentation is based on specific characters, and the split string is received as a character array
	String strD =  "The shade of green leaves is thick, and the pools, pavilions and water pavilions are all over. It's much cooler";
	String[] strE = new  String[3]; 
	
	strE= strD.split("，");
	
	for(String str:strE) {
		System.out.println(str);
	}
	}
}

Schedule

For more useful methods, please refer to the JDK API documentation

StringBuffer and StringBuilder

StringBuffer and StringBuilder are variable length strings. We can append the strings directly
String is an immutable length string. Each new assignment is equal to redefining it, which is very inefficient

The common methods of StringBuffer and StringBuilder are the same, generally including append, delete, insert and modify

The usage is as follows

public class StringBufferDemo {

	public static void main(String[] args) {
		//Variable length string
		//statement
		StringBuffer sb=new StringBuffer();

		//Add
		sb.append("one");
		sb.append("two");
		sb.append("three");
		sb.append("four");
		
		System.out.println(sb);
		
		//delete
		System.out.println("Start delete");
		System.out.println("From the beginning of index deletion to the end, the index is left closed and right open:"+sb.delete(1, 3));
		
		//insert
		System.out.println("Insert the index value of the first parameter into the index position and the content of the second parameter:"+sb.insert(1, "two or three"));
		
		//modify
		System.out.println("Change from start index to end index: "+sb.replace(1, 3, "seven or eight"));
		
		//Convert to String
		System.out.println("convert to String:"+sb.toString());
		
	}
	
}

The difference between StringBuffer and StringBuilder

There is little difference between the usage of StringBuffer and StringBuilder
The main difference is that StringBuffer is thread safe and StringBuilder is non thread safe
The use of both is generally divided into cases (single thread and multi thread)
The specific characteristics of the two are as follows

String class
➥ strings of immutable length
➥ when splicing, a new string is actually created to replace the original string, so the efficiency of string addition is very low

StringBuffer class
➥ thread safe, variable length strings
➥ it is used more in multithreading operation, and it is more efficient to append strings

StringBuilder class
➥ thread unsafe, variable length strings
➥ string appending is most efficient in a single threaded operation

Date time class

We usually use the Date class to handle time in milliseconds
Generally, it includes obtaining the current time and customizing the time, and passing in a long integer data
Use as follows

		Date date = new Date();
		//Gets the time when the data was created
		System.out.println(date);
		
		//Incoming milliseconds custom time
		Date dateA = new Date(600000000000l);
		System.out.println(dateA);
		
		//Gets the millisecond value of the time
		System.out.println("Time millisecond value:"+date.getTime());

However, the output time format is not in line with our aesthetics
We usually use the SimpleDateFormat class to convert the time format
The usage is as follows. Note that M represents month and M represents minute

		Date date = new Date();
		//Gets the time when the data was created
		System.out.println(date);
		
		//Incoming milliseconds custom time
		Date dateA = new Date(600000000000l);
		System.out.println(dateA);
		
		//Gets the millisecond value of the time
		System.out.println("Time millisecond value:"+date.getTime());
		
		//The format looks uncomfortable. We can convert it to conform to Chinese aesthetics
		//Time format conversion class
		SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
		System.out.println(simpleDateFormat.format(date));

Math operation class

This method has been used by everyone. Its methods are static methods, which can be accessed directly using the class name without instantiating the object
There are several common usages. You can see the use of random numbers

		 //use Π value
		 System.out.println(Math.PI);
		 //Maximum of two numbers
		 System.out.println(Math.max(20, 22));
		 //Minimum of two numbers
		 System.out.println(Math.min(50, 12));
		 //rounding
		 System.out.println(Math.round(36.7));
		 //absolute value
		 System.out.println(Math.abs(-11));		 
		 //Generate decimals between random numbers 0.0-1.0
		 System.out.println(Math.random());
		 //Between 0-10
		 System.out.println((int)(Math.random()*10));
		 //0-100
		 System.out.println((int)(Math.random()*100));
		 //Root seeking
		 System.out.println(Math.sqrt(4));
		 //Seeking square
		 System.out.println(Math.pow(2,10));

Packaging

In order to realize more complex data processing, Java arranges wrapper classes for 8 basic data types for complex data processing
The corresponding relationship is shown in the figure below

Packing and unpacking of packaging

Simply put, boxing is to package the basic data type into its packaging class instance for data processing. Disassembly is to get the data in the packaging class into the basic data type

Packing and unpacking can be done automatically

int i = 100;
		//Packing
		//Direct declaration auto packing common
		Integer k = i;
		Double l= 15.6;
		//Create manually
		Integer s = new Integer(12);
	
		//Unpacking method
		//Automatic unpacking
		i = s;
		//Manual unpacking
		int p = k.intValue();

Key role
The data types we receive from web pages are all string types, so we need to convert the data if we want to perform operations. Therefore, the data processing method (static method) of packaging class is particularly important
Automatic unpacking after sample data processing

		//Why use wrapper classes? A function that has to be used to convert a string into data in any format, parseInt, parsefloat, etc
		String strA="1234";
		String StrB = "true";
				
		int testA = Integer.parseInt(strA);
		boolean testB = Boolean.parseBoolean(StrB); 
		System.out.println(testA+12);
		if(testB) {
			System.out.println(testB);
		}

Arrays array class

Commonly used are basically ascending and searching
The specific usage is as follows

		//Define an array
		int[] testA = {1,8,95,33,99,152};
		
		//Arrays sort
		//In ascending order, notice that the array itself has changed
		Arrays.sort(testA);
		for(int i:testA) {
			System.out.println(i);
		}
		//Descending order
		
		//The value specified in the Arrays search must be an ordinal return index value
		System.out.println(Arrays.binarySearch(testA, 95));

System class

The commonly used methods to get the current time of the system and exit the millisecond error output are as follows

		//Error output
		System.err.println("Error output");
		
		//Returns the millisecond value of the current time
		System.out.println(System.currentTimeMillis());
		//System exit normal exit 0 abnormal exit
		System.exit(0);
		System.out.println("The system has exited");

Timed task

The specific method is not detailed
The details are as follows

import java.util.Timer;
import java.util.TimerTask;



public class MyTask  extends TimerTask{

	@Override
	public void run() {
		// Method stub automatically generated by TODO
		//Tasks requiring timing
		System.out.println("Output every two seconds");
	}
	
	public static void main(String[] args) {
		Timer timer = new Timer();
		MyTask myTask = new MyTask();
		//The first parameter is the task object. The second parameter is how many milliseconds to start. The third parameter is how many milliseconds to execute once. It is executed only once without writing
		timer.schedule(myTask, 1000,2000);
		
	}
	

}