Common API s

ava API is the classes provided to us in JDK. These classes encapsulate the underlying code implementation. We don't need to care about how these classes are implemented, we just need to learn how to use these classes.

There is a Src in the JDK installation directory Zip file. The contents of this file after decompression are the source files of all Java classes. You can view the source code of the corresponding class.

Math math

java.lang.Math class is a program function class of mathematical calculation provided in the whole Java. Using this Zeng Xu function class, you can easily perform some basic mathematical calculations: logarithm, trigonometric function, square root, module, etc. This class is in jdk1 0 is provided to users, and there is no constructor in this class.

1. Take absolute value

System.out.println(Math.abs(-123));
123

2. Round down

System.out.println(Math.floor(4.3));
4.0

3. Round up

System.out.println(Math.ceil(4.2));
5.0

4. Rounding

System.out.println(Math.round(4.5));
5

5. Square root

System.out.println(Math.sqrt(16,0));
4.0

6. Power operation

System.out.println(Math.pow(2.0,3.0));
8.0

7. Maximum

System.out.println(Math.max(3,5,7));
7

8. Minimum value

System.out.println(Math.min(3,5,7));
3

9. Random number

System.out.println(Math.random()*100);
0~100 between
System.out.println((int)(Math.random()*100));
0~100 Integer between

Math.rangdon() produces non repeating decimals between 0 and 1, but there will be repeated numbers after expansion and coercion.

System.out.println(UUID.randomUUID().toString());

The second way to generate random numbers: through UUID Randomuuid() produces a random number in hexadecimal and will not be repeated

Date class

java. util. The date class represents a specific moment, accurate to milliseconds.

Continue to refer to the description of the Date class. It is found that Date has multiple constructors, but some of them are outdated, but some of them can convert the millisecond value into a Date object.

Date date = new Date();
System.out.println(date);
		date.getYear();
// getYear will be crossed out by a black line, which means it is out of date. It is not recommended to use in the future, but it can be used

Date formatting

SimpleDateFormat  dFormat = new SimpleDateFormat("yyyy-MM-dd  HH:mm:ss");
String dateStr=dFormat.format(date);
System.out.println(dateStr)

String class

java.lang.String class is the implementation of some methods for storing strings.

String str = "IaskldfjakDFAS";
System.out.println(str.length());//Print the length of the string
System.out.println(str.charAt(2));//Print characters with subscript 2
System.out.println(str.toCharArray());//Converts a string to an array of character types
System.out.println(str.startsWith("i");//Determines whether to start with a specific string (characters in parentheses)
System.out.println(str.endsWith("t"));//Determines whether to end with a specific string (characters in parentheses)
System.out.println(str.indexOf("a"));//indexOf() looks for the first occurrence of a character or substring
System.out.println(str.lastIndexOf("a"));
 //lastIndesOf() looks for the last occurrence of a string or string
System.out.println(str.substring(2));//Intercept from the position marked 2 to the end
System.out.println(str.substring(2,5));
//Intercept from the position marked with 2 to the position of 5, but do not include the position of 5
System.out.println(str.replace("a","bj"));
//Replace with a character and then call all places in the string where a character appears
System.out.println(str.trim());//Remove the spaces at the beginning and end
String a="asdf,asdf,asdf,gasd,wer";
String [] b=a.split(",");//When characters in parentheses are read, string segmentation is performed
System.out.println(str.toUpperCase());//Convert characters to uppercase
System.out.println(str.toLowerCase());//Convert characters to lowercase

object

String s1 = "Hello";
String s2 = new String(s1);
System.out.println(s1.equals(s2));//The equals () method compares characters in a string object
System.out.println(s1 == s2);
// ==Operator to compare whether two objects refer to an instance and whether two objects point to a memory

Accuracy calibration

You can calibrate some numbers with missing accuracy

double a=3.14*2.0*5;
System.out.println(a);
Result: 31.400000000000002
 DecimalFormat dFormat = new DecimalFormat("0.000"); // Decimal table 6 two decimal places
 String result = dFormat.format(a);
	System.out.println(result);