Java's Utility class

1) Random class

Used to generate random numbers, all generated numbers are equal probabilities.

Nextint (): The resulting value is between all ranges of values of int (-231 ~ 231-1)

Nextint (int value): The value generated from 0 to value (contains 0, does not contain value).

2) Math class

Used for general mathematical operations.

Random (): Generates a random decimal between 0-1 (including 0, excluding 1).

ABS (Double): Take absolute value.

Pow (double, double): Calculates the n-th square of M

sqrt (double): Calculates the root of M.

Floor (Double): Gets the largest integer value smaller than M.

Ceil (double): Gets the smallest integer value larger than M.

Round (): Rounds rounding.

3) String class

IndexOf (): Gets the index position of one string in another string (whether a string contains another string).

LastIndexOf (): Gets the last index position of a string in another string.

SUBSTRING (): Takes a substring from the specified position of a string.

Concat (): concatenation of two strings into a single string.

Equals (): Compares the contents of two strings (note the difference from = =).

toLowerCase (): Converts a string to lowercase characters.

toUpperCase (): Converts a string to uppercase characters.

Equalsignorecase (): Compares whether two strings are the same (case insensitive).

4) StringBuilder and StringBuffer class

You need to use StringBuilder or stringbuffer when you need to do frequent concatenation of strings.

5) Date class and SimpleDateFormat class

The date class is used to manipulate dates, and SimpleDateFormat is used to format dates.

Note: Many of the methods available in the date class are obsolete methods and are recommended using the methods provided by the Calendar class.

When you get a month, either by date or by calender, it is between 0-11.

Get System Current Time
Date date = new Date ();
Format Time YYYY-MM-DD HH:mm:ss
SimpleDateFormat DateFormat = new SimpleDateFormat ("Yyyy-mm-dd HH:mm:ss");
String Text = Dateformat.format (date);
Knows a date string, if it is converted to a time object

String time = "2016-10-05 12:10:05";

Date date2 = Dateformat.parse (time);

6) Calendar class

Create a Calendar class object
Calender cal = Calendar.getinstance ();
Get year
Int year = Cal.get (calendar.year);
Get month
Int month = Cal.get (calendar.month);
Calendar.date, Calendar.hour, Calendar.minite, Calendar.second
Add time period (year, month, day, hour, minute, second)
Cal.add (calender.year, 1);//plus 1 years
Cal.add (Calender.hour,-2);//minus 2 hours
Calcender Conversion to date
Date date = Cal.gettime ();

Java's Utility class