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sqrt (x): square root
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pow (x, y): x to the power demand y
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ceil (x): rounding up
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floor (x): rounded down
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round (x): rounding
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random (): random number obtained in the range [0,1)
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max (x, y): find x, the maximum value of y
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min (x, y): minimization x, y of
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PI: pi
2，java.Math包
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BigDecimal: immutable arbitrary precision decimal
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They are to new objects to represent the data, performs arithmetic operation to add, subtract, multiply, divide or the like
3, java.util.Random
Random r = new Random();
r.nextDouble D = Double (); // [0,1)
int I = r.nextInt (); // int within a range
within // [0, x) range; int j = r.nextInt (x) integer
Date and time-related
1 java.util.Date
new Date (): Gets the system date and time
new Date (Long ms): The date and time acquired milliseconds
long getTime (): Get the date and time corresponding to the milliseconds from 1970-1-10: 0: 0
2 java.util.Calender
How to create / acquire objects Calender of?
(1), create a sub-class object: GregorianCalendar
(2) to obtain the specified time zone calendar object
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getInstance()
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getInstance (TimeZone time zone) or getInstance (Locale locale)
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getInstance(TimeZone, Locale)
(3) get (field names)
For example: int year = get (Calendar.YEAR);
java.text.DateFormat date and time format
Other sub-categories: SimpleDateFormat
y: Year
M: month
d: day
The first day of the year: D
H: When the 24-hour
h: 12 hour clock
m: min
s: sec
S: ms
E: week
SimpleDateFormat sf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
// the date to String
String str = sf.format (Date date of the object);
// parse string date
Date d = sf.parse (string);
java.time 包
1, localized date / time: LocalDate, LocalTime, LocalDateTime
now (): Gets the current date or time
of (x, x, x): Gets a specified date or time
withXxx (), plusXxx (), minusXxx (): modification date and time, to re-modified to receive the new object
isLeapYear (): determine whether it is a leap year
2, phase date or time: Date Period interval and duration Duration
between (x, y): two date or time
3, Format: DateTimeFormatter
Use predefined pattern: ISO_LOCAL_DATE
Use custom mode: e.g. yyyy-MM-dd HH: mm: ss
Use FormatStyle enumeration constants: LONG, MEDIUM, SHORT
And system-related classes
1, java.lang.System categories: System Tools
static long currentTimeMillis (): Get the value of the system time in milliseconds
static void exit (x): Exit JVM
static void arraycopy (subscript starting source array, the starting index of the original array, an array of targets, the target array, a total of several elements copy)
static void gc (): inform the garbage collector work
static String getProperty (system property name)
2, java.lang.Runtime categories: JVM runtime environment
static Runtime getRutime()
long totalMemory()
long freeMemory()
void gc()