J2se basics: 8. Common System Class 2

1: transformation between basic data and encapsulation types

A: Basic Data Type ---> encapsulation type (object type)

Boolean boolean_1 = new Boolean (true );

Byte ----> byte

Short ----> short

Char ----> character

Int ---> integer

Long --> long

Float --> float

Double --> double

B: encapsulation type ---> basic type

Integer. intvalue ---> int

Folat. floatvalue ---> float

Double. doublevalue ---> double

2: conversion between basic types and strings

Call the string. valueof () method.
String STR = string. valueof (any data type );

3: Convert string to basic data type

Call the parse method of the encapsulation class.
Int I = integer. parseint ("100 ");
Double D = double. parsedouble ("12.334 ");

4: some common characeter methods:

Character mainly works with the string object to perform some simple verification on the string object.
String. chatat (I );

Simple data verification

Java. util. RegEx package

Pattern: compile a regular expression.

Matcher: checks whether the string matches the regular expression.

5: The String object has the length method. The array has the Length attribute.


Math object

1: generate random number

A: use math object: Math. Random (); to obtain a random number between 0 and 1. This random number is always less than 0 and less than 1 decimal number.

B: Use a random object.

2: decimal operation

Math. Round (): the decimal point is not retained, and the numeric value is rounded down.

12.65 ---> 13

Math. Ceil (): retain the decimal place to obtain the ceiling closest to the number.

12.65 --> 13.0

Math. Floor (): the floor with the nearest digit is retained.

12.65 --> 12.0

Date object

1: how to get the system time:

A: system. currenttimemillis:

It indicates the number of milliseconds that have elapsed since January 1, lunar January 1970 yen.

B: Java. util. Date ();

Subclass: Java. SQL. Date: The year, month, and day when the Java program processes the date fields in the database.

Java. SQL. Time: the time, minute, and second used by Java programs to process the date fields in the database.

Java. SQL. timestamp: The year, month, hour, minute, and second used by Java programs to process date fields in the database.

C: Calendar class:

Calendar

Mutual conversion between the three objects.

Between long and date.

Long ---> date

Long L = 20171110888578l;

Date = new date (L );

Date ---> long

Date = new date ();

Long time_long = date. gettime ();

Conversion between long and calendar

Long ---> calendar

Long long_time = 121110864656l;

Calendar calendar = calendar. getinstance ();

Calendar. settimeinmillis (long_time );

Calendar ---> long

Calendar calendar = calendar. getinstance ();

Long time_long = calendar. gettimeinmillis ();

Conversion between date and calendar

Date ---> calendar

Calendar calendar = calendar. getinstance ();

Calendar. settime (new date ());

Calendar ---> date

Calendar calendar = calendar. getinstance ();

Date = calendar. gettime ();

Formatting time

Format ---> dateformat ---> simpledateformat ();

Convert date to string

Call simpledateformat. Format

String to date

Call simpledateformat. parse.

Precise computing object

Biginteger: exact integer calculation

Bigdecimal: exact decimal calculation.

Truncation and rounding.


Normal format

Numberformat = numberformat. getinstance ();

Percent format

Numberformat = numberformat. getpercentinstance ();

// Set the Maximum/minimum reserved integer bits.
Numberformat. setmaximumintegerdigits (4 );
Numberformat. setminimumintegerdigits ();

// Set the Maximum/minimum number of digits after the decimal point
Numberformat. setmaximumfractiondigits (2 );
Numberformat. setmaximumfractiondigits ();
Loan currency format
Numberformat = numberformat. getcurrencyinstance ();

Decimalformat

Number ---> string (rounding function)
Decimalformat = new decimalformat (".##");
String result = decimalformat. Format (d );