Tool class: system, runtime, math, date, and calendar

API ---Java. Lang.System:Both attributes and behaviors are static.

Long currenttimemillis ();// Returns the current time in milliseconds.

Exit ();// Exit the VM

PropertiesGetproperties (); // Obtain the attribute information of the current system

 

Properties prop = system. getproperties ();// Obtain the system property information and store the information in the properties set.

 

System. setproperty ("myname", "instructor bi ");// Add specific property information to the system property information set

// Temporary setting method: when running JVM, you can use JVM parameters to set System Properties temporarily. You can add-D <name >=< value> usage: Java-Dmyname = Xiaoming class name.

 

String name = system. getproperty ("OS. Name ");// Obtain the information of the specified attribute

 

// You want to know whether the system is one of the systems supported by the software.

Set <string> HS = new hashset <string> ();

HS. Add ("Windows XP ");

HS. Add ("Windows 7 ");

If (HS. Contains (name ))

System. Out. println ("supported ");

Else

System. Out. println ("not supported ");

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API ---Java. Lang.Runtime:Class.

But there are non-static methods. The object should be defined in this class and can be obtained through a static method. Use this object to call non-static methods. This method is static runtime getruntime ();

 

This Runtime is actually designed in the singleton design mode.

 

Class runtimedemo {

Public static void main (string [] ARGs) throws exception {

Runtime r = runtime. getruntime ();

PROCESS p = R.Exec("Notepad.exe systemdemo. Java"); // run the specified program

Thread. Sleep (4000 );

P. Destroy ();// Kill the process

}

}

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API ---Java.Util.Math:Tools used for mathematical operations, attributes and behaviors are static. This class is not allowed to inherit from final.

 

Static double Ceil (double A); // returns the smallest integer greater than the specified value.

Static double floor (double A); // returns the maximum integer smaller than the specified value.

Static long round (double A); // rounding to an integer

Static double POW (double A, double B); // B power of

Static double random (); // returns 0 ~ 1 pseudo-random number

 

Public static void main (string [] ARGs ){

Random r = new random ();

For (INT x = 0; x <10; X ++ ){

// Double D = math. Floor (math. Random () * 10 + 1 );

// Int d = (INT) (math. Random () * 10 + 1 );

Int d = R. nextint (10) + 1;

System. Out. println (d );

}

}

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API ---Java.Util.Date:Date class. The month ranges from 0 to 11;

/*

The conversion between the date object and the millisecond value.

1. Convert the date object to the millisecond value. The gettime method in the date class.

2. How do I convert the obtained millisecond value to a specific date?

The settime method in the date class. You can also use constructor.

*/

// Convert the date object to a millisecond Value

Date d = new date ();

Long time1 = D. gettime ();

Long time2 = system. currenttimemillis (); // millisecond value.

 

// Convert the millisecond value to a specific date

Long time = 1322709921312l;

Date d = new date ();

D. settime (time );

/*

Convert a date string to a date object: In the dateformat methodDate parse (string source);

*/

Public static void method () throws exception {

String str_time = "2011/10/25 ";

Dateformat df = new simpledateformat ("yyyy/mm/DD ");// Simpledateformat can be used as a custom format to complete formatting.

Date d = DF. parse (str_time );

}

/*

If you do not need to use a specific formatting style, you can use the static factory method in the dateformat class to obtain the specific encapsulated style objects.Getdateinstance (); getdatetimeinstance ();

*/

Date d = new date ();

Dateformat df = dateformat. getdateinstance (dateformat. Long );

DF = dateformat. getdatetimeinstance (dateformat. Long, dateformat. Long );

String str_time = DF. Format (d );

 

// Convert a date object to a string using the format method in the dateformat class.

// Create Date Format object.

Dateformat df = new simpledateformat ();// The creation of this object encapsulates a default date format. 11-12-1 pm

// If You Want to customize the date format. You can use the simpledateformat constructor. Pass the specific format as a parameter to the constructor. How to represent the middle-aged part of the date? You may need to participate in the format object document.

DF = new simpledateformat ("mm DD, yyyy hh: mm: SS ");

// Call the format method in dateformat. Format the existing date object.

String str_time = DF. Format (d );

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API ---Java.Util.Calendar:Calendar

Public static void method (){

Calendar c = calendar. getinstance ();

System. Out. println (C. Get (calendar. Year) + "year" + (C. Get (calendar. month) + 1) + "month"

+ Getnum (C. Get (calendar. day_of_month) + "day"

+ "Week" + getweek (C. Get (calendar. day_of_week )));

}

Public static string getnum (INT num ){

Return num> 9? Num + "": "0" + num;

}

Public static string getweek (INT index ){

/*

Table query: establishes a data correspondence.

Best: the number of data is determined and has a corresponding relationship. If one of the mappings is a number and can be used as the badge, an array can be used as a table.

*/

String [] weeks = {"", "day", "one", "two", "three", "four", "five", "Six "};

Return weeks [Index];

}

Tool class: system, runtime, math, date, and calendar