3. Java Basic class

1. The system class is used to obtain the property data of the systems commonly used methods: (1) Print array

int int int length);   // arrays.tostring (Array)

(2) Current system time ( common )

Currenttimemillis ();  // number of milliseconds from 1970 to present

(3) Exiting the Java Virtual machine

Exit (int status);   // 0 Normal exit, not 0 abnormal exit. 0;catch in a try block General pass 1

(4) Garbage collection

GC ();  // It is recommended that the JVM start the garbage collector garbage collection quickly (the JVM calls the Finalize () method of object to reclaim)

(5) Get environment variables

getenv (String name);

(6) Get System Properties, properties have List Property List property

GetProperties ();

2. Runtime class

Represents the Application Runtime environment

(1) Returns the running environment object for the current application

GetRuntime ();

(2)根据指定的路径执行可执行文件，返回Process对象可用destory()关闭程序

EXEC (String command);

(3)返回jvm空闲的内存

Freememory ();

(4)试图使用的的最大内存

MaxMemory ()

(5)jvm虚拟机的内存总量

TotalMemory ();

3. Date class Dates

Calendar class (not much, not formatted)

Calendar calendar = Calendar. getinstance (); System. Out.println (Cal.get (Calendar. year));   // +1 If you get the month .

Date formatting class SimpleDateFormat ( common )

Role:

(1) Convert the date to the specified format string format ();

Newnew SimpleDateFormat ("yyyy mm month DD day hh" mm min ss sec "= Dateformat.format (date);

(2) Convert the string to the corresponding date parse ()

String str1 = "September 19, 2016 16:30 43 seconds";  // format to be consistent with the above Date date1 = Dateformat.parse (STR1);

4. Math class

Provides a number of mathematical formulas

Ceil: Rounding up the floor: Rounding down Round: rounded random: Generates a stochastic number, greater than or equal to 0.0 less than 1.0

5. Random class

Next generates a random number

// Generate verification Code: Char [] str = {' Medium ', ' country ', ' good ', ' sound ', ' sound ', ' O 'newnew  Random ();  for int i=0; i<6; i++) {int index = random.nextint (str. length); Sb.append (str[index]);}

3. Java Basic class