Java The Complete Reference, Seventh Edition

Using currentTimeMillis( ) to Time Program Execution

One use of theSystemclass that you might find particularly interesting is to use the

currentTimeMillis( )method to time how long various parts of your program take to execute.

ThecurrentTimeMillis( )method returns the current time in terms of milliseconds since

midnight, January 1, 1970. To time a section of your program, store this value just before

beginning the section in question. Immediately upon completion, callcurrentTimeMillis( )

again. The elapsed time will be the ending time minus the starting time. The following

program demonstrates this:

// Timing program execution.

class Elapsed {

public static void main(String args[]) {

long start, end;

System.out.println("Timing a for loop from 0 to 1,000,000");

// time a for loop from 0 to 1,000,000

410 Part II: The Java Library

Method Description

static Channel inheritedChannel( )

throws IOException

Returns the channel inherited by the Java Virtual

Machine. Returnsnullif no channel is inherited.

static void load(Stringlibrar yFileName) Loads the dynamic librar y whose file is specified by

librar yFileName,which must specify its complete path.

static void loadLibrar y(Stringlibrar yName) Loads the dynamic librar y whose name is

associated withlibrar yName.

static String mapLibrar yName(Stringlib) Returns a platform-specific name for the librar y

namedlib.

static long nanoTime( ) Obtains the most precise timer in the system and

returns its value in terms of nanoseconds since

some arbitrar y starting point. The accuracy of the

timer is unknowable.

static void runFinalization( ) Initiates calls to thefinalize( )methods of unused

but not yet recycled objects.

static void setErr(PrintStreameStream) Sets the standarderrstream toeStream.

static void setIn(InputStreamiStream) Sets the standardinstream toiStream.

static void setOut(PrintStreamoStream) Sets the standardoutstream tooStream.

static void

setProperties(PropertiessysProperties)

Sets the current system properties as specified by

sysProperties.

static String setProperty(Stringwhich,

Stringv)

Assigns the valuevto the property namedwhich.

static void setSecurityManager(

SecurityManagersecMan)

Sets the security manager to that specified by

secMan.

TABLE 16-13 The Methods Defined bySystem(continued)