java.util.date - Java: Speeding up and slowing down time -

i'm trying figure out logic set arbitrary time, , have time "play back" @ different speeds (like .5x or 4x real time).

here logic have far, playback time @ normal speed:

import java.util.calendar;   public class clock {       long delta;     private float speed = 1f;      public clock(calendar startingtime) {         delta = system.currenttimemillis()-startingtime.gettimeinmillis();     }      private calendar adjustedtime() {         calendar cal = calendar.getinstance();          cal.settimeinmillis(system.currenttimemillis()-delta);          return cal;      }      public void setplaybackspeed(float speed){         this.speed  = speed;     }      public static void main(string[] args){           calendar calendar = calendar.getinstance();         calendar.set(2010, 4, 4, 4, 4, 4);         clock clock = new clock(calendar);          while(true){             system.out.println(clock.adjustedtime().gettime());             try {                 thread.sleep(1000);             } catch (interruptedexception e) {                 // todo auto-generated catch block                 e.printstacktrace();             }         }      }   } 

i'm having trouble figuring out "speed" attribute needs used in logic.

the following code gives example of how design such clock, has internal state of double spped , long starttime. exposes publish method gettime(), return adjusted time in millisecond since midnight, jan 1st, 1970. note adjustment happens after starttime.

the formula calculate adjusted time simple. first take real timedelta subtracting currenttimemillis() starttime, multiply value speed adjusted timedelta, add starttime result.

public class variablespeedclock {      private double speed;     private long starttime;      public variablespeedclock(double speed) {         this(speed, system.currenttimemillis());     }      public variablespeedclock(double speed, long starttime) {         this.speed = speed;         this.starttime = starttime;     }      public long gettime () {         return (long) ((system.currenttimemillis() - this.starttime) * this.speed + this.starttime);     }      public static void main(string [] args) throws interruptedexception {          long st = system.currenttimemillis();         variablespeedclock vsc = new variablespeedclock(2.3);          thread.sleep(1000);          system.out.println(vsc.gettime() - st);         system.out.println(system.currenttimemillis() - st);      } }