PreyPred.java

package tutorial;
 
import umontreal.ssj.simevents.*;
 
public class PreyPred {
   double r = 0.005, c = 0.00001, s = 0.01, d = 0.000005, h = 5.0;
   double x0 = 2000.0, z0 = 150.0;
   double horizon = 501.0;
   Simulator sim = new Simulator();
   Continuous x;
   Continuous z;
 
   public static void main(String[] args) {
      new PreyPred();
   }
 
   public PreyPred() {
      x = new Preys(sim);
      z = new Preds(sim);
      sim.init();
      new EndOfSim(sim).schedule(horizon);
      new PrintPoint(sim).schedule(h);
      (sim.continuousState()).selectRungeKutta4(h);
      x.startInteg(x0);
      z.startInteg(z0);
      sim.start();
   }
 
   public class Preys extends Continuous {
      public Preys(Simulator sim) {
         super(sim);
      }
 
      public double derivative(double t) {
         return (r * value() - c * value() * z.value());
      }
   }
 
   public class Preds extends Continuous {
      public Preds(Simulator sim) {
         super(sim);
      }
 
      public double derivative(double t) {
         return (-s * value() + d * x.value() * value());
      }
   }
 
   class PrintPoint extends Event {
      public PrintPoint(Simulator sim) {
         super(sim);
      }
 
      public void actions() {
         System.out.println(sim.time() + "  " + x.value() + "  " + z.value());
         this.schedule(h);
      }
   }
 
   class EndOfSim extends Event {
      public EndOfSim(Simulator sim) {
         super(sim);
      }
 
      public void actions() {
         sim.stop();
      }
   }
}