Predator Prey

Summary

This demo illustrates how the JMSL Library can be used to easily build an interactive display application using Java™, utilizing ordinary differential equations to describe and illustrate real-world situations. The predator-prey problem computes the population density of rabbits and foxes over time. The population density is governed by the following Ordinary Differential Equations:

dr/dt = 2r - 2rf
df/dt = -f + rf

where r is the density of rabbits, and f is the density of foxes. The chart shows the population density of foxes vs. rabbits. The curve is parameterized by time. The blue squares are computed values and the line is an interpolation between the computed values. The large red circle is the initial condition.

Usage

Click in the chart area to set a new initial condition for the problem. The solution is recomputed automatically. You can click and drag to update the solution continuously.

JMSL Library Math/Stat Classes

com.imsl.math.OdeRungeKutta - this class is used to compute the solution for this initial-value problem using the Runge-Kutta-Verner fifth-order and sixth-order method.

com.imsl.IMSLException - this class appears because OdeRungeKutta can throw a com.imsl.math.OdeRungeKutta.DidNotConvergeException. The exception is thrown by the solve() method and caught in the updateInitialPoint() method where it is reported by printing text in the chart area.

JMSL Library Charting Classes

The chart in this demo contains the initial condition marked as a large red circle and the solution as a blue line with blue filled squares as markers. All of the charting attributes are configured in the constructor.

To update the chart, the class implements MouseListener, MouseMotionListener and the updateInitialPoint() method is called on mouseClicked or mouseDragged events. Note that the MouseEvent is passed to that method.

Java Code

Reporting an exception by printing text in the chart area is unique to this demo. There are two parts to accomplishing this. First, a Data object must be created to hold the text. To hold the text, the setLabelType() method is set to Data.LABEL_TYPE_TITLE.

double ex[] = {2.5};
double ey[] = {4.0};
dataError = new Data(axis, ex, ey);
dataError.setLabelType(Data.LABEL_TYPE_TITLE);
dataError.setTextColor(Color.red);

Initially the title attribute for this data object is not set; when there is nothing to report, or is set to an empty string. Once an exception is thrown and needs to be reported, the title attribute is set to the string to display.

} catch (IMSLException exception) {
    StringBuffer sb = new StringBuffer(exception.toString());
    int nCharsPerLine = 30;
    for (int k = nCharsPerLine; k < sb.length(); k += nCharsPerLine) {
        for (int j = 0; j < nCharsPerLine; j++) {
            if (sb.charAt(k-j) == ' ') {
                sb.setCharAt(k-j, '\n'); k -= j;
                break;
            }
        }
    }
    dataError.setTitle(sb.toString());
}
repaint();

Link to Source Code

Running This Demo

Two alternatives are available to run this demo:

1) Use the source code in your development environment as any other Java code. More information is available in the How To.

2) An executable jar file containing all of the demos referenced in this guide is included in the jmsl/lib directory. On Windows, you may double-click the file to run it if files with a ".jar" extension are properly registered with javaw.exe. Alternatively, for both Windows and UNIX environments, the jar file may be executed from the command line using java -jar gallery.jar.

As list of buttons, one for each demo, is created. Demos can be subsetted as they relate to specific areas (Math, Stat, Finance, Charting) by choosing the appropriate selection on the JComboBox. To run the Additional Demos, select Quick Start in the JComboBox.