// test_passing_function.java  uses interface
// implement trapezoidal integration of function f(x) from xmin to xmax

public class test_passing_function
{
  public test_passing_function()
  {
    double xmin = 1.0;
    double xmax = 2.0;
    int n = 100;
    double h;
    double area;

    System.out.println("test_passing_function running");
    System.out.println("trapezoidal integration of f(x)=x*x  n="+n);
    System.out.println("xmin="+xmin+", xmax="+xmax);

    area = (user.f(xmin)+user.f(xmax))/2.0;
    h = (xmax-xmin)/(double)n;
    for(int i=1; i<n; i++) area += user.f(xmin+(double)i*h);
    area = h*area;
    System.out.println("area = "+area);
    System.out.println("test_passing_function finished");
  }

  public interface test_lib
  {
    // just the signature, no body
    // the user provides the body in class test_user
    double f(double x);
  }

  public static class test_user implements test_lib
  {
    public double f(double x)
    {
      return x*x; // just sample function
    }
  }

  public static test_user user=new test_user();

  public static void main (String[] args)
  {
    new test_passing_function();
  }
}