example6_2.java

清华大学2002年出版的《科学与工程数值计算算法Java》配套源码

import java.applet.*;
import java.awt.*;
import  corejava.*;

public class Example6_2 extends Applet
{
 public static double f(double x)
 {double y;
  y=x*Math.sin(x)+1/(x*x*x+1);
  return y; }

 public static double simpson(double a,double b)
  {double s1=0,s2=0,h,y1,y2,y3;
   int i,n=1;
   h=(b-a)/2;y1=f((a+b)/2);y2=0;y3=f(a)+f(b);s2=(y1*4+y3)*h/3;
   while(Math.abs(s1-s2)>1e-10&&Math.abs(h)>1e-10)
   {s1=s2;y2+=y1;y1=0;
    for(i=1;i<=2*n;i++)y1+=f(a+(i-1.0/2)*h);
    s2=(4*y1+2*y2+y3)*h/6;
    n*=2;h/=2;
   }
	return s2;
  }

public void paint(Graphics g)
{double y;
 y=simpson(0,Math.PI);
 g.drawString("积分值为"+y,10,20);}
}