Java(TM) Boutique - Tutorial - Encapsulating Algorithms with the Template Method Design Pattern

Listing 2. The Simpson's Rule Class

public class SimpsonsRule
	{
	private double a; // the lower limit of the integration
	private double b; // the upper limit of the integration
	private int n; // the number of points for the integration

	public SimpsonsRule()
		{
		setA(0);
		setB(1);
		setN(10);
		}

	public SimpsonsRule(double a,double b,int n)
		{
		setA(a);
		setB(b);
		setN(n);
		}

	public double integrate()
		{
		int n = getN();
		if(!isEven(n))
			setN(++n);
		double[] endpoints = getEndpoints();
		double h = calcH(endpoints[0],endpoints[1],n);
		double area = calcArea(n,h);
		return area;
		}

	private double getA()
		{
		return a;
		}

	private void setA(double a)
		{
		this.a = a;
		}

	private double getB()
		{
		return b;
		}

	private void setB(double b)
		{
		this.b = b;
		}

	private int getN()
		{
		return n;
		}

	private void setN(int n)
		{
		this.n = n;
		}

	private double[] getEndpoints()
		{
		double[] endpoints =
			{
			getA(),
			getB(),
			};
		return endpoints;
		}

	private double calcH(double a,double b,int n)
		{
		return (a + b) / (double)n;
		}

	private double calcArea(int n,double h)
		{
		double xi = 0.0;
		double area = 0.0;
		double factor = h / 3D;
		for(int i = 0;i <= n;++i)
			{
			xi = calcXi(i,h);
			if(i == 0 || i == n)
				area += calcFunction(xi);
			else if(isEven(i))
				area += 2D * calcFunction(xi);
			else
				area += 4D * calcFunction(xi);
			}
		return factor * area;
		}

	private double calcXi(int i,double h)
		{
		return getA() + i * h;
		}

	private double calcFunction(double x)
		{
		return Math.sin(x);
		}

	private boolean isEven(int n)
		{
		if(n % 2 == 0)
			return true;
		else
			return false;
		}
	}

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