public class Factorial
{
    public int fact(int num)
    {   int temp;
        // base case:
        if (num == 1)
            return 1;
        else
        {   temp = num * fact(num - 1);
            return temp;
        }  
    }
    public static void main(String[] args)
    {   Factorial f = new Factorial();
        int result = f.fact(5);
        System.out.println("result: " + result);        
    }
}

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// find the sum of the first n positive integers
public class Sum
{
    public int findSum(int num)
    {   // base case:
        if (num == 1) return 1;
        else
            return num + findSum(num - 1);        
    }
    public static void main(String[] args)
    {   Sum s = new Sum();
		// remember Gauss!
        int result = s.findSum(100); 
        System.out.println("result: " + result);
    }
}

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import javax.swing.JOptionPane;

public class Triangle
{   private int width;
    
    public Triangle(int a_width)
    {   width = a_width; }
    
    public int getArea()
    {   if (width == 1) return 1;
        Triangle tri2 = new Triangle(width - 1);
        int area2 = tri2.getArea();
        return width + area2;
    }
        
    public static void main(String[] args)
    {   String widthStr = JOptionPane.showInputDialog("Triangle width?");    
        int width = Integer.parseInt(widthStr);
        Triangle tri = new Triangle(width);
        int area = tri.getArea();
        String msg = "Triangle with width " + width;
        msg += " has an area of " + area;
        System.out.println(msg);
    }
}