My first java programs. Part 2

first_program2.java

import javax.swing.JFrame;
import org.math.plot.*;

public class Integral 
{
	private double a;
	private double b;
	private double c;
	
	// Constructor of the class
	public Integral(double a, double b, double c)
	{
		this.a = a;
		this.b = b;
		this.c = c;
	}
	
	// Returs a string with a simple description of what this class can do 
	public String description()
	{
		String content = "This class lets you generate lines and parabolas"
				+ "\nFurthermore you can:"
				+ "\nCalculate definite integrals"
				+ "\nEvaluate functions"
				+ "\nPlot functions"
				+ "\nOutput data (txt format)";
		return content;
	}
	
	// Lets you evaluate the line at x and optionally print the result (print=true)
	public double line(double x,boolean print)
	{
		double value = a*x + b;
		if(print)
		{
			System.out.println("Line: the value of y at x="+x+" is equal to: "+value);
		}
		return value;
	}

	
	// Same as above but for the parabola
	public double parabola(double x, boolean print)
	{
		double value = a*Math.pow(x,2) + b*x + c;
		if(print)
		{
			System.out.println("Parabola: the value of y at x="+x+" is equal to: "+value);
		}
		return value;
	}
	
	// Evaluate definite integral with a finite number of steps
		public double approxIntegral(double initialp, double finalp, double steps, String function)
		{
			double sum = 0;
			double interval = (finalp-initialp)/steps;
			
			if(function.equals("line"))
			{
				while(initialp < finalp)
				{
					sum = sum + interval*this.line(initialp,false);
					initialp += interval;			
				}
				return sum;
			}else if(function.equals("parabola"))
			{
				while(initialp < finalp)
				{
					sum = sum + interval*this.parabola(initialp,false);
					initialp += interval;			
				}
				return sum;
			}else
			{
				String error = "Error, please insert either of the following:\nparabola\nline";
				System.out.println(error);
				return 0;
			}
		}
		
		// Plot the functions
		public void plotFunction(String function,double from, double to,int npoints)
		{
			
			if((from >= to) || (from==to) || (npoints==0))
			{
				System.out.println("Check input please! Something is wrong.");
				return;
			}
			
			double step = Math.abs((to-from)/npoints);
			
			Plot2DPanel plot = new Plot2DPanel();
			double x[] = new double[npoints];
			double y[] = new double[npoints];
			
			if(function.equals("line"))
			{
				for(int i=0; i < npoints; i++ )
				{
					x[i] = from;
					y[i] = this.line(from,false);
					from += step;
				}
				plot.addLinePlot("Line 1",x,y);
				
			}else if(function.equals("parabola"))
			{
				for(int i=0; i < npoints; i++ )
				{
					x[i] = from;
					y[i] = this.parabola(from,false);
					from += step;
				}
				plot.addLinePlot("Parabola 1",x,y);
				
			}else if(function.equals("both"))
			{
				double y1[] = new double[npoints];
				
				for(int i=0; i < npoints; i++)
				{
					x[i] = from;
					y[i] = this.parabola(from, false);
					y1[i] = this.line(from, false);
					from += step;
				}
				plot.addLinePlot("Parabola 1",x,y);
				plot.addLinePlot("Line1",x,y1);
				
			}else
			{
				System.out.println("Please check your input, function's name seems not ok.");
				return;
			}
			
			// Plot name and frame settings
			plot.setAxisLabel(0,"X");
			plot.setAxisLabel(1,"Y");
			JFrame frame = new JFrame("Graph");
			frame.setContentPane(plot);
			frame.setVisible(true);
			return;
		}
		
		
		// Find intersections between line and parabola
		void findIntersections()
		{	
			double delta = Math.pow(this.b-this.a, 2)-4*this.a*(this.c-this.b);
			if(delta < 0)
			{
				System.out.println("Delta is negative, no solutions!");
			}else
			{
				double x1 = (this.a - this.b + Math.sqrt(delta))/(2*this.a);
				double x2 = (this.a - this.b - Math.sqrt(delta))/(2*this.a);
				System.out.println("Intersections at:"
							+ "\nx1: "+x1 + "y1: "+this.parabola(x1,false)
							+ "\nx2: "+x2 + "y2: "+this.parabola(x2,false));
			}
		}
		
		// Self explanatory ;)
		void findIntersectionsWithXaxis(String function)
		{
			if(function.equals("parabola"))
			{
				if(this.c==0)
				{
					double x2 = -this.b/this.a;
					System.out.println("x1: 0 y1: 0");
					System.out.println("x2: "+x2+" y2: "+this.parabola(x2,false));
				}
				else
				{
					double delta = Math.pow(this.b, 2)-4*this.a*this.c;
					if(delta < 0)
					{
						System.out.println("Negative delta, no solutions!");
						return;
					}else
					{
					double x1 = (-this.b + Math.sqrt(delta))/(2*this.a);
					double x2 = (-this.b - Math.sqrt(delta))/(2*this.a);
					System.out.println("x1: "+x2+"y1: "+this.parabola(x1,false));
					System.out.println("x2: "+x2+"y2: "+this.parabola(x2,false));
					}
				}
			}else if(function.equals("line"))
			{
				double x1 = -this.b/this.a;
				System.out.println("x1: "+x1+"y1: "+this.line(x1,false));
			}
		}
}