LinearEquation v3 by SbStudios

//Linear Equation Creator v3 (C)SbStudios, 2011.

//NOW WITH LINEAR (WEIRD) STANDARD FORM!

public class LinearEquation
{
    protected double slope, yInt, x1, x2, y1, y2;

    protected Boolean hasPoints = false, hasSlope = false,hasYInt = false;

    public LinearEquation(double start_slope, double start_yInt)
    {
        slope = start_slope;
        yInt = start_yInt;

        hasYInt = true;

        hasSlope = true;
    }

    public LinearEquation(double start_x1, double start_x2, double start_y1, double start_y2)
    {
        x1 = start_x1;
        x2 = start_x2;

        y1 = start_y1;
        y2 = start_y2;

        hasPoints = true;
    }

    public void calcSlope()
    {
        if (hasPoints)
        {
            slope = ((y2-y1)/(x2-x1));
            hasSlope = true;
        }

        else { System.out.println("Points haven't been given, slope not calculated."); }
    }

    public void calcYInt()
    {
        if (hasPoints&&hasSlope)
        {
            yInt = y1 - (slope*x1);
            hasYInt = true;
        }

        else { System.out.println("ERROR! This doesn't have: Points OR Slope"); }
    }

    public String getEquation()
    {
        if (hasYInt && hasSlope)
        {
            String desc = ("y="+slope+"x+"+yInt);
            return desc;
        }

        else
        {
            String descError = ("ERROR! This doesn't have: y Intercept OR Slope");
            return descError;
        }
    }

    public String getLOLequation()
    {
        if (hasYInt && hasSlope)
        {
            double negativeYInt = -1*yInt;

            String desc = (slope+"x"+"-y="+negativeYInt);
            return desc;
        }

        else
        {
            String descError = ("ERROR! This doesn't have: y Intercept OR Slope");
            return descError;
        }
    }

    public double getSlope() {return slope;}
    public double getYInt() {return yInt;}
    public double getX1() {return x1;}
    public double getX2() {return x2;}
    public double getY1() {return y1;}
    public double getY2() {return y2;}
}

//Linear Equation Creator v3 - BASIC TESTER (C)SbStudios, 2011.

import java.util.Scanner;

public class MainClass
{
    public static void main (String args[])
    {
        Scanner scan = new Scanner(System.in);

        System.out.println("LinEq Calc v2.0. Do you have:\n"
                + "\n"
                + "1) The slope and y Intercept\n"
                + "\n"
                + "2) x1, y1, and x2, y2\n");

        String option = scan.next();

        if (option.equals("1"))
        {
            System.out.println("Slope = ?");

            String initSlope = scan.next();

            //~~~~~~~~~~~~~~~~~~~~~~

            System.out.println("Y Intercept = ?");

            String initYInt = scan.next();

            //~~~~~~~~~~~~~~~~~~~~~~

            double slope = Double.parseDouble(initSlope);

            double yInt = Double.parseDouble(initYInt);

            //~~~~~~~~~~~~~~~~~~~~~~

            LinearEquation myEQ = new LinearEquation(slope,yInt);

            //~~~~~~~~~~~~~~~~~~~~~~

            String desc = myEQ.getEquation();

            System.out.println("Polynomial Standard Form Equation: "+desc);

            String desc2 = myEQ.getLOLequation();

            System.out.println("Standard Form Equation: "+desc2);

            //~~~~~~~~~~~~~~~~~~~~~~

            System.exit(0);
        }

        else if (option.equals("2"))
        {
            System.out.println("x1 = ?");

            String initx1 = scan.next();

            //~~~~~~~~~~~~~~~~~~~~~~

            System.out.println("y1 = ?");

            String initY1 = scan.next();

            //~~~~~~~~~~~~~~~~~~~~~~

            System.out.println("x2 = ?");

            String initx2 = scan.next();

            //~~~~~~~~~~~~~~~~~~~~~~

            System.out.println("y2 = ?");

            String initY2 = scan.next();

            //~~~~~~~~~~~~~~~~~~~~~~


            double x1 = Double.parseDouble(initx1);
            double Y1 = Double.parseDouble(initY1);

            double x2 = Double.parseDouble(initx2);
            double Y2 = Double.parseDouble(initY2);

            //~~~~~~~~~~~~~~~~~~~~~~

            LinearEquation myEQ = new LinearEquation(x1,x2,Y1,Y2);

            myEQ.calcSlope();
            myEQ.calcYInt();

            //~~~~~~~~~~~~~~~~~~~~~~

            String desc = myEQ.getEquation();

            System.out.println("Polynomial Standard Form Equation: "+desc);

            String desc2 = myEQ.getLOLequation();

            System.out.println("Standard Form Equation: "+desc2);

            //~~~~~~~~~~~~~~~~~~~~~~

            System.exit(0);
        }

        else
        {
            System.out.println("Please input a valid number (1 or 2). Quitting...");
            System.exit(0);
        }
    }
}