find all roots (lab)

package lab11;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Lab11
{
    public static double F(double a,double b,double c,double d,double x)
    {
        double result = (x-a)*(x-b)*(x-c)*(x-d);
        return result;

    }
    public static boolean checkSign(double x)
    {
        if(x<0)
        {
            return false;
        }
        return true;
    }
    public static void bisection(double a,double b,double c,double d,double x1,double x2)
    {

        int i = 1;
        double x0;
        double f2,f0,f1,t;
        double ESP = 0.001;

        System.out.println("\n__________________________________________________________________\n");
        System.out.println("\niteration\t x1\t       x2\t x0\t   f1\t   f2\t   f0");
        System.out.println("\n___________________________________________________________________\n");
        do
        {
            x0=(x1+x2)/2;
            f1=F(a,b,c,d,x1);
            f2=F(a,b,c,d,x2);
            f0=F(a,b,c,d,x0);
            System.out.println("\n"+" "+i+" "+x1+" "+x2+" "+x0+" "+f1+" "+f2+" "+f0);
            if(f1*f0<0)
            {
                x2=x0;
            }
            else
            {
                x1=x0;
            }
            i++;
        }
        while(Math.abs(f0)>ESP);
        System.out.println("\n__________________________________________________________\n");
        System.out.println("\n\nRoot = "+x0);
    }
    public static void main(String[] args)
    {
        System.out.println("Enter a,b,c,d,A,B,delX : ");
        Scanner scanner = new Scanner(System.in);

        double a = scanner.nextDouble();
        double b = scanner.nextDouble();
        double c = scanner.nextDouble();
        double d = scanner.nextDouble();
        double A = scanner.nextDouble();
        double B = scanner.nextDouble();
        double delX = scanner.nextDouble();

        double initialStart = A;
        double initialEnd = B;
        double stepSize = delX;
        List<Double> listX =new ArrayList();
        List<Double> listFX = new ArrayList();
        for(double i=initialStart;i<=initialEnd;i=i+stepSize)
        {
            double X = i;
            double FX = F(a,b,c,d,X);
            listX.add(X);
            listFX.add(FX);
        }

        //now find each two points that bracket root
        List<Double> listBracket = new ArrayList();
        for(int i=0;i<listX.size()-1;i++)
        {
            boolean previous = checkSign(listFX.get(i));
            boolean next = checkSign(listFX.get(i+1));
            if(previous!=next)
            {
                listBracket.add(listX.get(i));
                listBracket.add(listX.get(i+1));
            }
        }

        //finally do bisection between them
        for(int i=0;i<listBracket.size();i=i+2)
        {
            double start = listBracket.get(i);
            double end = listBracket.get(i+1);
            start = start -1;
            end = end +1;
            bisection(a,b,c,d,start,end);
            //System.out.println(listBracket.get(i));

        }




    }
}