HW2.java

import java.util.Scanner;
import java.io.*;

public class HW2 {
  public static void main(String[] args) throws IOException {
    BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));

    String list1 = bf.readLine();
    String[] l1 = list1.split(" ");
    double[] li1 = new double[l1.length];
    for (int i = 0; i < l1.length; i++) {
      li1[i] = Double.valueOf(l1[i]);
    }

    double x1 = li1[0]; 
    double y1 = li1[1]; 
    double x2 = li1[2]; 
    double y2 = li1[3];
    double x3 = li1[4]; 
    double y3 = li1[5]; 
    double x4 = li1[6]; 
    double y4 = li1[7]; 

    isIntersect(x1,y1,x2,y2,x3,y3,x4,y4);
  }

  public static double getSlope(double x1, double y1, double x2, double y2) {
    return (y2 - y1) / (x2 - x1);
  }

  public static void isIntersect(double x1, double y1, double x2, double y2, double x3, double y3, double x4, double y4) {
    
    double m1 = getSlope(x1,y1,x2,y2);
    double m2 = getSlope(x3,y3,x4,y4);

    double x1_const = -(m1 * x1) + y1;
    double x2_const = -(m2 * x3) + y3;

    double x = (x2_const - x1_const) / (m1 - m2);
    double y = m1 * (x - x1) + y1;
    
    if (m1 == Double.POSITIVE_INFINITY || m1 == Double.NEGATIVE_INFINITY) {
      x = x1;
      y = m2 * (x - x3) + y3;
    } else if (m2 == Double.POSITIVE_INFINITY || m2 == Double.NEGATIVE_INFINITY) {
      x = x3;
      y = m1 * (x - x1) + y1;
    } 

    if (!Double.isNaN(x) || !Double.isNaN(y)) {
      if (x > x1 && x > x2 || x < x1 && x < x2) {
        System.out.println("No intersect");
      } else if (y > y1 && y > y2 || y < y1 && y < y2) {
        System.out.println("No intersect");
      } else if (x > x3 && x > x4 || x < x3 && x < x4) {
        System.out.println("No intersect");
      } else if (y > y3 && y > y4 || y < y3 && y < y4) {
        System.out.println("No intersect");
      } else {
        System.out.println("Intersect at (" + x + ", " + y + ").");
      }
    } else {
      System.out.println("Two line parallel.");
    }
  }
}