# Java: Test whether two lines PQ and RS are parallel

## Java Basic: Exercise-222 with Solution

Write a Java program to test whether two lines PQ and RS are parallel. The four points are P(x1, y1), Q(x2, y2), R(x3, y3), and S(x4, y4).

Input:
−100 ≤ x1, y1, x2, y2, x3, y3, x4, y4 ≤ 100
Each value is a real number with at most 5 digits after the decimal point.

Sample Solution:

Java Code:

``````// Importing necessary Java utilities for input
import java.util.*;

// Main class named "Main"
class Main {
// Main method to execute the program
public static void main(String args[]){
// Creating a Scanner object for user input
Scanner in = new Scanner(System.in);

// Prompting the user to input coordinates for point P(x1, y1)
System.out.println("Input P(x1, y1), separated by a space.");
double x1 = in.nextDouble(), y1 = in.nextDouble();

// Prompting the user to input coordinates for point Q(x2, y2)
System.out.println("Input Q(x2, y2), separated by a space.");
double x2 = in.nextDouble(), y2 = in.nextDouble();

// Prompting the user to input coordinates for point R(x3, y3)
System.out.println("Input R(x3, y3), separated by a space.");
double x3 = in.nextDouble(), y3 = in.nextDouble();

// Prompting the user to input coordinates for point S(x4, y4)
System.out.println("Input S(x4, y4), separated by a space.");
double x4 = in.nextDouble(), y4 = in.nextDouble();

// Calculating differences between coordinates to represent vectors
double p1 = x2 - x1, p2 = y2 - y1, q1 = x4 - x3, q2 = y4 - y3,
r1 = x3 - x1, r2 = y3 - y1, s1 = x4 - x1, s2 = y4 - y1;

// Checking if the cross product of vectors P-Q and R-S is close to zero
if(Math.abs(p1*q2 - p2*q1) < 1e-9)
// Output if the cross product is close to zero, indicating parallel lines
System.out.println("Two lines are parallel.");
else
// Output if the cross product is not close to zero, indicating non-parallel lines
System.out.println("Two lines are not parallel.");
}
}
```
```

Sample Output:

```Input P(x1,y1),separated by a space.
5 6
Input Q(x2,y2),separated by a space.
4 2
Input R(x3,y3),separated by a space.
5 3
Input S(x4,y4),separated by a space.
5 6
Two lines are not parallel.
```

Flowchart: Java Code Editor: