Java: Test whether two lines PQ and RS are parallel

Last update on May 13 2025 13:00:08 (UTC/GMT +8 hours)

Test If Two Lines Are Parallel

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:

For more Practice: Solve these Related Problems:

• Write a Java program to test if two lines are parallel and also check if they are collinear.
• Write a Java program to determine if two lines are parallel when given in vector form.
• Write a Java program to test if two lines are parallel by comparing their slopes computed from given points.
• Write a Java program to check if two lines are parallel using the cross product of their direction vectors.

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