In the previous article, we have discussed Java Program to Find Slope of a Line

In this article we will see how to check if three points are collinear or not.

## Java Program to Check if Three Points are Collinear

Before jumping into the program directly, let’s see first how we get the slope of a line.

Approach-1: Formula for area of triangle : 0.5 * [x1 * (y2 - y3) + x2 * (y3 - y1) + x3 * (y1 - y2)] If formula for area of triangle is equal to zero then three points lie on a straight line. Or, we can say 3 points are collinear.

Approach-2: Slope of any pair of points : (y3 - y2)/(x3 - x2) = (y2 - y1)/(x2 - x1) When slope of any pair of points are same as other pair then three points lie on a straight line. Or, we can say 3 points are collinear.

Let’s see different ways to check three points are collinear or not.

### Method-1: Java Program to Check if Three Points are Collinear By Finding Area of Triangle Formed

**Approach:**

- Take the value for three points.
- Find area of triangle formed by using formula
`0.5 * [x1 * (y2 - y3) + x2 * (y3 - y1) + x3 * (y1 - y2)]`

- Check if the area of triangle formed is equal to zero then three points are collinear.

**Program:**

public class Main { // Main method public static void main(String args[]) { double x1 = 1, x2 = 1, x3 = 1, y1 = 1, y2 = 4, y3 = 5; //checkCollinear() method called to check //if 3 points lie on a stright line or not. checkCollinear(x1, y1, x2, y2, x3, y3); } // checkCollinear Method to check if point collinear or not static void checkCollinear(double x1, double y1, double x2, double y2, double x3, double y3) { //calculating area double area = 0.5*(x1 * (y2 - y3) + x2 * (y3 - y1) + x3 * (y1 - y2)); System.out.println("Area formed by triangle:"+area); if (area == 0) System.out.println("Area is equal to zero. So, 3 lines are collinear."); else System.out.println("Area is not equal to zero. So, 3 lines are not collinear."); } }

Output: Area formed by triangle: 0.0 Area is equal to zero. So, 3 lines are collinear.

### Method-2: Java Program to Check if Three Points are Collinear By Finding Slope of Pairs

**Approach:**

- Take the value for three points.
- Check slope of any pair of point is equal to slope of other point i.e.
`((y3 - y2) * (x2 - x1) == (y2 - y1) * (x3 - x2))`

- If both slope are equal then three points are collinear.

**Program:**

import java.io.*; public class Main { // Main Method public static void main (String[] args) { double x1 = 1, x2 = 1, x3 = 1, y1 = 1, y2 = 4, y3 = 5; //checkCollinear() method called to check //if 3 points lie on a stright line or not. checkColliner(x1, y1, x2, y2, x3, y3); } // Methodn to check if 3 points collinear or not static void checkColliner(double x1, double y1, double x2, double y2, double x3, double y3) { if ((y3 - y2) * (x2 - x1) == (y2 - y1) * (x3 - x2)) System.out.println("Slope are equal. So, 3 lines are collinear."); else System.out.println("Slope are not equal. So, 3 lines are not collinear."); } }

Output: Slope are equal. So, 3 lines are collinear.

Have you mastered basic programming topics of java and looking forward to mastering advanced topics in a java programming language? Go with these ultimate Advanced java programs examples with output & achieve your goal in improving java coding skills.

**Related Java Programs:**