Storing X and Y Coordinates in Java

1. Overview

From plotting points on a map to capturing the exact location where a user clicks their mouse, we often need to store (x, y) coordinates.

Java doesn’t have a coordinates construct that’s in common use, although storing coordinates is not that uncommon. In this tutorial, we’ll compare records, POJOs, and arrays as potential ways to store these coordinates and explore the tradeoffs of each.

2. Using a Point Record

Records in Java are designed to be simple data carriers.  Furthermore, they’re immutable and automatically generate constructors, getters, equals(), hashCode(), and toString() methods, significantly reducing boilerplate code.

Thus, records are ideal for representing simple data aggregates, such as (x, y) coordinate pairs.

2.1. Defining a Record

This one line creates the record with all necessary methods:

public record PointRecord(double x, double y) {}

We declare each coordinate as a double because it can hold a decimal value, whereas an int cannot.

2.2. Example Record

Let’s verify that a given PointRecord instance stores the (x, y) coordinates correctly:

@Test
void givenAPointRecord_whenUsingAccessorMethods_thenRecordReturnsCoordinatesCorrectly() {
    PointRecord point = new PointRecord(30.5, 40.0);

    assertEquals(30.5, point.x(), "X coordinate should be 30.5");
    assertEquals(40.0, point.y(), "Y coordinate should be 40.0");
}

In this example, we create a PointRecord instance with coordinates (30.5, 40.0). Then, we test the assertion that its accessor methods return the correct coordinates.

Let’s verify that PointRecords with the same coordinates are equal:

@Test
void givenTwoRecordsWithSameCoordinates_whenComparedForEquality_thenShouldBeEqual() {
    PointRecord point1 = new PointRecord(7.0, 8.5);
    PointRecord point2 = new PointRecord(7.0, 8.5);

    assertEquals(point1, point2, "Records with same coordinates should be equal");
}

3. Using a Point POJO

A POJO is a standard Java class that encapsulates data. This approach is flexible and widely understood as it’s a classic object-oriented solution.

3.1. Defining a Point Class

We define private fields for the coordinates and public methods (getters) to access them:

public class PointPojo {
    private final double x;
    private final double y;

    public PointPojo(double x, double y) {
        this.x = x;
        this.y = y;
    }

    // standard setters and getters

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        PointPojo point = (PointPojo) o;
        return Double.compare(point.x, x) == 0 &&
          Double.compare(point.y, y) == 0;
    }

    @Override
    public int hashCode() {
        return Objects.hash(x, y);
    }
}

3.2. Example Point

Let’s verify that a given PointPojo instance stores the (x, y) coordinates correctly:

@Test
void givenAPoint_whenUsingGetter_thenPointReturnsCoordinatesCorrectly() {
    PointPojo point = new PointPojo(10.0, 20.5);

    assertEquals(10.0, point.getX(), "X coordinate should be 10.0");
    assertEquals(20.5, point.getY(), "Y coordinate should be 20.5");
}

In this example, we create a PointPojo instance with coordinates (10.0, 20.5). Afterward, we test the assertion that its getters return the correct coordinates. Additionally, we verify that PointPojos with the same coordinates are equal:

@Test
void givenTwoPointsWithSameCoordinates_whenComparedForEquality_thenShouldBeEqual() {
    PointPojo point1 = new PointPojo(5.1, -3.5);
    PointPojo point2 = new PointPojo(5.1, -3.5);
        
    assertEquals(point1, point2, "Points with same coordinates should be equal");
}

4. Using java.awt

Java’s Abstract Window Toolkit (AWT) library includes a class specifically for this purpose. The AWT library is especially useful in GUI applications.

AWT has a class called java.awt.Point; however, it uses only integers. The java.awt.Point2D.Double class offers double precision.

Unless we are building an AWT-based app, though, we should favor other solutions.

5. Using an Array

The most minimalistic approach is to use a double array of two elements. By convention, the first element stores the x coordinate, and the second element stores the y coordinate.

To demonstrate, let’s use an array to represent coordinates (15.0, 25.0):

@Test
void givenArrayOfCoordinates_whenAccessingArrayIndices_thenReturnsCoordinatesAtCorrectIndices() {
    double[] coordinates = new double[2];
    coordinates[0] = 15.0; // x
    coordinates[1] = 25.0; // y

    assertEquals(15.0, coordinates[0], "Index 0 should be the X coordinate");
    assertEquals(25.0, coordinates[1], "Index 1 should be the Y coordinate");
}

6. Choosing the Best Method

All four methods may be suitable depending on our use case. Primarily, we need to consider how we want to use the (x, y) coordinates once stored.

Accordingly, if we want to extend our data structure or are using Java 15 or earlier, we should create a POJO. One example of this would be extending our PointPojo class to take on three coordinates (x, y, z). A downside is the extra boilerplate.

Alternatively, we can choose records if we are using Java 16 or later. These are succinct and well-suited to the task.

Further, if our top priority is to be fast, we can sacrifice encapsulation and readability for performance by using a 2-cell double array. However, we need to be aware of how we implement an array since nothing enforces that the array must have exactly two elements or which index corresponds to which coordinate.

7. Conclusion

In this article, we explored four ways of storing a pair of (x, y) coordinates representing a point in Java in the Cartesian coordinate system.