Kotlin Christmas

It’s in the small things

A 2 minute read written by
Herman Møyner Lund
10.12.2019

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Kotlin brings with it a lot of good things like Null Safety and full interoperability with Java, but data classes might be one the things you most often notice during day to day work.

Most larger applications, especially those consuming or producing REST apis need lots of DTOs (data transfer objects). These are used to represent plaint data objects which is only used to hold data.

In java an example of such a class could be:

import java.util.Objects;public class ChristmasTree {
    private int height;
    private int radius;
    private int price;public int getHeight() {
        return height;
    }

    public void setHeight(int height) {
        this.height = height;
    }

    public int getRadius() {
        return radius;
    }

    public void setRadius(int radius) {
        this.radius = radius;
    }

    public int getPrice() {
        return price;
    }

    public void setPrice(int price) {
        this.price = price;
    }

    public ChristmasTree(int height, int radius, int price) {
        this.height = height;
        this.radius = radius;
        this.price = price;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        ChristmasTree that = (ChristmasTree) o;
        return height == that.height &&
            radius == that.radius &&
            price == that.price;
    }

    @Override
    public int hashCode() {
        return Objects.hash(height, radius, price);
    }

    @Override
    public String toString() {
        return "ChristmasTree{" +
            "height=" + height +
            ", radius=" + radius +
            ", price=" + price +
            '}';
    }}

It is possible to use third party libraries or an IDE to help with reduce the amount of code, or generate most of the boilerplate code. Still I prefer to not have the extra lines of code, and if you are going to add or remove fields you still have to regenerate constructors, getters/setters and equals()/hashcode() methods.

Kotlins Data classes

Using data classes in Kotlin the same DTO as above becomes:

 data class ChristmasTree(val height: Number, val radius: Number, val price: Number )

The compiler automatically derives

  • getters and setters for mutable properties and getters for immutable properties.
  • equals()
  • hashCode()
  • toString()
  • componentN()
  • copy()

Creating new a new christmas tree object is done like this:

val christmasTree = ChristmasTree(200, 100, 500)

Using and accessing the values is done like this:

println(christmasTree.height)   // 200
println(christmasTree.radius)   // 100
println(christmasTree.price)    // 500

The toString-method prints the values like this:

println(christmasTree.toString()) // ChristmasTree(height=200, radius=100, price=500)

Summary

Kotlin data classes gives us less and more concise code, and with this more maintainable applications.

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