data class in Kotlin

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In this blog, we will learn about the data class in Kotlin. We will also learn how it works internally.

What is a data class in Kotlin?

A data class in Kotlin is a special kind of class mainly used to hold data. It automatically generates useful methods based on the properties we define in the primary constructor. These include methods like equals(), hashCode(), toString(), copy(), and componentN().

All of these happen without writing a lot of boilerplate code.

Note: It automatically generates useful methods based on the properties we define in the primary constructor. It is important to note that we have only mentioned the primary constructor.

Let's understand it with an example:

Let's create a data class Developer:

data class Developer(var name: String, var age: Int)

That's it; our data class is ready.

Now, we will try to understand the above code.

data is the keyword used to create a data class in Kotlin.
We have two properties in the primary constructor: name and age.

Data classes in Kotlin make it easier to store data without all the extra code needed in Java to create a POJO. This means we don't have to write getters, setters, or methods like equals() and hashCode(), making our model classes cleaner and easier to read.

How does it work internally?

The Kotlin compiler automatically generates the methods for a data class. When you declare a class as a data class, the compiler generates the following methods based on the properties defined in the primary constructor:

equals()
hashCode()
toString()
copy()
componentN()

Now, we need to decompile this code. For that, we will have to convert this Kotlin source file to a Java source file.

Steps to convert from Kotlin source file to Java source file and decompile in Android Studio:

Tools > Kotlin > Show Kotlin Bytecode. You will get the bytecode of your Kotlin file.
Now click on the Decompile button to get your Java code from the bytecode.

Here, I have kept only the important lines of the code and removed other lines for brevity.

We will get the following output:

public final class Developer {

   private String name;
   private int age;

   public final String getName() {
      return this.name;
   }

   public final void setName(String var1) {
      this.name = var1;
   }

   public final int getAge() {
      return this.age;
   }

   public final void setAge(int var1) {
      this.age = var1;
   }

   public Developer(String name, int age) {
      super();
      this.name = name;
      this.age = age;
   }

   public final String component1() {
      return this.name;
   }

   public final int component2() {
      return this.age;
   }

   public final Developer copy(@NotNull String name, int age) {
      return new Developer(name, age);
   }

   public String toString() {
      return "Developer(name=" + this.name + ", age=" + this.age + ")";
   }

   public int hashCode() {
      String var10000 = this.name;
      return (var10000 != null ? var10000.hashCode() : 0) * 31 + Integer.hashCode(this.age);
   }

   public boolean equals(Object var1) {
      if (this != var1) {
         if (var1 instanceof Developer) {
            Developer var2 = (Developer)var1;
            if (Intrinsics.areEqual(this.name, var2.name) && this.age == var2.age) {
               return true;
            }
         }

         return false;
      } else {
         return true;
      }
   }
}

Earlier, we used to write all of these methods in Java. Now, in Kotlin, these methods are generated automatically, reducing the need for boilerplate code. The Kotlin compiler handles this for us.

Things to take care of while creating data class:

Primary constructor: Must have at least one parameter.
Properties: All primary constructor parameters must be val or var.
Limitations: The data class cannot be abstract, open, sealed, or inner.

Use Cases:

Model Classes
DTOs (Data Transfer Objects)
Representing State
Immutable Objects

One question for you: Can we extend a data class? For the answer, refer to the limitations mentioned above.

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That's it for now.

Thanks

Amit Shekhar
Co-Founder @ Outcome School

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What is a data class in Kotlin?

How does it work internally?

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