Data class

One of main motivation of Kotlin is to reduce repetitive coding. One that many of us have spent a significant amount of time and effort is the class mechanism. However, creating classes that primarily hold data still requires a considerable amount of repetitive code. This is where data class simplify Kotlin code and perform common tasks.

Two features of data class

You can define a data class using data keyword. Each constructor parameter must be preceded by var or val.

data class Sample(
  val arg1: Int,
  val arg2: Int
)

val s1 = Sample(10, 20)
val s2 = Sample(10, 20)
print(s1)	// "Sample(arg1=10, arg2=20)"
print(s1 == s2)	// true

Based on the sample above, you can see two features:

1. On print statement, you can notice that the String produced by s1. The data class display objects in a nice, readable format without requiring any additional code.
2. In normal class, in order to achieve the comparison between s1 and s2, we have to implement equals() function. In data classes, this function is automatically generated. It compares the values of all properties specified as constructor parameters.

copy

Another useful function generated for every data class is copy(), which creates a new object containing the data from the current object. However, it also allows you to change selected values in the process.

data class Contact(
  val name: String,
  val number: String,
  val address: String
)

val contact = Contact("Mike", "12345", "US")
val newContact = contact.copy(address = "UK")

All arguments have default values that are equal to the current values, so you provide only the ones you want to replace.

hashCode

hashCode() is used in conjunction with equal() to rapidly look up a Key in a HashMap or HashSet. Creating a correct hashCode() by hand is tricky and error-prone, so let the data class to do it for you.

Conclusion

In conclusion, data class simplifies and reduces a lot of boilerplate code and functions such as copy(), equals() and hashCode(). You should use data class over normal class if the purpose is to purely hold the data.