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| Credit: Android Academics |
What is a Data Class in Kotlin?
toString(),
equals(), hashCode(), and copy() for
you, significantly reducing boilerplate code.
Key Features of Data Classes
- Primary Constructor: A Kotlin data class must have at least one parameter in the primary constructor.
-
Val/Var Properties: The properties defined in the primary
constructor are automatically declared as
val(read-only) orvar(mutable). -
Generated Functions: The compiler generates standard
functions like
equals(),hashCode(), andtoString().
Syntax of a Data Class
equals(), hashCode(), and
toString() methods based on the properties defined in the
primary constructor.
data class User(val id: Int, val name: String, val age: Int)
User data class with three
properties: id, name, and email.
Equivalent Java Class
User class:
public class User {
private int id;
private String name;
private String email;
public User(int id, String name, String email) {
this.id = id;
this.name = name;
this.email = email;
}
public int getId() {
return id;
}
public String getName() {
return name;
}
public String getEmail() {
return email;
}
@Override
public String toString() {
return "User{id=" + id + ", name='" + name + "', email='" + email + "'}";
}
@Override
public boolean equals(Object obj) {
if (this == obj) return true;
if (!(obj instanceof User)) return false;
User user = (User) obj;
return id == user.id &&
name.equals(user.name) &&
email.equals(user.email);
}
@Override
public int hashCode() {
return Objects.hash(id, name, email);
}
}
A POJO in Java is a regular class that contains fields, constructors, and getter/setter methods. It doesn’t provide any methods automatically, so you have to implement them manually.
Comparing Features: Kotlin Data Class vs. Java POJO
1. Boilerplate Code
Kotlin Data Classes: The data class automatically generates
methods like toString(), equals(), and
hashCode(), drastically reducing boilerplate.
Java POJOs: You must manually implement these methods, leading to more lines of code and potential errors.
2. Immutability
val (immutable) or
var (mutable). This encourages the use of immutable data
objects, which can lead to safer code.
data class User(val name: String, val age: Int)
Java POJOs: To control mutability, explicitly define getters (and setters for mutable properties).
3. Copying Objects
copy() function
allows for easy duplication of instances with modifications.
data class User(val id: Int, val name: String, val email: String)
data class User(val id: Int, val name: String, val email: String)
4. Destructuring Declarations
val user = User("Alice", 30)
val (name, age) = user
Java POJO: Java does not have built-in support for destructuring. You need to manually access the fields using getter methods.
5. Default Values
data class User(val name: String = "Unknown", val age: Int = 0)
Java POJO: To achieve similar functionality in Java, you’d need to implement multiple constructors or provide default values in the fields.
Example of Using the Kotlin Data Classes
Now, let's create an instance of the User class and see the
generated methods in action:
Kotlin Usage
fun main() {
val user1 = User(1, "Alice", "alice@example.com")
val user2 = User(2, "Bob", "bob@example.com")
// Print the user details
println(user1) // Output: User(id=1, name=Alice, email=alice@example.com)
// Check equality
val user3 = User(1, "Alice", "alice@example.com")
println(user1 == user3) // Output: true
// Using the copy method
val user4 = user1.copy(name = "Alice Smith")
println(user4) // Output: User(id=1, name=Alice Smith, email=alice@example.com)
}
Java Usage
public class Main {
public static void main(String[] args) {
User user1 = new User(1, "Alice", "alice@example.com");
User user2 = new User(2, "Bob", "bob@example.com");
// Print user details
System.out.println(user1);
// Check equality
User user3 = new User(1, "Alice", "alice@example.com");
System.out.println(user1.equals(user3)); // true
// Copy and modify (requires a new constructor)
User user4 = new User(user1.getId(), "Alice Smith", user1.getEmail());
System.out.println(user4);
}
}
Explanation of the Example
-
Creating Instances: We create instances of the
Userclass with different data. -
Printing the Object: The
toString()method automatically formats the object as a string. -
Checking Equality: The
equals()method allows us to compare two instances for value equality. -
Copying an Object: The
copy()method allows for creating a new instance with some properties modified.
Conclusion
Data classes in Kotlin provide a powerful and efficient way to handle data. They reduce boilerplate code, promote immutability, and come with built-in functionality that simplifies data management. Whether you're building a small application or a large-scale system, understanding kotlin data classes will enhance your Kotlin programming experience.