What is the difference between:

Definition 1

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

Definition 2

class Person (var name:String, var age:Int)

Definition 3

class Person (){
    var name:String = ""
    var age:Int = 1
}

In the 3 cases when I use the autocomplete, I saw the same methods available like a POJO... is this the same but in 3 different ways?

Difference in equals, hashCode, & toString

the most important difference between definition 1 and definitions 2 & 3 is that in definition 1, the equals, hashcode and toString methods are overridden for you:

• equals and hashCode methods test for structural equality
• toString method returns a nice, human-friendly string

Code example:

NOTE: in Kotlin, the == operator calls an object's .equals() method. see operator overloading on kotlinlang.org for more info.

data class Person1 (var name:String, var age:Int)
class Person2 (var name:String, var age:Int)

@Test fun test1()
{
    val alice1 = Person1("Alice", 22)
    val alice2 = Person1("Alice", 22)
    val bob = Person1("bob", 23)

    // alice1 and alice2 are structurally equal, so this returns true.
    println(alice1 == alice2)   // true

    // alice1 and bob are NOT structurally equal, so this returns false.
    println(alice1 == bob)      // false

    // the toString method for data classes are generated for you.
    println(alice1)     // Person1(name=Alice, age=22)
}

@Test fun test2()
{
    val alice1 = Person2("Alice", 22)
    val alice2 = Person2("Alice", 22)
    val bob = Person2("bob", 23)

    // even though alice1 and alice2 are structurally equal, this returns false.
    println(alice1 == alice2) // false
    println(alice1 == bob)    // false

    // the toString method for normal classes are NOT generated for you.
    println(alice1)  // Person2@1ed6993a
}

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Difference in constructors

another difference between definitions 1 & 2 and definition 3 is that:

• definitions 1 & 2 both have a constructor that takes 2 parameters
• definition 3 only has a no argument constructor that assigns default values to the class members.

Code example:

data class Person1 (var name:String, var age:Int)
class Person2 (var name:String, var age:Int)
class Person3 ()
{
    var name:String = ""
    var age:Int = 1
}

@Test fun test3()
{
    Person1("alice",22)     // OK
    Person2("bob",23)       // OK
    Person3("charlie",22)   // error

    Person1()   // error
    Person2()   // error
    Person3()   // OK
}

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The copy method

Finally, another difference between definition 1 and definitions 2 & 3 is that in definition 1, a copy method is generated for it. Here's an example of how it can be used:

val jack = Person1("Jack", 1)
val olderJack = jack.copy(age = 2)

// jack.age = 1
// olderJack.age = 2

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Check out the official documentation for data classes on kotlinlang.org!