A quick question regarding `data class` and `set.c...

# getting-started

[Kai Yuan]

A quick question regarding data class and set.contains(e)
If I didn't understand it wrong, set.contains(e) checks elementInSet.equals(e). Also, data class creates equals() method for the properties in the primary constructor.
Now, I'm wondering why I get this output:

data class Foo(var v: Int)

fun test() {
    val mySet = mutableSetOf(Foo(1))
    mySet.forEach {
        it.v += 1
    }
    println("mySet contains: $mySet")
    val f = Foo(2)
    println("$f in Set? ${f in mySet}")
    println("equals? ${f == mySet.toList().first()}")
}

Output:

mySet contains: [Foo(v=2)]
Foo(v=2) in Set? false
equals? true

Why f in mySet returns false?

[mkrussel]

mutableSetOf creates a linked hash set and it not suitable for storing mutable objects. I'm not aware of any efficient set that can handle mutable objects.

When you change v you change the hashcode of the Foo object and the set can no longer find it.

[Kai Yuan]

I also thought about the hashcode diff. However, I checked the Java Set.contains, it says

boolean contains(Object o)

Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that (o==null ? e==null : o.equals(e)).

[mkrussel]

It does use equals, but it uses hashcode to reduce the number of items to consider for equals. So it will only use equals when the hashcode puts them in the same bucket.

[CLOVIS]

The hashCode contract ensures that using the hash code to make fast equality checks is always legal, so it doesn't have to be documented everywhere

It's not legal in Java or Kotlin to add an object whose identity changes over time as a key (value in a set, key in a map…)

[Kai Yuan]

Does it mean Kotlin's Set.contains() may behave different from Java's Set.contains() ?

[CLOVIS]

Set is an interface, HashSet and LinkedHashSet don't behave the same way.

[mkrussel]

No Java's HashSet behaves the same way, and TreeSet actual breaks that contract.

[CLOVIS]

"returns true if and only if this set contains an element e such that (o==null ? e==null : o.equals(e)).

Does not mean that the function calls only equals. It means that, for a well implemented object, it behaves the same way as if it did.

[CLOVIS]

All Set implementations in Java and Kotlin do respect that contract

[Kai Yuan]

Thanks guys. I think I misunderstood the javaDoc, I thought it calls equals() literally.
So the lesson I learned is to avoid using Set to store mutable elements.

[CLOVIS]

All implementations are allowed to make shortcuts in their actual code, as long as the result is the same (and that's what you're seeing here: you broke the "storing mutable elements", and it was relying on that rule to make the equals check faster)

[mkrussel]

Or don't modify them after storing them.

Date is a mutable object, but I would still store them in Sets because I know that I would not modify them and I pretended they were immutable.

[mkrussel]

From the Set class documentation.

Note: Great care must be exercised if mutable objects are used as set elements. The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set. A special case of this prohibition is that it is not permissible for a set to contain itself as an element.

[Kai Yuan]

Yep, it took me half day to finally spot the cause in my real project. Really hard to find ... 💢

[CLOVIS]

One of the reasons many people advocate for using only immutable objects 🙂