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Sorry if this is obvious but I just realized I wasn’t sure — is equality in a `Set` compared via `equals()`?

Afaik HashSet/HashMap first check with hashCode() then, if hashCodes are similar, use equals()

The Java definition of the `Set` interface defines equality using `equals()` , but the Kotlin definition is lousy here (honestly most of Kotlin's stdlib KDoc is really less precise than the Java counterpart, potentially because it could vary across platforms).

So technically in Kotlin it depends on the `Set`'s implementation, but it's a pretty safe bet to say that equality should be based on `equals()` in most implementations (at least on the JVM). The most common one (`HashSet`) uses `hashCode()` and then `equals()` to break ties as <@U029CN8U0R5> said.

hashCode is required to be consistent with equals (this is documented in both Kotlin <https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/hash-code.html|https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/hash-code.html> and Java <https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#hashCode--)|https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#hashCode--)>
thus hashCode is just an optimization for Set, Map; the collection has the same behavior as if it used just equals

a couple exceptions I just remembered: <https://docs.oracle.com/javase/8/docs/api/java/util/IdentityHashMap.html|java.util.IdentityHashMap> does break the contract that keys use `equals`, because it uses object identity instead. also <https://docs.oracle.com/javase/8/docs/api/java/util/SortedSet.html|SortedSet>/<https://docs.oracle.com/javase/8/docs/api/java/util/SortedMap.html|SortedMap> can be inconsistent with equals depending on the implementation of Comparable.compareTo or Comparator.compare. but the point stands that sets and map keys are generally expected to compare by equals

TIL there is an `IdentityHashMap`, thanks!