Java8 time comparison in Kotlin

Question

I stumbled across some strange behaviour when comparing Java8 time objects. The below does not appear to be valid code.

val t1 = LocalTime.now()
val t2 = LocalTime.now()
val foo: Int = t1 > t2

Yet hovering over the undersquiggled code shows that the overridden function return type is correct:

Any ideas?

t1 > t2 returns a boolean. The > operator uses compareTo under the hood, but it's not compareTo. — gpunto
– gpunto, Commented Apr 14, 2022 at 23:22

SJoshi · Accepted Answer · 2022-04-14 23:59:33Z

2

According to the documentation, when using the natively overloaded operators (comparison operators specifically) - Kotlin doesn't just call compareTo but rather performs a compareTo against 0 (thus ending up as a bool).

https://kotlinlang.org/docs/operator-overloading.html#comparison-operators

Comparison operators

Expression Translated to

a > b a.compareTo(b) > 0

a < b a.compareTo(b) < 0

a >= b a.compareTo(b) >= 0

a <= b a.compareTo(b) <= 0

All comparisons are translated into calls to compareTo, that is required to return Int.

The documentation snippet you attached is admittedly a bit confusing.

answered Apr 14, 2022 at 23:59

SJoshi

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Java8 time comparison in Kotlin

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Comparison operators

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Expression	Translated to
`a > b`	`a.compareTo(b) > 0`
`a < b`	`a.compareTo(b) < 0`
`a >= b`	`a.compareTo(b) >= 0`
`a <= b`	`a.compareTo(b) <= 0`

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Comparison operators

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