Aren t == and `equals` synonyms in kotlin ```data class Foo kotlinlang #getting-started

data class Foo(val value: Double)

Foo(3.0) == 3.0  // fails to compile with "Operator '==' cannot be applied to 'Foo' and 'Double'"
Foo(3.0).equals(3)  // compiles

Vlad Krava

01/21/2022, 1:56 PM

It is indeed, however, Kotlin here introduces a type check on a compiler level, unlike

equals

method (function), which needs to do this when overriding it according to best practices Think of this behaviour as like a safeguard against naughty developers 😅

holgerbrandl

01/21/2022, 1:58 PM

But could I add a covariant(?) version of `equals`to allow comparing

Foo

against other types with

==

Vlad Krava

01/21/2022, 2:04 PM

For comparison of

Foo

Boo

let’s say better use your custom function (not
equals
though but some sort of
equivalent
) Leave

==

and

equals

for equality-check of the same time.

Joffrey

01/21/2022, 2:15 PM

It must be noted that

equals()

has to be symmetric by contract. If

Foo(3.0).equals(3.0)

is true, then

3.0.equals(Foo(3.0))

should be true too. But, since you can't override

Double.equals()

3.0.equals(Foo(3.0))

will be false. So, your

Foo.equals()

is technically broken if you accept

Double

there. So I agree with Vlad, you should probably use your own custom method for it.

➕ 2

holgerbrandl

01/21/2022, 2:39 PM

Thanks for the clarification!

ephemient

01/21/2022, 9:53 PM

==

and

equals

are not exactly synonyms in some corner cases:

Double.NaN == Double.NaN // false, as defined by IEEE 754
Double.NaN.equals(Double.NaN) // true

0.0 == -0.0 // true, as defined by IEEE 754
0.0.equals(-0.0) // false

although you can force the use of

equals

Double.NaN as Any == Double.NaN // true
0.0 as Any == -0.0 // false

showing that this is really due to Double trying to maintain consistency between

.hashCode()

and

.equals()

, which necessarily breaks consistency between boxed

.equals()

and primitive

==

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