Kotlin - reference equality doesn't always work

Question

According to the docs, === performs referential equality.

Given the following referential equality comparisons, this doesn't seem to work in all cases:

val x = "A"
val y = "A"

println(x === y) // "true"
println("A" === "A") // "true"

I would expect both of those cases to return false.

Yet this example returns false as expected:

val x = readLine()!! // "A"
val y = readLine()!! // "A"

println(x === y) // "false"

So why does referential equality comparison work for the latter case, but not the former?

Because two equal string literals are likely to be compiled to the same object. — Oliver Charlesworth, Dec 19 '18 at 21:14
to add to the link, the difference here is that it is not known at compile time what readLine() will read — Tim, Dec 19 '18 at 21:16

score 3 · Accepted Answer · answered Dec 19 '18 at 21:17

The === basically means "are these objects of the same type and do they point to the same memory address?"

In your first example, both x and y point to a constant A, which, as a String constant, there is a single instance of, so they return true.

When you read from a file, an allocation takes place for the string read, and thus, x and y point to different memory addresses, so they are equal (== returns true), but not identical (=== return false).

Kotlin - reference equality doesn't always work

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