Lambda confusion in Kotlin, it == x vs equals(x)

Damián Rafael Lattenero :

I'm doing a simple test to understand why my code wasn't working.

    val v = listOf("1", "2", "2", "3")
    assertThat((v.filter { equals("2") }).size , `is`(2))

it fails with reason "size is 0"

but when I change it for:

    val v = listOf("1", "2", "2", "3")
    assertThat((v.filter { it == "2" }).size , `is`(2))

In Java I could do:

.filter(Objects::equals)

It returns the expected result. Anybody can understand why is this behaviour???

ysakhno :

What you need to write is

    assertThat((v.filter { it.equals("2") }).size , `is`(2))

although note that IntelliJ will immediately suggest substituting the equals call with == if you have the 'Can be replaced with binary operator' inspection enabled.

Also, no you wouldn't be able to write this specific call as .filter(Objects::equals) in Java, because static Objects#equals takes 2 parameters, while the method filter provides only 1 to its lambda argument. But you still can use function references in Kotlin too (with the appropriate functions for the lambda expected). For instance, you could filter all non-blank strings like this:

    val v = listOf("1", "", "2", "   ", "\t", "3")
    println(v.filter(String::isNotBlank))