How to implement a java.util.function.Predicate as Kotlin lambda?

Question

I need to pass a java.util.function.Predicate to a Java function. How can I implement it as Lambda in Kotlin?

The Java-Function I need to call:

public void foo(Predicate<String> p)

Java Lambda implemenation ✔ :

foo(text-> true)

Kotlin Lambda implemenation ❌:

foo{text:String -> true}  
    ^^^^^^^^^^^^ 
Type mismatch.
Required: Predicate<String>
Found:    (String) → Boolean

Kotlin-Version 1.2.21

I just tried this, it works fine. – karandeep singh Feb 16 '18 at 14:26 — karandeep singh, Feb 16 '18 at 14:26
@karandeepsingh which version do you use? – Chriss Feb 22 '18 at 15:06 — Chriss, Feb 22 '18 at 15:06

Chriss · Accepted Answer · 2021-08-12T12:31:42.640

20

Since Kotlin 1.4

foo({text -> true  })

or

foo {text -> true}

Before Kotlin 1.4

These variants work:

foo(Predicate {text -> true  })  
foo(Predicate {true})
foo({true  }as Predicate<String>)

edited Aug 12 '21 at 12:31

answered Feb 22 '18 at 15:06

Chriss

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so ugly we need to specify Predicate in Kotlin – jpganz18 Oct 17 '19 at 12:29
1

@jpganz18 this has been resolved in Kotlin 1.4 – AlexO Dec 10 '20 at 14:56

score 3 · Answer 2 · answered Jul 12 '21 at 11:18

If in case want to declare as a property:

private val normal = Predicate<Int> { true }
private val even = Predicate<Int> { it % 2 == 0 }
private val odd = even.negate()

fun main() {
 println("Normal count ${get(normal)}")
 println("Even count ${get(even)}")
 println("Odd count ${get(odd)}")

}

fun get(predicate: Predicate<Int>): Int {
 val filter = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10).filter { predicate.test(it)}  
 println(filter)
 val map = filter.map { it * 2 }
 println(map)
 return map.sum()
}

sjahan · Answer 3 · 2018-02-16T15:25:35.003

1

It works fine, here is a test that compiles with a Java List and the filter method that takes a Predicate as parameter:

val test = Arrays.asList("Hello", "Bye", "World!")
println(test)
println(test.filter { it.startsWith("W") })

Is the last sample code (foo{text:String -> true}) your code that does not compile?

Between the braces, your are supposed to pass the implementation of the lambda, not the type/interface of it. This part is inferred for you!

edited Feb 16 '18 at 15:25

answered Feb 16 '18 at 14:28

sjahan

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The `filter` function is an extension function in Kotlin and takes actually not a `java.util.function.Predicate` argument, that's why your sample works for your sample but not the one in question. – Chriss Feb 22 '18 at 14:52
Oh! I didn't know that. I'm not currently in front of my computer, but I think I double checked it and IntelliJ confirmed me it was a predicate! Nice to know anyway, thanks for your comment :) – sjahan Feb 23 '18 at 16:00

score 1 · Answer 4 · answered Dec 10 '20 at 14:55

As of Kotlin 1.4, there is support for Single Abstract Method (SAM) conversion for Java interfaces: https://kotlinlang.org/docs/reference/java-interop.html#sam-conversions

From the example in the question, the following works now:

foo { text -> true }

How to implement a java.util.function.Predicate as Kotlin lambda?

4 Answers4