how do I declare a static val in Kotlin?

# getting-started

[y]

how do I declare a static val in Kotlin?

[Vitor Hugo Schwaab]

class Foo {
    companion object {
        const val BAR = "bar" // Const for primitives
        val someData = SomeData.default
    }
}

Everything inside an object is the equivalent to static in other languages.

Kotlin also supports top-level definitions, so you don't need to define inside a class:

const val BAR = "bar"

class Foo {
    fun doSomething() {
        println("BAR = $BAR")
    }
}

More about it:
• Companion objects
• Objects

[y]

so is it not possible to declare a local static?
for example, a Regex object in a frequently-invoked closure.

[y]

thanks, appreciated.

[Vitor Hugo Schwaab]

You can do something like:

class Parser {
    
    fun isInputValid(input: String): Boolean {
        return REGEX_PATTERN.containsMatchIn(input)
    }
    
    private companion object {
        val REGEX_PATTERN = """[0-9]+""".toRegex()
    }
    
}

[Vitor Hugo Schwaab]

In order to avoid instantiating the Regex multiple times

[y]

I see. that’s what I expected. hoped there was a less boilerplatey way to do this.

[y]

again - thanks. you guys are really helpful

[y]

(this is a general comment about the replies here, heh)

[Landry Norris]

If you want less boilerplate, you can do

class Parser {
    
    fun isInputValid(input: String): Boolean {
        return REGEX_PATTERN.containsMatchIn(input)
    }
}

private val REGEX_PATTERN = """[0-9]+""".toRegex()

[Landry Norris]

This makes REGEX_PATTERN a global variable that is scoped to the file.

[Klitos Kyriacou]

I see that the language designers have gone to some lengths to explicitly prohibit creating companion objects inside a local class.

[Stephan Schroeder]

if you want even less code, you can do

class Parser {
    
    fun isInputValid(input: String): Boolean = REGEX_PATTERN.containsMatchIn(input)
}

private val REGEX_PATTERN = """[0-9]+""".toRegex()

and yes, technically you can also drop the ": Boolean" but I like methods explicitly stating their return value 🤷‍♂️
and the Parser class looks kind of empty, the method isInputValid could be a top level function as well (without being nested in a class), in that case it should have a more descriptive name though.

fun isInputValid(input: String): Boolean =
    REGEX_PATTERN.containsMatchIn(input)

private val REGEX_PATTERN =
    """[0-9]+""".toRegex()

Alternatively you use Parser as kind of a namespace and put isInputValid also into the companion class

class Parser {
    companion object {
        @JvmStatic // not necessary, but an optimization  
        fun isInputValid(input: String): 
            Boolean = REGEX_PATTERN.containsMatchIn(input)
        @JvmStatic
        private val REGEX_PATTERN = """[0-9]+""".toRegex()
    }
}