Basic Kotlin cheatsheet / read-only

• 💡 Use filter / on top of the page to look for a particular topic

• Unsorted Basics

  • val positives = list.filter { it > 0 }
    
    for ((k, v) in map) {
        println("$k -> $v")
    }
    
    val list = listOf("a", "b", "c") // read-only list
    val map = mapOf("a" to 1, "b" to 2, "c" to 3) // read-only map
    
    // Lazy property
    val p: String by lazy {  
        // compute the string
    }
    
    // Singleton
    object Resource {
        val name = "Name"
    }
    

• • String templates

  • Multiline

• • var a: String = "abc"
    a = null // compilation error
    
    var b: String? = "abc"
    b = null // ok
    
    b?.length // call length if b is not null, return null otherwise
    
    val l = b!!.length // If you really want NPE
    
    val l = b?.length ?: -1  // l = -1 if b is null, or b.length otherwise
    
    fun foo(node: Node): String? {
        val parent = node.getParent() ?: return null
        val name = node.getName() ?: throw IllegalArgumentException("name expected")
        // ...
    }
    
    val aInt: Int? = a as? Int  // Safe type cast
    

• • WHILE

• • Unit = analog of void, can be omitted for return value of a function

  • Default parameters

  • Variable parameters

• • Simplest form, like daemon thread

  • Timeout

  • Cancellable job

  • Async execution of 2 functions