Sample for demonstrating extension functions in kotlin

DateExtensions.kt

package extensions

import java.text.SimpleDateFormat
import java.util.*

private val calendar: Calendar by lazy {
    Calendar.getInstance()
}

fun Date.isSunday(): Boolean {
    calendar.time = this
    return calendar.get(Calendar.DAY_OF_WEEK) == Calendar.SUNDAY
}

fun main(args: Array<String>) {
    val Subat_4_1979 = SimpleDateFormat("dd/MM/yyyy").parse("04/02/1979")
    if (Subat_4_1979.isSunday()) {
        println("Bir pazar gunu dogmusum.")
    }
}