Comparison of `!!` operator, `requireNotNull()`, `checkNotNull()` and null-safe operators: `?.`, `?:`

NotNullAssertionOperator.kt

import org.junit.Test

class NotNullAssertionOperator {

    @Test
    @Throws(Exception::class)
    fun exclamation() {
        fun square(x: Double?): Double = x!!.times(x)

        square(null)
    }

    @Test
    @Throws(Exception::class)
    fun require() {
        fun square(x: Double?): Double = requireNotNull(x) { "Value is null" }.times(x)

        square(null)
    }

    @Test
    @Throws(Exception::class)
    fun check() {
        fun square(x: Double?): Double = checkNotNull(x) { "Value is null" }.times(x)

        square(null)
    }

    @Test
    @Throws(Exception::class)
    fun safe() {
        fun square(x: Double?): Double = x?.times(x) ?: 0.0

        square(null)
    }
}