Generate a random alphanumeric string in Kotlin

Issue: BAEL-1913

core-kotlin/src/test/kotlin/com/baeldung/random/RandomStringUnitTest.kt

@@ -0,0 +1,62 @@
+import org.apache.commons.lang3.RandomStringUtils
+import org.junit.Before
+import org.junit.jupiter.api.BeforeAll
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+import java.security.SecureRandom
+import java.util.concurrent.ThreadLocalRandom
+import kotlin.experimental.and
+import kotlin.streams.asSequence
+import kotlin.test.assertEquals
+
+const val STRING_LENGTH = 10
+const val ALPHANUMERIC_REGEX = "[a-zA-Z0-9]+"
+
+class RandomStringUnitTest {
+    private val charPool : List<Char> = ('a'..'z') + ('A'..'Z') + ('0'..'9')
+
+    @Test
+    fun givenAStringLength_whenUsingJava_thenReturnAlphanumericString() {
+        var randomString = ThreadLocalRandom.current()
+                .ints(STRING_LENGTH.toLong(), 0, charPool.size)
+                .asSequence()
+                .map(charPool::get)
+                .joinToString("")
+
+        assert(randomString.matches(Regex(ALPHANUMERIC_REGEX)))
+        assertEquals(STRING_LENGTH, randomString.length)
+    }
+
+    @Test
+    fun givenAStringLength_whenUsingKotlin_thenReturnAlphanumericString() {
+        var randomString = (1..STRING_LENGTH).map { i -> kotlin.random.Random.nextInt(0, charPool.size) }
+                .map(charPool::get)
+                .joinToString("")
+
+        assert(randomString.matches(Regex(ALPHANUMERIC_REGEX)))
+        assertEquals(STRING_LENGTH, randomString.length)
+    }
+
+    @Test
+    fun givenAStringLength_whenUsingApacheCommon_thenReturnAlphanumericString() {
+        var randomString = RandomStringUtils.randomAlphanumeric(STRING_LENGTH)
+
+        assert(randomString.matches(Regex(ALPHANUMERIC_REGEX)))
+        assertEquals(STRING_LENGTH, randomString.length)
+    }
+
+    @Test
+    fun givenAStringLength_whenUsingRandomForBytes_thenReturnAlphanumericString() {
+        val random = SecureRandom()
+        val bytes = ByteArray(STRING_LENGTH)
+        random.nextBytes(bytes)
+
+        var randomString = (0..bytes.size - 1).map { i ->
+            charPool.get((bytes[i] and 0xFF.toByte() and charPool.size.toByte()).toInt())
+        }.joinToString("")
+
+        assert(randomString.matches(Regex(ALPHANUMERIC_REGEX)))
+        assertEquals(STRING_LENGTH, randomString.length)
+    }
+
+}