KTLN-899: Format number with thousand separator in Kotlin

core-kotlin-modules/core-kotlin-12/pom.xml

@@ -14,6 +14,21 @@
         <version>1.0.0-SNAPSHOT</version>
     </parent>
 
+    <dependencies>
+        <dependency>
+            <groupId>io.kotest</groupId>
+            <artifactId>kotest-runner-junit5</artifactId>
+            <version>${kotest.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>io.kotest</groupId>
+            <artifactId>kotest-property-jvm</artifactId>
+            <version>${kotest.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
     <build>
         <sourceDirectory>src/main/kotlin</sourceDirectory>
         <testSourceDirectory>src/test/kotlin</testSourceDirectory>
@@ -34,4 +49,10 @@
             </plugin>
         </plugins>
     </build>
+
+    <properties>
+        <kotest.version>5.9.0</kotest.version>
+        <maven-surefire-plugin.version>2.22.2</maven-surefire-plugin.version>
+    </properties>
+
 </project>

core-kotlin-modules/core-kotlin-12/src/main/kotlin/com/baeldung/formatting/NumberFormat.kt

@@ -0,0 +1,43 @@
+package com.baeldung.formatting
+
+import java.text.DecimalFormat
+import java.text.DecimalFormatSymbols
+import java.util.*
+
+/**
+ * Interface for the different formatting implementation.
+ */
+interface NumberFormat {
+
+    /**
+     * Returns provided number as string with thousands separator.
+     */
+    fun formatted(number: Int): String
+}
+
+object FormatByDecimalFormat : NumberFormat {
+    override fun formatted(number: Int): String =
+        DecimalFormat("#,###")
+            .format(number)
+            .replace(",", ".")
+}
+
+object FormatByDecimalFormatGermany : NumberFormat {
+    override fun formatted(number: Int): String =
+        DecimalFormat("#,###", DecimalFormatSymbols(Locale.GERMANY))
+            .format(number)
+}
+
+object FormatByStringFormat : NumberFormat {
+    override fun formatted(number: Int): String =
+        String.format(Locale.GERMANY, "%,d", number)
+}
+
+object FormatByChunking : NumberFormat {
+    override fun formatted(number: Int): String =
+        number.toString()
+            .reversed() // 15000 -> 00051
+            .chunked(3) // [000] [51]
+            .joinToString(".") // 000.51
+            .reversed() // 15.000
+}

core-kotlin-modules/core-kotlin-12/src/test/kotlin/com/baeldung/formatting/NumberFormatUnitTest.kt

@@ -0,0 +1,50 @@
+package com.baeldung.formatting
+
+import io.kotest.core.spec.style.ShouldSpec
+import io.kotest.property.Arb
+import io.kotest.property.arbitrary.positiveInt
+import io.kotest.property.checkAll
+import org.assertj.core.api.Assertions.assertThat
+
+class NumberFormatUnitTest : ShouldSpec({
+
+    // Dynamic generation of tests for each implementation
+    nameToImplementationPairs.forEach { (name, function) ->
+
+        // Property based test (for each implementation)
+        should("return correctly formatted string with $name implementation") {
+            checkAll(Arb.positiveInt()) { number ->
+                var result = function(number)
+
+                assertThat(result).containsPattern("^(\\d{1,3}(\\.\\d{3})*|\\d+)$")
+                assertThat(number.toString()).isEqualTo(result.replace(".", ""))
+            }
+        }
+
+        givenToExpectedPairs.forEach { (givenNumber, expectedString) ->
+
+            // Parameterised; Example based test
+            should("return '$expectedString' for $givenNumber with $name implementation") {
+                assertThat(function(givenNumber)).isEqualTo(expectedString)
+            }
+        }
+
+    }
+})
+
+// Examples to check against, with given number and expected string
+private val givenToExpectedPairs = listOf(
+    0 to "0",
+    12 to "12",
+    456 to "456",
+    100_000 to "100.000",
+    1_234_567 to "1.234.567"
+)
+
+// Different implementations of the formatting feature with the display name
+private val nameToImplementationPairs = listOf(
+    "FormatByChunking" to { it: Int -> FormatByChunking.formatted(it) },
+    "FormatByStringFormat" to { it: Int -> FormatByStringFormat.formatted(it) },
+    "FormatByDecimalFormatGermany" to { it: Int -> FormatByDecimalFormatGermany.formatted(it) },
+    "FormatByDecimalFormat" to { it: Int -> FormatByDecimalFormat.formatted(it) }
+)