Added the code for double to string conversion without scientific notation (#859)

* Added the code for double to string conversion without scientific notation

* removed println

Author: yadavan88

core-kotlin-modules/core-kotlin-numbers-2/src/test/kotlin/com/baeldung/decimalformat/DecimalFormatUnitTest.kt

@@ -0,0 +1,71 @@
+package com.baeldung.decimalformat
+
+import org.junit.Test
+import java.math.BigDecimal
+import java.text.DecimalFormat
+import java.text.DecimalFormatSymbols
+import java.text.NumberFormat
+import java.util.*
+import kotlin.test.assertFalse
+
+fun formatDoubleUsingFormat(num: Double): String {
+    return "%.8f".format(Locale.US, num)
+}
+
+fun formatUsingBigDecimal(num: Double): String {
+    return BigDecimal.valueOf(num).toPlainString()
+}
+
+fun formatUsingString(num: Double): String {
+    return String.format(Locale.US, "%.6f", num)
+}
+
+fun formatUsingNumberFormat(num: Double): String {
+    val numberFormat = NumberFormat.getInstance(Locale.US)
+    numberFormat.maximumFractionDigits = 8
+    return numberFormat.format(num)
+}
+
+fun formatUsingDecimalFormat(num: Double): String {
+    val symbols = DecimalFormatSymbols(Locale.US)
+    val df = DecimalFormat("#.###############", symbols)
+    return df.format(num)
+}
+
+class DecimalFormatUnitTest {
+
+    @Test
+    fun `format double using format method`() {
+        val num = 0.000123456789
+        val result = formatDoubleUsingFormat(num)
+        assertFalse(result.contains("E"))
+    }
+
+    @Test
+    fun `format double using bigdecimal method`() {
+        val num: Double = 0.000123456789
+        val result = formatUsingBigDecimal(num)
+        assertFalse(result.contains("E"))
+    }
+
+    @Test
+    fun `format double using DecimalFormat method`() {
+        val num: Double = 0.000123456789
+        val result = formatUsingDecimalFormat(num)
+        assertFalse(result.contains("E"))
+    }
+
+    @Test
+    fun `format double using String method`() {
+        val num: Double = 0.000123456789
+        val result = formatUsingString(num)
+        assertFalse(result.contains("E"))
+    }
+
+    @Test
+    fun `format double using NumberFormat method`() {
+        val num: Double = 0.000123456789
+        val result = formatUsingNumberFormat(num)
+        assertFalse(result.contains("E"))
+    }
+}