Bael 756 (#1513)

* BAEL-756 code for kotlin null-safety article

* BAEL-756 fix typo

* BAEL-756 increment version of Kotlin

* BAEL-756 remove duplicate form pom

kotlin/pom.xml

@@ -90,10 +90,9 @@
 
     <properties>
         <junit.version>4.12</junit.version>
-        <kotlin-maven-plugin.version>1.0.6</kotlin-maven-plugin.version>
-        <kotlin-test-junit.version>1.0.6</kotlin-test-junit.version>
-        <kotlin-stdlib.version>1.0.6</kotlin-stdlib.version>
-        <kotlin-maven-plugin.version>1.0.6</kotlin-maven-plugin.version>
+        <kotlin-maven-plugin.version>1.1.1</kotlin-maven-plugin.version>
+        <kotlin-test-junit.version>1.1.1</kotlin-test-junit.version>
+        <kotlin-stdlib.version>1.1.1</kotlin-stdlib.version>
     </properties>
 
 </project>

kotlin/src/test/kotlin/com/baeldung/kotlin/NullSafetyTest.kt

@@ -0,0 +1,140 @@
+package com.baeldung.kotlin
+
+import org.junit.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertNull
+import kotlin.test.assertTrue
+
+
+class NullSafetyTest {
+    @Test
+    fun givenNonNullableField_whenAssignValueToIt_thenNotNeedToCheckAgainstNull() {
+        //given
+        var a: String = "value"
+        //a = null compilation error
+
+        //then
+        assertEquals(a.length, 5)
+    }
+
+    @Test
+    fun givenNullableField_whenReadValue_thenNeedToCheckAgainstNull() {
+        //given
+        var b: String? = "value"
+        b = null
+
+        //when
+        if (b != null) {
+
+        } else {
+            assertNull(b)
+        }
+    }
+
+    @Test
+    fun givenComplexObject_whenUseSafeCall_thenShouldChainCallsResultingWithValue() {
+        //given
+        val p: Person? = Person(Country("ENG"))
+
+        //when
+        val res = p?.country?.code
+
+        //then
+        assertEquals(res, "ENG")
+    }
+
+    @Test
+    fun givenComplexObject_whenUseSafeCall_thenShouldChainCallsResultingWithNull() {
+        //given
+        val p: Person? = Person(Country(null))
+
+        //when
+        val res = p?.country?.code
+
+        //then
+        assertNull(res)
+    }
+
+    @Test
+    fun givenCollectionOfObjects_whenUseLetOperator_thenShouldApplyActionOnlyOnNonNullValue() {
+        //given
+        val firstName = "Tom"
+        val secondName = "Michael"
+        val names: List<String?> = listOf(firstName, null, secondName)
+
+        //when
+        var res = listOf<String?>()
+        for (item in names) {
+            item?.let { res = res.plus(it) }
+        }
+
+        //then
+        assertEquals(2, res.size)
+        assertTrue { res.contains(firstName) }
+        assertTrue { res.contains(secondName) }
+    }
+
+    @Test
+    fun givenNullableReference_whenUseElvisOperator_thenShouldReturnValueIfReferenceIsNotNull() {
+        //given
+        val value: String? = "name"
+
+        //when
+        val res = value?.length ?: -1
+
+        //then
+        assertEquals(res, 4)
+    }
+
+    @Test
+    fun givenNullableReference_whenUseElvisOperator_thenShouldReturnDefaultValueIfReferenceIsNull() {
+        //given
+        val value: String? = null
+
+        //when
+        val res = value?.length ?: -1
+
+        //then
+        assertEquals(res, -1)
+    }
+
+    @Test
+    fun givenNullableField_whenUsingDoubleExclamationMarkOperatorOnNull_thenThrowNPE() {
+        //given
+        var b: String? = "value"
+        b = null
+
+        //when
+        assertFailsWith<NullPointerException> {
+            b!!.length
+        }
+    }
+
+    @Test
+    fun givenNullableField_whenUsingDoubleExclamationMarkOperatorOnNotNull_thenReturnValue() {
+        //given
+        val b: String? = "value"
+
+        //then
+        assertEquals(b!!.length, 5)
+    }
+
+    @Test
+    fun givenNullableList_whenUseFilterNotNullMethod_thenRemoveALlNullValues() {
+        //given
+        val list: List<String?> = listOf("a", null, "b")
+
+        //when
+        val res = list.filterNotNull()
+
+        //then
+        assertEquals(res.size, 2)
+        assertTrue { res.contains("a") }
+        assertTrue { res.contains("b") }
+    }
+}
+
+data class Person(val country: Country?)
+
+data class Country(val code: String?)