fix/Add list extension (#148)

* Add list extension

* Fix test function description

* Update java version

* Revert jdk change.

.travis.yml

@@ -1,7 +1,6 @@
 language: android
 dist: xenial
 
-
 before_cache:
   - rm -f  $HOME/.gradle/caches/modules-2/modules-2.lock
   - rm -fr $HOME/.gradle/caches/*/plugin-resolution/

kt-extensions/src/main/java/com/mindera/skeletoid/kt/extensions/utils/Collection.kt

@@ -1,5 +1,3 @@
 package com.mindera.skeletoid.kt.extensions.utils
 
-fun <T: Any> Collection<T>.nullIfEmpty(): Collection<T>? {
-    return if (isEmpty()) null else this
-}
+fun <T> Collection<T>.nullIfEmpty() = if (isEmpty()) null else this

kt-extensions/src/main/java/com/mindera/skeletoid/kt/extensions/utils/ListExt.kt

@@ -0,0 +1,3 @@
+package com.mindera.skeletoid.kt.extensions.utils
+
+fun <T> List<T>.nullIfEmpty() = if (isEmpty()) null else this

kt-extensions/src/test/java/com/mindera/skeletoid/kt/extensions/utils/CollectionTest.kt

@@ -7,13 +7,13 @@ class CollectionTest {
 
     @Test
     fun `test nullIfEmpty with non empty collection`() {
-        val collection = listOf("One", "Two", "Three")
+        val collection = setOf("One", "Two", "Three")
         Assert.assertNotNull(collection.nullIfEmpty())
     }
 
     @Test
     fun `test nullIfEmpty with empty collection`() {
-        val collection = listOf<String>()
+        val collection = setOf<String>()
         Assert.assertNull(collection.nullIfEmpty())
     }
 }

kt-extensions/src/test/java/com/mindera/skeletoid/kt/extensions/utils/ListExtTest.kt

@@ -0,0 +1,19 @@
+package com.mindera.skeletoid.kt.extensions.utils
+
+import org.junit.Assert
+import org.junit.Test
+
+class ListExtTest {
+
+    @Test
+    fun `test nullIfEmpty with non empty list`() {
+        val list = listOf("One", "Two", "Three")
+        Assert.assertNotNull(list.nullIfEmpty())
+    }
+
+    @Test
+    fun `test nullIfEmpty with empty list`() {
+        val list = listOf<String>()
+        Assert.assertNull(list.nullIfEmpty())
+    }
+}