[BAEL-872] map and flatMap difference (#1892)

* injecting beans

* XML-based configuration replaced with Java Config.

* [BAEL-431] Exploring TestRestTemplate.

* Revert of evaluation task "XML-based configuration replaced with Java Config."

This reverts commit 66471cf0574c85f8ff514ec4caf5ba44ebba1a74.

* Revert of evaluation task "injecting beans"

This reverts commit d2ac20185e636245bc0ae0b4ccb952965de88e28.

* [BAEL-431] fix to the tests in TestRestTemplateBasicLiveTest.

* [BAEL-431] added more meaningful user and password for auth.

* [BAEL-820] examples of wait() and sleep() methods.

* [BAEL-820] wait() and sleep() examples.

* [BAEL-829] number of occurences of a char in a String.

* [BAEL-829] printlns changed to assertions.

* [BAEL-829] improved names of the tests.

* [BAEL-872] map and flatMap difference.

* removed duplicated countingChars class.

* [BAEL-872] changed Object into List.

* [BAEL-872] improved code for map() and flatMap() article.

core-java/src/test/java/com/baeldung/java8/Java8MapAndFlatMap.java

@@ -0,0 +1,52 @@
+package com.baeldung.java8;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.Optional;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static java.util.Arrays.asList;
+import static org.junit.Assert.assertEquals;
+
+public class Java8MapAndFlatMap {
+
+    @Test
+    public void givenStream_whenCalledMap_thenProduceList() {
+        List<String> myList = Stream.of("a", "b")
+                .map(String::toUpperCase)
+                .collect(Collectors.toList());
+        assertEquals(asList("A", "B"), myList);
+    }
+
+    @Test
+    public void givenStream_whenCalledFlatMap_thenProduceFlattenedList() throws Exception {
+        List<List<String>> list = Arrays.asList(Arrays.asList("a"), Arrays.asList("b"));
+        System.out.println(list);
+
+        System.out.println(list
+                .stream().flatMap(Collection::stream)
+                .collect(Collectors.toList()));
+    }
+
+    @Test
+    public void givenOptional_whenCalledMap_thenProduceOptional() {
+        Optional<String> s = Optional.of("test");
+        assertEquals(Optional.of("TEST"), s.map(String::toUpperCase));
+    }
+
+    @Test
+    public void givenOptional_whenCalledFlatMap_thenProduceFlattenedOptional() {
+        assertEquals(Optional.of(Optional.of("STRING")), Optional
+                .of("string")
+                .map(s -> Optional.of("STRING")));
+
+        assertEquals(Optional.of("STRING"), Optional
+                .of("string")
+                .flatMap(s -> Optional.of("STRING")));
+    }
+
+}