code snippets for the `Java 9 Stream API improvements` article (#809)

* code snippets for the `Java 9 Stream API improvements` article

* code snippets for the `Java 9 Stream API improvements` article [2 attempt]

* removed the first attempt

core-java-9/src/test/java/com/baeldung/java9/language/stream/StreamFeaturesTest.java

@@ -0,0 +1,119 @@
+package com.baeldung.java9.language.stream;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static java.lang.Integer.*;
+import static org.junit.Assert.assertEquals;
+
+public class StreamFeaturesTest {
+
+    public static class TakeAndDropWhileTest {
+
+        public Stream<String> getStreamAfterTakeWhileOperation() {
+            return Stream
+                    .iterate("", s -> s + "s")
+                    .takeWhile(s -> s.length() < 10);
+        }
+
+        public Stream<String> getStreamAfterDropWhileOperation() {
+            return Stream
+                    .iterate("", s -> s + "s")
+                    .takeWhile(s -> s.length() < 10)
+                    .dropWhile(s -> !s.contains("sssss"));
+        }
+
+        @Test
+        public void testTakeWhileOperation() {
+            List<String> list = getStreamAfterTakeWhileOperation().collect(Collectors.toList());
+
+            assertEquals(10, list.size());
+
+            assertEquals("", list.get(0));
+            assertEquals("ss", list.get(2));
+            assertEquals("sssssssss", list.get(list.size() - 1));
+        }
+
+        @Test
+        public void testDropWhileOperation() {
+            List<String> list = getStreamAfterDropWhileOperation().collect(Collectors.toList());
+
+            assertEquals(5, list.size());
+
+            assertEquals("sssss", list.get(0));
+            assertEquals("sssssss", list.get(2));
+            assertEquals("sssssssss", list.get(list.size() - 1));
+        }
+
+    }
+
+    public static class IterateTest {
+
+        private Stream<Integer> getStream() {
+            return Stream.iterate(0, i -> i < 10, i -> i + 1);
+        }
+
+        @Test
+        public void testIterateOperation() {
+            List<Integer> list = getStream().collect(Collectors.toList());
+
+            assertEquals(10, list.size());
+
+            assertEquals(valueOf(0), list.get(0));
+            assertEquals(valueOf(5), list.get(5));
+            assertEquals(valueOf(9), list.get(list.size() - 1));
+        }
+
+    }
+
+    public  static class OfNullableTest {
+
+        private List<String> collection = Arrays.asList("A", "B", "C");
+        private Map<String, Integer> map = new HashMap<>() {{
+           put("A", 10);
+           put("C", 30);
+        }};
+
+        private Stream<Integer> getStreamWithOfNullable() {
+            return collection.stream()
+                    .flatMap(s -> Stream.ofNullable(map.get(s)));
+        }
+
+        private Stream<Integer> getStream() {
+            return collection.stream()
+                            .flatMap(s -> {
+                                Integer temp = map.get(s);
+                                return temp != null ? Stream.of(temp) : Stream.empty();
+                            });
+        }
+
+        private List<Integer> testOfNullableFrom(Stream<Integer> stream) {
+            List<Integer> list = stream.collect(Collectors.toList());
+
+            assertEquals(2, list.size());
+
+            assertEquals(valueOf(10), list.get(0));
+            assertEquals(valueOf(30), list.get(list.size() - 1));
+
+            return list;
+        }
+
+        @Test
+        public void testOfNullable() {
+
+            assertEquals(
+                    testOfNullableFrom(getStream()),
+                    testOfNullableFrom(getStreamWithOfNullable())
+            );
+
+        }
+
+    }
+
+}