BAEL-885 - How to merge two Java streams? (#1884)

Added examples using:
* JDK8
* StreamEx library
* JOOl library

libraries/pom.xml

@@ -276,6 +276,16 @@
             <version>2.3.0</version>
         </dependency>
 
+        <dependency>
+            <groupId>one.util</groupId>
+            <artifactId>streamex</artifactId>
+            <version>0.6.5</version>
+        </dependency>
+        <dependency>
+            <groupId>org.jooq</groupId>
+            <artifactId>jool</artifactId>
+            <version>0.9.12</version>
+        </dependency>
     </dependencies>
     <properties>
         <multiverse.version>0.7.0</multiverse.version>

libraries/src/test/java/com/baeldung/stream/JoolMergeStreamsTest.java

@@ -0,0 +1,35 @@
+package com.baeldung.stream;
+
+import org.jooq.lambda.Seq;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.Arrays;
+
+import static junit.framework.TestCase.assertEquals;
+
+public class JoolMergeStreamsTest {
+    @Test
+    public void givenTwoStreams_whenMergingStreams_thenResultingStreamContainsElementsFromBothStreams() {
+        Seq<Integer> seq1 = Seq.of(1, 3, 5);
+        Seq<Integer> seq2 = Seq.of(2, 4, 6);
+
+        Seq<Integer> resultingSeq = seq1.append(seq2);
+
+        assertEquals(Arrays.asList(1, 3, 5, 2, 4, 6),
+                     resultingSeq.toList());
+    }
+
+    @Test
+    public void givenThreeStreams_whenAppendingAndPrependingStreams_thenResultingStreamContainsElementsFromAllStreams() {
+        Seq<String> seq = Seq.of("foo", "bar");
+        Seq<String> openingBracketSeq = Seq.of("[");
+        Seq<String> closingBracketSeq = Seq.of("]");
+
+        Seq<String> resultingStream = seq.append(closingBracketSeq)
+                                         .prepend(openingBracketSeq);
+
+        Assert.assertEquals(Arrays.asList("[", "foo", "bar", "]"),
+                            resultingStream.toList());
+    }
+}

libraries/src/test/java/com/baeldung/stream/MergeStreamsTest.java

@@ -0,0 +1,53 @@
+package com.baeldung.stream;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.junit.Assert.assertEquals;
+
+public class MergeStreamsTest {
+
+    @Test
+    public void givenTwoStreams_whenMergingStreams_thenResultingStreamContainsElementsFromBothStreams() {
+        Stream<Integer> stream1 = Stream.of(1, 3, 5);
+        Stream<Integer> stream2 = Stream.of(2, 4, 6);
+
+        Stream<Integer> resultingStream = Stream.concat(stream1,
+                                                        stream2);
+
+        assertEquals(Arrays.asList(1, 3, 5, 2, 4, 6),
+                     resultingStream.collect(Collectors.toList()));
+    }
+
+    @Test
+    public void givenThreeStreams_whenMergingStreams_thenResultingStreamContainsElementsFromAllStreams() {
+        Stream<Integer> stream1 = Stream.of(1, 3, 5);
+        Stream<Integer> stream2 = Stream.of(2, 4, 6);
+        Stream<Integer> stream3 = Stream.of(18, 15, 36);
+
+        Stream<Integer> resultingStream = Stream.concat(Stream.concat(stream1,
+                                                                      stream2),
+                                                        stream3);
+
+        assertEquals(Arrays.asList(1, 3, 5, 2, 4, 6, 18, 15, 36),
+                     resultingStream.collect(Collectors.toList()));
+    }
+
+    @Test
+    public void givenFourStreams_whenMergingStreams_thenResultingStreamContainsElementsFromAllStreams() {
+        Stream<Integer> stream1 = Stream.of(1, 3, 5);
+        Stream<Integer> stream2 = Stream.of(2, 4, 6);
+        Stream<Integer> stream3 = Stream.of(18, 15, 36);
+        Stream<Integer> stream4 = Stream.of(99);
+
+        Stream<Integer> resultingStream = Stream.of(stream1, stream2, stream3, stream4).flatMap(Function.identity());
+
+        assertEquals(Arrays.asList(1, 3, 5, 2, 4, 6, 18, 15, 36, 99),
+                     resultingStream.collect(Collectors.toList()));
+
+    }
+}

libraries/src/test/java/com/baeldung/stream/StreamExMergeStreamsTest.java

@@ -0,0 +1,51 @@
+package com.baeldung.stream;
+
+import one.util.streamex.StreamEx;
+import org.junit.Test;
+
+import java.util.Arrays;
+
+import static org.junit.Assert.assertEquals;
+
+public class StreamExMergeStreamsTest {
+
+    @Test
+    public void givenTwoStreams_whenMergingStreams_thenResultingStreamContainsElementsFromBothStreams() {
+        StreamEx<Integer> stream1 = StreamEx.of(1, 3, 5);
+        StreamEx<Integer> stream2 = StreamEx.of(2, 4, 6);
+
+        StreamEx<Integer> resultingStream = stream1.append(stream2);
+
+        assertEquals(Arrays.asList(1, 3, 5, 2, 4, 6),
+                     resultingStream.toList());
+    }
+
+    @Test
+    public void givenFourStreams_whenMergingStreams_thenResultingStreamContainsElementsFromAllStreams() {
+        StreamEx<Integer> stream1 = StreamEx.of(1, 3, 5);
+        StreamEx<Integer> stream2 = StreamEx.of(2, 4, 6);
+        StreamEx<Integer> stream3 = StreamEx.of(18, 15, 36);
+        StreamEx<Integer> stream4 = StreamEx.of(99);
+
+        StreamEx<Integer> resultingStream = stream1.append(stream2)
+                                                   .append(stream3)
+                                                   .append(stream4);
+
+        assertEquals(Arrays.asList(1, 3, 5, 2, 4, 6, 18, 15, 36, 99),
+                     resultingStream.toList());
+
+    }
+
+    @Test
+    public void givenThreeStreams_whenAppendingAndPrependingStreams_thenResultingStreamContainsElementsFromAllStreams() {
+        StreamEx<String> stream1 = StreamEx.of("foo", "bar");
+        StreamEx<String> openingBracketStream = StreamEx.of("[");
+        StreamEx<String> closingBracketStream = StreamEx.of("]");
+
+        StreamEx<String> resultingStream = stream1.append(closingBracketStream)
+                                                  .prepend(openingBracketStream);
+
+        assertEquals(Arrays.asList("[", "foo", "bar", "]"),
+                     resultingStream.toList());
+    }
+}