Added a Java file on combining collections (#849)

BAEL-35 Added a Java file on combining collections

core-java/src/test/java/org/baeldung/java/collections/CollectionsConcatenateUnitTest.java

@@ -0,0 +1,116 @@
+package org.baeldung.java.collections;
+
+import static java.util.Arrays.asList;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.List;
+import java.util.NoSuchElementException;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Lists;
+
+public class CollectionsConcatenateUnitTest {
+
+    @Test
+    public void givenUsingJava8_whenConcatenatingUsingConcat_thenCorrect() {
+        Collection<String> collectionA = asList("S", "T");
+        Collection<String> collectionB = asList("U", "V");
+        Collection<String> collectionC = asList("W", "X");
+
+        Stream<String> combinedStream = Stream.concat(Stream.concat(collectionA.stream(), collectionB.stream()), collectionC.stream());
+        Collection<String> collectionCombined = combinedStream.collect(Collectors.toList());
+
+        Assert.assertEquals(asList("S", "T", "U", "V", "W", "X"), collectionCombined);
+    }
+
+    @Test
+    public void givenUsingJava8_whenConcatenatingUsingflatMap_thenCorrect() {
+        Collection<String> collectionA = asList("S", "T");
+        Collection<String> collectionB = asList("U", "V");
+
+        Stream<String> combinedStream = Stream.of(collectionA, collectionB).flatMap(Collection::stream);
+        Collection<String> collectionCombined = combinedStream.collect(Collectors.toList());
+
+        Assert.assertEquals(asList("S", "T", "U", "V"), collectionCombined);
+    }
+
+    @Test
+    public void givenUsingGuava_whenConcatenatingUsingIterables_thenCorrect() {
+        Collection<String> collectionA = asList("S", "T");
+        Collection<String> collectionB = asList("U", "V");
+
+        Iterable<String> combinedIterables = Iterables.unmodifiableIterable(Iterables.concat(collectionA, collectionB));
+        Collection<String> collectionCombined = Lists.newArrayList(combinedIterables);
+
+        Assert.assertEquals(asList("S", "T", "U", "V"), collectionCombined);
+    }
+
+    @Test
+    public void givenUsingJava7_whenConcatenatingUsingIterables_thenCorrect() {
+        Collection<String> collectionA = asList("S", "T");
+        Collection<String> collectionB = asList("U", "V");
+
+        Iterable<String> combinedIterables = concat(collectionA, collectionB);
+        Collection<String> collectionCombined = makeListFromIterable(combinedIterables);
+        Assert.assertEquals(Arrays.asList("S", "T", "U", "V"), collectionCombined);
+    }
+
+    public static <E> Iterable<E> concat(Iterable<? extends E> list1, Iterable<? extends E> list2) {
+        return new Iterable<E>() {
+            public Iterator<E> iterator() {
+                return new Iterator<E>() {
+                    protected Iterator<? extends E> listIterator = list1.iterator();
+                    protected Boolean checkedHasNext;
+                    protected E nextValue;
+                    private boolean startTheSecond;
+
+                    public void theNext() {
+                        if (listIterator.hasNext()) {
+                            checkedHasNext = true;
+                            nextValue = listIterator.next();
+                        } else if (startTheSecond)
+                            checkedHasNext = false;
+                        else {
+                            startTheSecond = true;
+                            listIterator = list2.iterator();
+                            theNext();
+                        }
+                    }
+
+                    public boolean hasNext() {
+                        if (checkedHasNext == null)
+                            theNext();
+                        return checkedHasNext;
+                    }
+
+                    public E next() {
+                        if (!hasNext())
+                            throw new NoSuchElementException();
+                        checkedHasNext = null;
+                        return nextValue;
+                    }
+
+                    public void remove() {
+                        listIterator.remove();
+                    }
+                };
+            }
+        };
+    }
+
+    public static <E> List<E> makeListFromIterable(Iterable<E> iter) {
+        List<E> list = new ArrayList<E>();
+        for (E item : iter) {
+            list.add(item);
+        }
+        return list;
+    }
+}