[BAEL-1847] Iterate Backwards through a List (#4540)

* [BAEL-1847] Iterate Backwards through a List

* [BAEL-1847] Updated unit tests

core-java-collections/src/main/java/com/baeldung/java/list/ReverseIterator.java

@@ -0,0 +1,80 @@
+package com.baeldung.java.list;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.ListIterator;
+
+import org.apache.commons.collections4.iterators.ReverseListIterator;
+
+import com.google.common.collect.Lists;
+
+/**
+ * Provides methods for iterating backward over a list.
+ */
+public class ReverseIterator {
+
+    /**
+     * Iterate using the for loop.
+     *
+     * @param list the list
+     */
+    public void iterateUsingForLoop(final List<String> list) {
+
+        for (int i = list.size(); i-- > 0; ) {
+            System.out.println(list.get(i));
+        }
+    }
+
+    /**
+     * Iterate using the Java {@link ListIterator}.
+     *
+     * @param list the list
+     */
+    public void iterateUsingListIterator(final List<String> list) {
+
+        final ListIterator listIterator = list.listIterator(list.size());
+        while (listIterator.hasPrevious()) {
+            System.out.println(listIterator.previous());
+        }
+    }
+
+    /**
+     * Iterate using Java {@link Collections} API.
+     *
+     * @param list the list
+     */
+    public void iterateUsingCollections(final List<String> list) {
+
+        Collections.reverse(list);
+        for (final String item : list) {
+            System.out.println(item);
+        }
+    }
+
+    /**
+     * Iterate using Apache Commons {@link ReverseListIterator}.
+     *
+     * @param list the list
+     */
+    public void iterateUsingApacheReverseListIterator(final List<String> list) {
+
+        final ReverseListIterator listIterator = new ReverseListIterator(list);
+        while (listIterator.hasNext()) {
+            System.out.println(listIterator.next());
+        }
+    }
+
+    /**
+     * Iterate using Guava {@link Lists} API.
+     *
+     * @param list the list
+     */
+    public void iterateUsingGuava(final List<String> list) {
+
+        final List<String> reversedList = Lists.reverse(list);
+        for (final String item : reversedList) {
+            System.out.println(item);
+        }
+    }
+
+}

core-java-collections/src/test/java/com/baeldung/java/list/ReverseIteratorUnitTest.java

@@ -0,0 +1,87 @@
+package com.baeldung.java.list;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.ListIterator;
+
+import org.apache.commons.collections4.iterators.ReverseListIterator;
+import org.apache.commons.lang3.StringUtils;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import com.google.common.collect.Lists;
+
+public class ReverseIteratorUnitTest {
+
+    private final ReverseIterator reverseIterator = new ReverseIterator();
+
+    private List<String> list;
+
+    private final String originalString = "ABCDE";
+
+    @BeforeEach
+    void setUp() {
+
+        list = Lists.newArrayList("A", "B", "C", "D", "E");
+    }
+
+    @Test
+    void whenIteratingUsingForLoop_thenCorrect() {
+
+        String reverseString = "";
+        for (int i = list.size(); i-- > 0; ) {
+            reverseString += list.get(i);
+        }
+        assertEquals(reverseString, StringUtils.reverse(originalString));
+    }
+
+    @Test
+    void whenIteratingUsingListIterator_thenCorrect() {
+
+        String reverseString = "";
+        final ListIterator listIterator = list.listIterator(list.size());
+        while (listIterator.hasPrevious()) {
+            reverseString += listIterator.previous();
+        }
+        assertEquals(reverseString, StringUtils.reverse(originalString));
+    }
+
+    @Test
+    void whenIteratingUsingCollections_thenCorrect() {
+
+        String reverseString = "";
+        Collections.reverse(list);
+        for (final String item : list) {
+            reverseString += item;
+        }
+        assertEquals(reverseString, StringUtils.reverse(originalString));
+
+        assertEquals("E", list.get(0));
+    }
+
+    @Test
+    void whenIteratingUsingApacheReverseListIterator_thenCorrect() {
+
+        String reverseString = "";
+        final ReverseListIterator listIterator = new ReverseListIterator(list);
+        while (listIterator.hasNext()) {
+            reverseString += listIterator.next();
+        }
+        assertEquals(reverseString, StringUtils.reverse(originalString));
+    }
+
+    @Test
+    void whenIteratingUsingGuava_thenCorrect() {
+
+        String reverseString = "";
+        final List<String> reversedList = Lists.reverse(list);
+        for (final String item : reversedList) {
+            reverseString += item;
+        }
+        assertEquals(reverseString, StringUtils.reverse(originalString));
+
+        assertEquals("A", list.get(0));
+    }
+}