From 04c7b01450d3b27bcb08eaa445ef1e4b215cf578 Mon Sep 17 00:00:00 2001
From: Azhwani <13301425+azhwani@users.noreply.github.com>
Date: Tue, 2 May 2023 08:29:16 +0200
Subject: [PATCH] BAEL-6435: Reverse a String in Java 8 using Lambda and
 Streams (#13910)

---
 .../reverse/ReverseStringExamples.java        | 38 ++++++++++++++-
 .../ReverseStringExamplesUnitTest.java        | 48 ++++++++++++++++---
 2 files changed, 78 insertions(+), 8 deletions(-)

diff --git a/core-java-modules/core-java-string-algorithms/src/main/java/com/baeldung/reverse/ReverseStringExamples.java b/core-java-modules/core-java-string-algorithms/src/main/java/com/baeldung/reverse/ReverseStringExamples.java
index 5236f14ccd..07663ab7d1 100644
--- a/core-java-modules/core-java-string-algorithms/src/main/java/com/baeldung/reverse/ReverseStringExamples.java
+++ b/core-java-modules/core-java-string-algorithms/src/main/java/com/baeldung/reverse/ReverseStringExamples.java
@@ -1,5 +1,9 @@
 package com.baeldung.reverse;
 
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
+
 import org.apache.commons.lang3.StringUtils;
 
 public class ReverseStringExamples {
@@ -46,11 +50,43 @@ public class ReverseStringExamples {
         }
 
         return output.toString()
-            .trim();
+          .trim();
     }
 
     public static String reverseTheOrderOfWordsUsingApacheCommons(String sentence) {
         return StringUtils.reverseDelimited(sentence, ' ');
     }
 
+    public static String reverseUsingIntStreamRangeMethod(String str) {
+        if (str == null) {
+            return null;
+        }
+
+        char[] charArray = str.toCharArray();
+        return IntStream.range(0, str.length())
+          .mapToObj(i -> charArray[str.length() - i - 1])
+          .collect(StringBuilder::new, StringBuilder::append, StringBuilder::append)
+          .toString();
+    }
+
+    public static String reverseUsingStreamOfMethod(String str) {
+        if (str == null) {
+            return null;
+        }
+
+        return Stream.of(str)
+          .map(string -> new StringBuilder(string).reverse())
+          .collect(Collectors.joining());
+    }
+
+    public static String reverseUsingCharsMethod(String str) {
+        if (str == null) {
+            return null;
+        }
+
+        return str.chars()
+          .mapToObj(c -> (char) c)
+          .reduce("", (a, b) -> b + a, (a2, b2) -> b2 + a2);
+    }
+
 }
diff --git a/core-java-modules/core-java-string-algorithms/src/test/java/com/baeldung/reverse/ReverseStringExamplesUnitTest.java b/core-java-modules/core-java-string-algorithms/src/test/java/com/baeldung/reverse/ReverseStringExamplesUnitTest.java
index c122163174..b3685a49da 100644
--- a/core-java-modules/core-java-string-algorithms/src/test/java/com/baeldung/reverse/ReverseStringExamplesUnitTest.java
+++ b/core-java-modules/core-java-string-algorithms/src/test/java/com/baeldung/reverse/ReverseStringExamplesUnitTest.java
@@ -1,10 +1,11 @@
 package com.baeldung.reverse;
 
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+
 import org.apache.commons.lang3.StringUtils;
 import org.junit.Test;
 
-import static org.junit.Assert.assertEquals;
-
 public class ReverseStringExamplesUnitTest {
 
     private static final String STRING_INPUT = "cat";
@@ -19,7 +20,7 @@ public class ReverseStringExamplesUnitTest {
         String reversedEmpty = ReverseStringExamples.reverse(StringUtils.EMPTY);
 
         assertEquals(STRING_INPUT_REVERSED, reversed);
-        assertEquals(null, reversedNull);
+        assertNull(reversedNull);
         assertEquals(StringUtils.EMPTY, reversedEmpty);
     }
 
@@ -30,7 +31,7 @@ public class ReverseStringExamplesUnitTest {
         String reversedEmpty = ReverseStringExamples.reverseUsingStringBuilder(StringUtils.EMPTY);
 
         assertEquals(STRING_INPUT_REVERSED, reversed);
-        assertEquals(null, reversedNull);
+        assertNull(reversedNull);
         assertEquals(StringUtils.EMPTY, reversedEmpty);
     }
 
@@ -41,7 +42,7 @@ public class ReverseStringExamplesUnitTest {
         String reversedEmpty = ReverseStringExamples.reverseUsingApacheCommons(StringUtils.EMPTY);
 
         assertEquals(STRING_INPUT_REVERSED, reversed);
-        assertEquals(null, reversedNull);
+        assertNull(reversedNull);
         assertEquals(StringUtils.EMPTY, reversedEmpty);
     }
 
@@ -52,7 +53,7 @@ public class ReverseStringExamplesUnitTest {
         String reversedEmpty = ReverseStringExamples.reverseTheOrderOfWords(StringUtils.EMPTY);
 
         assertEquals(REVERSED_WORDS_SENTENCE, reversed);
-        assertEquals(null, reversedNull);
+        assertNull(reversedNull);
         assertEquals(StringUtils.EMPTY, reversedEmpty);
     }
 
@@ -63,7 +64,40 @@ public class ReverseStringExamplesUnitTest {
         String reversedEmpty = ReverseStringExamples.reverseTheOrderOfWordsUsingApacheCommons(StringUtils.EMPTY);
 
         assertEquals(REVERSED_WORDS_SENTENCE, reversed);
-        assertEquals(null, reversedNull);
+        assertNull(reversedNull);
+        assertEquals(StringUtils.EMPTY, reversedEmpty);
+    }
+
+    @Test
+    public void whenReverseStringUsingIntStreamRangeMethod_ThenCorrectStringIsReturned() {
+        String reversed = ReverseStringExamples.reverseUsingIntStreamRangeMethod(STRING_INPUT);
+        String reversedNull = ReverseStringExamples.reverseUsingIntStreamRangeMethod(null);
+        String reversedEmpty = ReverseStringExamples.reverseUsingIntStreamRangeMethod(StringUtils.EMPTY);
+
+        assertEquals(STRING_INPUT_REVERSED, reversed);
+        assertNull(reversedNull);
+        assertEquals(StringUtils.EMPTY, reversedEmpty);
+    }
+
+    @Test
+    public void whenReverseStringUsingCharsMethod_ThenCorrectStringIsReturned() {
+        String reversed = ReverseStringExamples.reverseUsingCharsMethod(STRING_INPUT);
+        String reversedNull = ReverseStringExamples.reverseUsingCharsMethod(null);
+        String reversedEmpty = ReverseStringExamples.reverseUsingCharsMethod(StringUtils.EMPTY);
+
+        assertEquals(STRING_INPUT_REVERSED, reversed);
+        assertNull(reversedNull);
+        assertEquals(StringUtils.EMPTY, reversedEmpty);
+    }
+
+    @Test
+    public void whenReverseStringUsingStreamOfMethod_ThenCorrectStringIsReturned() {
+        String reversed = ReverseStringExamples.reverseUsingStreamOfMethod(STRING_INPUT);
+        String reversedNull = ReverseStringExamples.reverseUsingStreamOfMethod(null);
+        String reversedEmpty = ReverseStringExamples.reverseUsingStreamOfMethod(StringUtils.EMPTY);
+
+        assertEquals(STRING_INPUT_REVERSED, reversed);
+        assertNull(reversedNull);
         assertEquals(StringUtils.EMPTY, reversedEmpty);
     }