added examples about how to reverse a string in java (#7219)

java-strings-2/src/main/java/com/baeldung/string/reverse/ReverseStringExamples.java

@@ -0,0 +1,56 @@
+package com.baeldung.string.reverse;
+
+import org.apache.commons.lang3.StringUtils;
+
+public class ReverseStringExamples {
+
+    public static String reverse(String input) {
+        if (input == null) {
+            return null;
+        }
+
+        String output = "";
+
+        for (int i = input.length() - 1; i >= 0; i--) {
+            output = output + input.charAt(i);
+        }
+
+        return output;
+    }
+
+    public static String reverseUsingStringBuilder(String input) {
+        if (input == null) {
+            return null;
+        }
+
+        StringBuilder output = new StringBuilder(input).reverse();
+
+        return output.toString();
+    }
+
+    public static String reverseUsingApacheCommons(String input) {
+        return StringUtils.reverse(input);
+    }
+
+    public static String reverseTheOrderOfWords(String sentence) {
+        if (sentence == null) {
+            return null;
+        }
+
+        StringBuilder output = new StringBuilder();
+        String[] words = sentence.split(" ");
+
+        for (int i = words.length - 1; i >= 0; i--) {
+            output.append(words[i]);
+            output.append(" ");
+        }
+
+        return output.toString()
+            .trim();
+    }
+
+    public static String reverseTheOrderOfWordsUsingApacheCommons(String sentence) {
+        return StringUtils.reverseDelimited(sentence, ' ');
+    }
+
+}

java-strings-2/src/test/java/com/baeldung/string/reverse/ReverseStringExamplesUnitTest.java

@@ -0,0 +1,70 @@
+package com.baeldung.string.reverse;
+
+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";
+    private static final String STRING_INPUT_REVERSED = "tac";
+    private static final String SENTENCE = "The quick brown fox jumps over the lazy dog";
+    private static final String REVERSED_WORDS_SENTENCE = "dog lazy the over jumps fox brown quick The";
+
+    @Test
+    public void whenReverseIsCalled_ThenCorrectStringIsReturned() {
+        String reversed = ReverseStringExamples.reverse(STRING_INPUT);
+        String reversedNull = ReverseStringExamples.reverse(null);
+        String reversedEmpty = ReverseStringExamples.reverse(StringUtils.EMPTY);
+
+        assertEquals(STRING_INPUT_REVERSED, reversed);
+        assertEquals(null, reversedNull);
+        assertEquals(StringUtils.EMPTY, reversedEmpty);
+    }
+
+    @Test
+    public void whenReverseUsingStringBuilderIsCalled_ThenCorrectStringIsReturned() throws Exception {
+        String reversed = ReverseStringExamples.reverseUsingStringBuilder(STRING_INPUT);
+        String reversedNull = ReverseStringExamples.reverseUsingStringBuilder(null);
+        String reversedEmpty = ReverseStringExamples.reverseUsingStringBuilder(StringUtils.EMPTY);
+
+        assertEquals(STRING_INPUT_REVERSED, reversed);
+        assertEquals(null, reversedNull);
+        assertEquals(StringUtils.EMPTY, reversedEmpty);
+    }
+
+    @Test
+    public void whenReverseUsingApacheCommonsIsCalled_ThenCorrectStringIsReturned() throws Exception {
+        String reversed = ReverseStringExamples.reverseUsingApacheCommons(STRING_INPUT);
+        String reversedNull = ReverseStringExamples.reverseUsingApacheCommons(null);
+        String reversedEmpty = ReverseStringExamples.reverseUsingApacheCommons(StringUtils.EMPTY);
+
+        assertEquals(STRING_INPUT_REVERSED, reversed);
+        assertEquals(null, reversedNull);
+        assertEquals(StringUtils.EMPTY, reversedEmpty);
+    }
+
+    @Test
+    public void whenReverseTheOrderOfWordsIsCalled_ThenCorrectStringIsReturned() {
+        String reversed = ReverseStringExamples.reverseTheOrderOfWords(SENTENCE);
+        String reversedNull = ReverseStringExamples.reverseTheOrderOfWords(null);
+        String reversedEmpty = ReverseStringExamples.reverseTheOrderOfWords(StringUtils.EMPTY);
+
+        assertEquals(REVERSED_WORDS_SENTENCE, reversed);
+        assertEquals(null, reversedNull);
+        assertEquals(StringUtils.EMPTY, reversedEmpty);
+    }
+
+    @Test
+    public void whenReverseTheOrderOfWordsUsingApacheCommonsIsCalled_ThenCorrectStringIsReturned() {
+        String reversed = ReverseStringExamples.reverseTheOrderOfWordsUsingApacheCommons(SENTENCE);
+        String reversedNull = ReverseStringExamples.reverseTheOrderOfWordsUsingApacheCommons(null);
+        String reversedEmpty = ReverseStringExamples.reverseTheOrderOfWordsUsingApacheCommons(StringUtils.EMPTY);
+
+        assertEquals(REVERSED_WORDS_SENTENCE, reversed);
+        assertEquals(null, reversedNull);
+        assertEquals(StringUtils.EMPTY, reversedEmpty);
+    }
+
+}