Merge pull request #3582 from shouvikbhattacharya/BAEL-1525-Check-if-a-String-is-a-palindrome

BAEL-1525: Check if a String is a palindrome

core-java/src/main/java/com/baeldung/string/Palindrome.java

@@ -0,0 +1,53 @@
+package com.baeldung.string;
+
+public class Palindrome {
+
+    public boolean isPalindrome(String text) {
+        text = text.replaceAll("\\s+", "")
+            .toLowerCase();
+        int length = text.length();
+        int forward = 0;
+        int backward = length - 1;
+        boolean palindrome = true;
+        while (backward > forward) {
+            char forwardChar = text.charAt(forward++);
+            char backwardChar = text.charAt(backward--);
+            if (forwardChar != backwardChar)
+                return false;
+        }
+        return palindrome;
+    }
+
+    public boolean isPalindromeReverseTheString(String text) {
+        String reverse = "";
+        text = text.replaceAll("\\s+", "").toLowerCase();
+        char[] plain = text.toCharArray();
+        for (int i = plain.length - 1; i >= 0; i--)
+            reverse += plain[i];
+        return reverse.equals(text);
+    }
+
+    public boolean isPalindromeUsingStringBuilder(String text) {
+        StringBuilder plain = new StringBuilder(text);
+        StringBuilder reverse = plain.reverse();
+        return reverse.equals(plain);
+    }
+
+    public boolean isPalindromeUsingStringBuffer(String text) {
+        StringBuffer plain = new StringBuffer(text);
+        StringBuffer reverse = plain.reverse();
+        return reverse.equals(plain);
+    }
+
+    public boolean isPalindromeRecursive(String text, int forward, int backward) {
+        if (forward == backward)
+            return true;
+        if ((text.charAt(forward)) != (text.charAt(backward)))
+            return false;
+        if (forward < backward + 1) {
+            return isPalindromeRecursive(text, forward + 1, backward - 1);
+        }
+
+        return true;
+    }
+}

core-java/src/test/java/com/baeldung/string/WhenCheckingPalindrome.java

@@ -0,0 +1,114 @@
+package com.baeldung.string;
+
+import java.util.Arrays;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class WhenCheckingPalindrome {
+
+    private String[] words = {
+            "Anna",
+            "Civic",
+            "Kayak",
+            "Level",
+            "Madam",
+    };
+
+    private String[] sentences = {
+            "Sore was I ere I saw Eros",
+            "Euston saw I was not Sue",
+            "Too hot to hoot",
+            "No mists or frost Simon",
+            "Stella won no wallets"
+    };
+    
+    private Palindrome palindrome = new Palindrome();
+    
+    @Test
+    public void whenWord_shouldBePalindrome() {
+        for(String word:words)
+            Assert.assertTrue(palindrome.isPalindrome(word));
+    }
+
+    @Test
+    public void whenSentence_shouldBePalindrome() {
+        for(String sentence:sentences)
+            Assert.assertTrue(palindrome.isPalindrome(sentence));
+    }
+    
+    @Test
+    public void whenReverseWord_shouldBePalindrome() {
+        for(String word:words)
+            Assert.assertTrue(palindrome.isPalindromeReverseTheString(word));
+    }
+    
+    @Test
+    public void whenReverseSentence_shouldBePalindrome() {
+        for(String sentence:sentences)
+            Assert.assertTrue(palindrome.isPalindromeReverseTheString(sentence));
+    }
+    
+    @Test
+    public void whenStringBuilderWord_shouldBePalindrome() {
+        for(String word:words)
+            Assert.assertTrue(palindrome.isPalindromeUsingStringBuilder(word));
+    }
+    
+    @Test
+    public void whenStringBuilderSentence_shouldBePalindrome() {
+        for(String sentence:sentences)
+            Assert.assertTrue(palindrome.isPalindromeUsingStringBuilder(sentence));
+    }
+    
+    @Test
+    public void whenStringBufferWord_shouldBePalindrome() {
+        for(String word:words)
+            Assert.assertTrue(palindrome.isPalindromeUsingStringBuffer(word));
+    }
+    
+    @Test
+    public void whenStringBufferSentence_shouldBePalindrome() {
+        for(String sentence:sentences)
+            
+            Assert.assertTrue(palindrome.isPalindromeUsingStringBuffer(sentence));
+    }
+    
+    @Test
+    public void whenPalindromeRecursive_wordShouldBePalindrome() {
+        for(String word:words) {
+            word = word.replaceAll("\\s+", "").toLowerCase();
+            int backward = word.length()-1;
+            
+            Assert.assertTrue(palindrome.isPalindromeRecursive(word,0,backward));
+        }
+    }
+    
+    @Test
+    public void whenPalindromeRecursive_sentenceShouldBePalindrome() {
+        for(String sentence:sentences) {
+            sentence = sentence.replaceAll("\\s+", "").toLowerCase();
+            int backward = sentence.length()-1;
+            
+            Assert.assertTrue(palindrome.isPalindromeRecursive(sentence,0,backward));
+        }
+    }
+    
+    @Test
+    public void whenPalindromeStreams_wordShouldBePalindrome() {
+        String[] expected = Arrays.stream(words)
+            .filter(palindrome::isPalindrome)
+            .toArray(String[]::new);
+        
+        Assert.assertArrayEquals(expected, words);
+    }
+    
+    @Test
+    public void whenPalindromeStreams_sentenceShouldBePalindrome() {
+        String[] expected = Arrays.stream(sentences)
+            .filter(palindrome::isPalindrome)
+            .toArray(String[]::new);
+        
+        Assert.assertArrayEquals(expected, sentences);
+    }
+}