Java String Programs: Reverse, Palindrome, Vowels & Duplicates

Reverse a String

public class ReverseString {
    public static void main(String[] args) {
        String original = "Testing";
        String reversed = "";

        for (int i = original.length() - 1; i >= 0; i--) {
            reversed = reversed + original.charAt(i);
        }

        System.out.println("Reversed String: " + reversed);
    }
}

Logic

Traverse the string from the last character to the first and append every character to build the reversed string.

Using StringBuilder

StringBuilder sb = new StringBuilder();

for (int i = original.length() - 1; i >= 0; i--) {
    sb.append(original.charAt(i));
}

// or

new StringBuilder(original).reverse().toString();

StringBuilder is preferred because it avoids creating multiple intermediate String objects.

Palindrome Check

A palindrome string reads the same in both forward and reverse directions.

Logic

• Reverse the string.
• Compare it with the original string using:

.equals()

If both strings are equal, the string is a palindrome.

Another approach uses two pointers:

• Compare the first and last characters.
• Move both pointers toward the center.
• Stop immediately if a mismatch is found.

Time Complexity: O(n)

Compare Two Strings

Use the appropriate comparison method based on the requirement.

Content Comparison

.equals()

Case-Insensitive Comparison

.equalsIgnoreCase()

Lexicographical Comparison

.compareTo()

Avoid using:

==

because it compares object references rather than string content.

Count Vowels & Consonants

public class CountVowelsConsonants {

    public static void main(String[] args) {

        String str = "Automation";

        int vowels = 0;
        int consonants = 0;

        str = str.toLowerCase();

        for (int i = 0; i < str.length(); i++) {

            char ch = str.charAt(i);

            if (ch >= 'a' && ch <= 'z') {

                if (ch == 'a' || ch == 'e' || ch == 'i'
                        || ch == 'o' || ch == 'u') {
                    vowels++;
                } else {
                    consonants++;
                }
            }
        }

        System.out.println("Vowels: " + vowels +
                ", Consonants: " + consonants);
    }
}

Logic

• Convert the string to lowercase.
• Process only alphabetic characters.
• Increment the vowel count for vowels.
• Increment the consonant count for remaining alphabetic characters.

Character Frequency & Duplicate Characters

Duplicate Characters

char[] chars = str.toCharArray();

for (int i = 0; i < chars.length; i++) {

    int count = 1;

    if (chars[i] == '0')
        continue;

    for (int j = i + 1; j < chars.length; j++) {

        if (chars[i] == chars[j]) {
            count++;
            chars[j] = '0';
        }
    }

    if (count > 1)
        System.out.println(chars[i] + " -> " + count);
}

Logic

• Convert the string into a character array.
• Count repeated characters.
• Mark processed characters so they are not counted again.
• Print only characters that appear more than once.

Optimized Approach

Use a HashMap.

map.put(ch, map.getOrDefault(ch, 0) + 1);

This approach performs frequency counting in a single traversal.

Remove Duplicate Characters

Use a LinkedHashSet<Character> to preserve insertion order while eliminating duplicate characters.

First Non-Repeated Character

Logic

• Count the frequency of every character.
• Traverse the string again.
• Return the first character whose frequency equals 1.

A LinkedHashMap preserves insertion order while counting frequencies.

Time Complexity: O(n)

Only Digits / Only Alphabets

Only Digits

Use:

Character.isDigit(ch)

or

str.matches("[0-9]+")

Only Alphabets

Use:

Character.isLetter(ch)

or

str.matches("[a-zA-Z]+")

Swap Characters

Since String objects are immutable:

• Convert the string into a character array.
• Swap the required characters.
• Build the string again.

Frequently Asked Questions

How do you reverse a string in Java?

Traverse the string from the last index to the first or use:

new StringBuilder(str).reverse().toString();

How do you check whether a string is a palindrome?

Reverse the string and compare it with the original using:

.equals()

Alternatively, compare characters using the two-pointer approach.

Why should .equals() be used instead of ==?

• == compares object references.
• .equals() compares string content.
• .equalsIgnoreCase() performs case-insensitive comparison.

How do you find the first non-repeated character?

Count character frequencies using a LinkedHashMap and return the first character whose frequency equals 1.

How do you count character frequency efficiently?

Use a HashMap with:

map.put(ch, map.getOrDefault(ch, 0) + 1);

This performs frequency counting in linear time.