Java Basics Programs - Learning with Nitheesh

Reverse String

A program to reverse a given string.

Reverse String

Write a program to reverse a given string.

Input

"hello"

Output

Reversed string: olleh

public class ReverseString { 

    public static void main(String[] args) {

        String str = "hello";

        String reversed = new StringBuilder(str).reverse().toString();

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

    }

}

Explanation:

Create a StringBuilder with the input string.
Call reverse() and convert back to String.
Print the reversed string.

Palindrome Check

A program to check whether a given string is a palindrome.

Palindrome Check

Write a program to check whether a given string is a palindrome.

Input

"madam"

Output

madam is a Palindrome

public class PalindromeCheck {

    public static void main(String[] args) {

        String str = "madam";

        String reversed = new StringBuilder(str).reverse().toString();

        if (str.equals(reversed)) {

            System.out.println(str + " is a Palindrome");

        } else {

            System.out.println(str + " is not a Palindrome");

        }

    }

}

Explanation:

Reverse the input string using StringBuilder.
Check if reversed equals original.
Print result accordingly.

Reverse String

A program to reverse a given string.

Reverse String

Write a program to reverse a given string.

Input

"hello"

Output

Reversed string: olleh

public class ReverseString {

    public static void main(String[] args) {

        String str = "hello";

        String reversed = new StringBuilder(str).reverse().toString();

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

    }

}

Explanation:

Create a StringBuilder with the input string.
Call reverse() and convert back to String.
Print the reversed string.

Palindrome Check

A program to check whether a given string is a palindrome.

Palindrome Check

Write a program to check whether a given string is a palindrome.

Input

"madam"

Output

madam is a Palindrome

public class PalindromeCheck {

    public static void main(String[] args) {

        String str = "madam";

        String reversed = new StringBuilder(str).reverse().toString();

        if (str.equals(reversed)) {

            System.out.println(str + " is a Palindrome");

        } else {

            System.out.println(str + " is not a Palindrome");

        }

    }

}

Explanation:

Reverse the input string using StringBuilder.
Check if reversed equals original.
Print result accordingly.

Vowels and Consonants Count

A program to count vowels and consonants in a string.

Vowels and Consonants Count

Write a program to count the number of vowels and consonants in a string.

Input

"programming"

Output

Vowels: 3
Consonants: 8

public class VowelConsonantCount {

    public static void main(String[] args) {

        String str = "programming".toLowerCase();

        int vowels = 0, consonants = 0;

        for (char c : str.toCharArray()) {

            if (Character.isLetter(c)) {

                if ("aeiou".indexOf(c) != -1) vowels++;

                else consonants++;

            }

        }

        System.out.println("Vowels: " + vowels);

        System.out.println("Consonants: " + consonants);

    }

}

Explanation:

Convert string to lowercase for uniformity.
Loop through characters and check if they are letters.
Use index check for vowels, count others as consonants.

Case Conversion

A program to convert a string to uppercase and lowercase.

Case Conversion

Write a program to convert a string to uppercase and lowercase.

Input

"Java Programming"

Output

Uppercase: JAVA PROGRAMMING
Lowercase: java programming

public class CaseConversion {

    public static void main(String[] args) {

        String str = "Java Programming";

        System.out.println("Uppercase: " + str.toUpperCase());

        System.out.println("Lowercase: " + str.toLowerCase());

    }

}

Explanation:

Use toUpperCase() to convert to uppercase.
Use toLowerCase() to convert to lowercase.
Print both results.

String Length

A program to find the length of a string without using .length().

String Length

Write a program to find the length of a string without using .length().

Input

"hello"

Output

Length of the string: 5

public class StringLength {

    public static void main(String[] args) {

        String str = "hello";

        int count = 0;

        for (char c : str.toCharArray()) {

            count++;

        }

        System.out.println("Length of the string: " + count);

    }

}

Explanation:

Convert the string to a character array.
Count the number of characters using a loop.
Print the final count.

Word Count

A program to count the number of words in a sentence.

Word Count

Write a program to count the number of words in a sentence.

Input

"Java is fun"

Output

Number of words: 3

public class WordCount {

    public static void main(String[] args) {

        String str = "Java is fun";

        String[] words = str.trim().split("\\s+");

        System.out.println("Number of words: " + words.length);

    }

}

Explanation:

Trim the string to remove leading/trailing spaces.
Split the string using space(s) as the delimiter.
Count the resulting array elements as words.

Anagram Check

A program to check if two strings are anagrams.

Anagram Check

Write a program to check if two strings are anagrams (contain the same characters in different order).

Input

"listen", "silent"

Output

The strings are anagrams

import java.util.Arrays;

public class AnagramCheck {

    public static void main(String[] args) {

        String str1 = "listen";

        String str2 = "silent";

        char[] arr1 = str1.toCharArray();

        char[] arr2 = str2.toCharArray();

        Arrays.sort(arr1);

        Arrays.sort(arr2);

        if (Arrays.equals(arr1, arr2)) {

            System.out.println("The strings are anagrams");

        } else {

            System.out.println("The strings are not anagrams");

        }

    }

}

Explanation:

Convert both strings to character arrays.
Sort the arrays alphabetically.
Compare the arrays for equality.

Remove Duplicates

A program to remove duplicate characters from a string.

Remove Duplicates

Write a program to remove duplicate characters from a string.

Input

"programming"

Output

String after removing duplicates: progamin

public class RemoveDuplicates {

    public static void main(String[] args) {

        String str = "programming";

        String result = "";

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

            if (!result.contains(String.valueOf(str.charAt(i)))) {

                result += str.charAt(i);

            }

        }

        System.out.println("String after removing duplicates: " + result);

    }

}

Explanation:

Initialize an empty result string.
Iterate through each character in the original string.
Append the character only if it's not already in the result string.

Most Frequent Character

A program to find the most frequent character in a string.

Most Frequent Character

Write a program to find the character that appears most frequently in a string.

Input

"banana"

Output

Most frequent character: a

import java.util.HashMap;

public class MostFrequentCharacter {

    public static void main(String[] args) {

        String str = "banana";

        HashMap freqMap = new HashMap<>();

        for (char ch : str.toCharArray()) {

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

        }

        char mostFreq = ' ';

        int max = 0;

        for (char ch : freqMap.keySet()) {

            if (freqMap.get(ch) > max) {

                max = freqMap.get(ch);

                mostFreq = ch;

            }

        }

        System.out.println("Most frequent character: " + mostFreq);

    }

}

Explanation:

Use a HashMap to count the frequency of each character.
Find the character with the maximum count.

Swap First & Last

A program to swap the first and last characters of a string.

Swap First & Last

Write a program to swap the first and last character of a string.

Input

"hello"

Output

Swapped string: oellh

public class SwapFirstLast {

    public static void main(String[] args) {

        String str = "hello";

        if (str.length() <= 1) {

            System.out.println("Swapped string: " + str);

        } else {

            char first = str.charAt(0);

            char last = str.charAt(str.length() - 1);

            String middle = str.substring(1, str.length() - 1);

            String swapped = last + middle + first;

            System.out.println("Swapped string: " + swapped);

        }

    }

}

Explanation:

Extract the first and last characters.
Form a new string with swapped positions.

Digits Only Check

A program to check if a string contains only numeric digits.

Digits Only Check

Write a program to check if a given string contains only numeric digits.

Input

"12345"

Output

The string contains only digits.

public class DigitsOnlyCheck {

    public static void main(String[] args) {

        String str = "12345";

        boolean isDigitOnly = str.matches("\\d+");

        if (isDigitOnly) {

            System.out.println("The string contains only digits.");

        } else {

            System.out.println("The string contains non-digit characters.");

        }

    }

}

Explanation:

Use the regular expression \d+ to check for digit-only content.

Character Count

A program to count the occurrences of a character in a string.

Character Count

Write a program to count how many times a given character appears in a string.

Input

"banana", 'a'

Output

Character 'a' appears 3 times.

public class CharacterCount {

    public static void main(String[] args) {

        String str = "banana";

        char target = 'a';

        int count = 0;

        for (char ch : str.toCharArray()) {

            if (ch == target) {

                count++;

            }

        }

        System.out.println("Character '" + target + "' appears " + count + " times.");

    }

}

Explanation:

Iterate through the string using a for-each loop.
Compare each character with the target character.
Increment the counter when there's a match.

First Non-Repeating Character

A program to find the first non-repeating character in a string.

First Non-Repeating Character

Write a program to find the first non-repeating character in a string.

Input

"swiss"

Output

First non-repeating character: w

import java.util.HashMap;

public class FirstNonRepeatingCharacter {

    public static void main(String[] args) {

        String str = "swiss";

        HashMap freqMap = new HashMap<>();

        for (char ch : str.toCharArray()) {

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

        }

        for (char ch : str.toCharArray()) {

            if (freqMap.get(ch) == 1) {

                System.out.println("First non-repeating character: " + ch);

                return;

            }

        }

        System.out.println("No non-repeating character found.");

    }

}

Explanation:

Use a HashMap to count the frequency of each character.
Traverse the string and return the first character with a frequency of 1.

Check Rotations

A program to check if one string is a rotation of another.

Check Rotations

Write a program to check if one string is a rotation of another.

Input

"hello", "lohel"

Output

The strings are rotations of each other.

public class CheckRotation {

    public static void main(String[] args) {

        String str1 = "hello";

        String str2 = "lohel";

        // Check if the lengths are the same and if str2 is a substring of str1 concatenated with str1

        if (str1.length() == str2.length() && (str1 + str1).contains(str2)) {

            System.out.println("The strings are rotations of each other.");

        } else {

            System.out.println("The strings are not rotations of each other.");

        }

    }

}

Explanation:

Check if the lengths of the strings are equal.
Concatenate the first string with itself and check if the second string is a substring of this result.

Remove Whitespaces

A program to remove all spaces from a given string.

Remove Whitespaces

Write a program to remove all spaces from a given string.

Input

"Java Programming"

Output

JavaProgramming

public class RemoveWhitespaces {

    public static void main(String[] args) {

        String str = "Java Programming";

        // Remove all whitespaces using replaceAll

        String result = str.replaceAll("\\s", "");

        System.out.println("String after removing whitespaces: " + result);

    }

}

Explanation:

Use the `replaceAll()` method with the regular expression "\\s" to remove all spaces.
The result is stored in a new string and printed.

Check if Pangram

A program to check if a string contains every letter of the alphabet at least once.

Check if Pangram

Write a program to check if a string contains every letter of the alphabet at least once.

Input

"The quick brown fox jumps over the lazy dog"

Output

The string is a pangram.

public class PangramChecker {

    public static void main(String[] args) {

        String str = "The quick brown fox jumps over the lazy dog";

        // Convert string to lowercase and remove non-alphabetic characters

        str = str.toLowerCase().replaceAll("[^a-z]", "");

        // Use a set to track the letters in the string

        boolean[] alphabet = new boolean[26];

        int index = 0;

        // Traverse the string and mark the characters that appear

        for (char ch : str.toCharArray()) {

            index = ch - 'a';

            alphabet[index] = true;

        }

        // Check if all letters are present

        boolean isPangram = true;

        for (boolean letter : alphabet) {

            if (!letter) {

                isPangram = false;

                break;

            }

        }

        if (isPangram) {

            System.out.println("The string is a pangram.");

        } else {

            System.out.println("The string is not a pangram.");

        }

    }

}

Explanation:

Convert the string to lowercase and remove non-alphabetic characters.
Use a boolean array to track which letters have appeared in the string.
Check if all letters are present to determine if the string is a pangram.

Replace a Word

A program to replace a specific word in a sentence.

Replace a Word

Write a program to replace a specific word in a sentence.

Input

"Java is fun", replace "fun" with "awesome"

Output

Java is awesome

public class ReplaceWordInSentence {

    public static void main(String[] args) {

        String sentence = "Java is fun";

        String wordToReplace = "fun";

        String replacementWord = "awesome";

        // Replace the word in the sentence

        String modifiedSentence = sentence.replace(wordToReplace, replacementWord);

        // Output the result

        System.out.println(modifiedSentence);

    }
}

Explanation:

Use the `replace()` method to replace a specific word in the sentence.
The method takes two arguments: the word to be replaced and the word to replace it with.

Longest Word in Sentence

A program to find the longest word in a given sentence.

Longest Word in Sentence

Write a program to find the longest word in a given sentence.

Input

"I love programming"

Output

Longest word: programming

public class LongestWordInSentence {

    public static void main(String[] args) {

        String sentence = "I love programming";

        // Split the sentence into words

        String[] words = sentence.split(" ");

        String longestWord = "";

        // Loop through the words to find the longest

        for (String word : words) {

            if (word.length() > longestWord.length()) {

                longestWord = word;

            }

        }

        // Output the longest word

        System.out.println("Longest word: " + longestWord);

    }

}

Explanation:

Use the `split()` method to split the sentence into words based on spaces.
Loop through each word and check its length to find the longest word.
Output the longest word found.

Convert String to Char Array

A program to convert a given string into a character array and print each character.

Convert String to Char Array

Write a program to convert a given string into a character array and print each character.

Input

"hello"

Output

h e l l o

public class StringToCharArray {

    public static void main(String[] args) {

        String str = "hello";

        // Convert string to character array

        char[] charArray = str.toCharArray();

        // Print each character

        for (char ch : charArray) {

            System.out.print(ch + " ");

        }

    }

}

Explanation:

Use the `toCharArray()` method to convert the string into a character array.
Loop through the character array and print each character.

Valid Email Address

A program to check whether a given string is a valid email address.

Valid Email Address

Write a program to check whether a given string is a valid email address.

Input

"example@email.com"

Output

Valid

import java.util.regex.Matcher;

import java.util.regex.Pattern;

public class ValidEmail {

    public static void main(String[] args) {

        String email = "example@email.com";

        // Regular expression for validating email

        String emailRegex = "^[a-zA-Z0-9_+&*-]+(?:\\.[a-zA-Z0-9_+&*-]+)*@(?:[a-zA-Z0-9-]+\\.)+[a-zA-Z]{2,7}$";

        Pattern pattern = Pattern.compile(emailRegex);

        Matcher matcher = pattern.matcher(email);

        if (matcher.matches()) {

            System.out.println("Valid");

        } else {

            System.out.println("Invalid");

        }

    }

}

Explanation:

Use a regular expression (regex) to match the pattern of a valid email address.
Check whether the given email matches the regex pattern using the `matcher.matches()` method.

Java String Programs

Reverse String

Palindrome Check

Reverse String

Palindrome Check

Vowels and Consonants Count

Case Conversion

String Length

Word Count

Anagram Check

Remove Duplicates

Most Frequent Character

Swap First & Last

Digits Only Check

Character Count

First Non-Repeating Character

Check Rotations

Remove Whitespaces

Check if Pangram

Replace a Word

Longest Word in Sentence

Convert String to Char Array

Valid Email Address