Java Strings - Complete Tutorial

public class StringManipulation {
    public static void main(String[] args) {
        String text = "   Java Programming Language   ";
        
        System.out.println("Original: \"" + text + "\"");
        System.out.println("Length: " + text.length());
        System.out.println("Trimmed: \"" + text.trim() + "\"");
        System.out.println("Uppercase: \"" + text.toUpperCase() + "\"");
        System.out.println("Lowercase: \"" + text.toLowerCase() + "\"");
        
        System.out.println("\n=== Substring Operations ===");
        String sentence = "The quick brown fox jumps over the lazy dog";
        System.out.println("Original: " + sentence);
        System.out.println("Substring(4, 9): " + sentence.substring(4, 9));  // quick
        System.out.println("Substring(10): " + sentence.substring(10));      // brown fox...
        
        System.out.println("\n=== Search Operations ===");
        System.out.println("Index of 'fox': " + sentence.indexOf("fox"));      // 16
        System.out.println("Last index of 'the': " + sentence.lastIndexOf("the"));  // 31
        System.out.println("Contains 'brown': " + sentence.contains("brown"));      // true
        System.out.println("Starts with 'The': " + sentence.startsWith("The"));     // true
        System.out.println("Ends with 'dog': " + sentence.endsWith("dog"));         // true
        
        System.out.println("\n=== Replace Operations ===");
        String original = "I love Java. Java is awesome!";
        System.out.println("Original: " + original);
        System.out.println("Replace 'Java' with 'Python': " + 
                          original.replace("Java", "Python"));
        System.out.println("Replace all spaces: " + 
                          original.replaceAll(" ", "-"));
        System.out.println("Replace first 'Java': " + 
                          original.replaceFirst("Java", "Python"));
        
        System.out.println("\n=== Split and Join ===");
        String csv = "John,Doe,30,New York";
        String[] parts = csv.split(",");
        System.out.println("Split by comma:");
        for(int i = 0; i < parts.length; i++) {
            System.out.println("  Part " + i + ": " + parts[i]);
        }
        
        String joined = String.join(" - ", parts);
        System.out.println("Joined with ' - ': " + joined);
        
        System.out.println("\n=== Character Operations ===");
        String word = "Hello";
        System.out.println("Word: " + word);
        System.out.println("First char: " + word.charAt(0));
        System.out.println("Last char: " + word.charAt(word.length() - 1));
        
        System.out.println("\nCharacters in reverse:");
        for(int i = word.length() - 1; i >= 0; i--) {
            System.out.print(word.charAt(i));
        }
        
        System.out.println("\n\n=== Pattern Matching ===");
        String email = "user@example.com";
        String phone = "123-456-7890";
        String date = "2023-12-25";
        
        System.out.println("Email '" + email + "' matches pattern: " + 
                          email.matches("^[\\w-\\.]+@([\\w-]+\\.)+[\\w-]{2,4}$"));
        System.out.println("Phone '" + phone + "' matches pattern: " + 
                          phone.matches("\\d{3}-\\d{3}-\\d{4}"));
        System.out.println("Date '" + date + "' matches pattern: " + 
                          date.matches("\\d{4}-\\d{2}-\\d{2}"));
        
        System.out.println("\n=== Modern Java Methods (Java 11+) ===");
        String spaced = "  Hello World  ";
        System.out.println("Original: \"" + spaced + "\"");
        System.out.println("strip(): \"" + spaced.strip() + "\"");
        System.out.println("stripLeading(): \"" + spaced.stripLeading() + "\"");
        System.out.println("stripTrailing(): \"" + spaced.stripTrailing() + "\"");
        System.out.println("isBlank(): " + spaced.isBlank());
        System.out.println("isEmpty(): " + spaced.isEmpty());
        System.out.println("\"Hi\".repeat(3): " + "Hi".repeat(3));
        
        String multiline = "Line1\nLine2\nLine3";
        System.out.println("\nMultiline string lines count: " + 
                          multiline.lines().count());
    }
}

import java.util.Scanner;

public class UserValidation {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        System.out.println("=== User Registration Form ===");
        
        // Name validation
        String name;
        while(true) {
            System.out.print("Enter full name (2-50 chars): ");
            name = scanner.nextLine().trim();
            
            if(name.length() < 2 || name.length() > 50) {
                System.out.println("Name must be 2-50 characters!");
                continue;
            }
            
            if(!name.matches("^[a-zA-Z\\s]+$")) {
                System.out.println("Name can only contain letters and spaces!");
                continue;
            }
            
            break;
        }
        
        // Email validation
        String email;
        while(true) {
            System.out.print("Enter email: ");
            email = scanner.nextLine().trim().toLowerCase();
            
            if(!email.matches("^[\\w-\\.]+@([\\w-]+\\.)+[\\w-]{2,4}$")) {
                System.out.println("Invalid email format! Use: user@example.com");
                continue;
            }
            
            break;
        }
        
        // Password validation
        String password;
        while(true) {
            System.out.print("Enter password (min 8 chars, with digit & special char): ");
            password = scanner.nextLine();
            
            if(password.length() < 8) {
                System.out.println("Password must be at least 8 characters!");
                continue;
            }
            
            if(!password.matches(".*\\d.*")) {
                System.out.println("Password must contain at least one digit!");
                continue;
            }
            
            if(!password.matches(".*[!@#$%^&*].*")) {
                System.out.println("Password must contain at least one special character!");
                continue;
            }
            
            // Confirm password
            System.out.print("Confirm password: ");
            String confirmPassword = scanner.nextLine();
            
            if(!password.equals(confirmPassword)) {
                System.out.println("Passwords don't match!");
                continue;
            }
            
            break;
        }
        
        // Phone validation
        String phone;
        while(true) {
            System.out.print("Enter phone (XXX-XXX-XXXX): ");
            phone = scanner.nextLine().trim();
            
            // Remove non-digits
            String digitsOnly = phone.replaceAll("[^\\d]", "");
            
            if(digitsOnly.length() != 10) {
                System.out.println("Phone must have 10 digits!");
                continue;
            }
            
            // Format phone
            phone = digitsOnly.replaceFirst("(\\d{3})(\\d{3})(\\d{4})", "$1-$2-$3");
            break;
        }
        
        // Display user information
        System.out.println("\n=== Registration Successful ===");
        System.out.println("Name: " + name);
        System.out.println("Email: " + email);
        System.out.println("Phone: " + phone);
        System.out.println("Password: " + "*".repeat(password.length()));
        
        // Generate username (first initial + last name)
        String[] nameParts = name.split(" ");
        String username = "";
        if(nameParts.length >= 2) {
            username = nameParts[0].charAt(0) + nameParts[nameParts.length - 1];
        } else {
            username = name.toLowerCase().replaceAll("\\s", "");
        }
        username = username.toLowerCase();
        System.out.println("Suggested username: " + username);
        
        scanner.close();
    }
}

public class TextProcessor {
    
    // Count words in text
    public static int countWords(String text) {
        if(text == null || text.isBlank()) {
            return 0;
        }
        String[] words = text.trim().split("\\s+");
        return words.length;
    }
    
    // Count characters (excluding spaces)
    public static int countCharacters(String text) {
        if(text == null) return 0;
        return text.replaceAll("\\s", "").length();
    }
    
    // Reverse string
    public static String reverse(String text) {
        if(text == null) return null;
        return new StringBuilder(text).reverse().toString();
    }
    
    // Check palindrome
    public static boolean isPalindrome(String text) {
        if(text == null) return false;
        String clean = text.toLowerCase().replaceAll("[^a-z0-9]", "");
        String reversed = new StringBuilder(clean).reverse().toString();
        return clean.equals(reversed);
    }
    
    // Count vowel occurrences
    public static int countVowels(String text) {
        if(text == null) return 0;
        String lower = text.toLowerCase();
        int count = 0;
        for(char c : lower.toCharArray()) {
            if("aeiou".indexOf(c) != -1) {
                count++;
            }
        }
        return count;
    }
    
    // Remove duplicate words
    public static String removeDuplicates(String text) {
        if(text == null || text.isBlank()) return text;
        String[] words = text.split("\\s+");
        StringBuilder result = new StringBuilder();
        java.util.LinkedHashSet uniqueWords = new java.util.LinkedHashSet<>();
        
        for(String word : words) {
            uniqueWords.add(word.toLowerCase());
        }
        
        for(String word : uniqueWords) {
            result.append(word).append(" ");
        }
        
        return result.toString().trim();
    }
    
    // Capitalize first letter of each word
    public static String capitalizeWords(String text) {
        if(text == null || text.isBlank()) return text;
        String[] words = text.split("\\s+");
        StringBuilder result = new StringBuilder();
        
        for(String word : words) {
            if(!word.isEmpty()) {
                result.append(Character.toUpperCase(word.charAt(0)))
                      .append(word.substring(1).toLowerCase())
                      .append(" ");
            }
        }
        
        return result.toString().trim();
    }
    
    // Extract email addresses from text
    public static java.util.List extractEmails(String text) {
        java.util.List emails = new java.util.ArrayList<>();
        if(text == null) return emails;
        
        java.util.regex.Pattern pattern = 
            java.util.regex.Pattern.compile("[\\w-\\.]+@([\\w-]+\\.)+[\\w-]{2,4}");
        java.util.regex.Matcher matcher = pattern.matcher(text);
        
        while(matcher.find()) {
            emails.add(matcher.group());
        }
        
        return emails;
    }
    
    public static void main(String[] args) {
        String sampleText = "Hello world! Hello again. The quick brown fox jumps over the lazy dog. " +
                           "Contact me at john@example.com or support@company.org for more info.";
        
        System.out.println("=== Text Processing Demo ===\n");
        System.out.println("Original Text:");
        System.out.println(sampleText);
        System.out.println("\n" + "=".repeat(50) + "\n");
        
        System.out.println("Word Count: " + countWords(sampleText));
        System.out.println("Character Count (no spaces): " + countCharacters(sampleText));
        System.out.println("Vowel Count: " + countVowels(sampleText));
        
        System.out.println("\nReversed Text:");
        System.out.println(reverse(sampleText));
        
        System.out.println("\nCapitalized Text:");
        System.out.println(capitalizeWords(sampleText));
        
        System.out.println("\nWithout Duplicate Words:");
        System.out.println(removeDuplicates(sampleText));
        
        System.out.println("\nPalindrome Check:");
        System.out.println("\"racecar\" is palindrome: " + isPalindrome("racecar"));
        System.out.println("\"hello\" is palindrome: " + isPalindrome("hello"));
        System.out.println("\"A man, a plan, a canal: Panama\" is palindrome: " + 
                          isPalindrome("A man, a plan, a canal: Panama"));
        
        System.out.println("\nExtracted Emails:");
        java.util.List emails = extractEmails(sampleText);
        for(String email : emails) {
            System.out.println("  - " + email);
        }
        
        // Performance test
        System.out.println("\n" + "=".repeat(50));
        System.out.println("Performance Test: Building Large String");
        
        int iterations = 10000;
        long startTime, endTime;
        
        // Using String
        startTime = System.currentTimeMillis();
        String strResult = "";
        for(int i = 0; i < iterations; i++) {
            strResult += "word" + i + " ";
        }
        endTime = System.currentTimeMillis();
        System.out.println("String concatenation: " + (endTime - startTime) + "ms");
        
        // Using StringBuilder
        startTime = System.currentTimeMillis();
        StringBuilder sbResult = new StringBuilder();
        for(int i = 0; i < iterations; i++) {
            sbResult.append("word").append(i).append(" ");
        }
        endTime = System.currentTimeMillis();
        System.out.println("StringBuilder: " + (endTime - startTime) + "ms");
    }
}