String Methods in Java

public class StringMethodsIntro {
    public static void main(String[] args) {
        String message = "Hello World";
        // Call different methods on the string
        System.out.println("Original: " + message);
        System.out.println("Length: " + message.length());
        System.out.println("Uppercase: " + message.toUpperCase());
        System.out.println("Lowercase: " + message.toLowerCase());
        System.out.println("First char: " + message.charAt(0));
        System.out.println("Contains 'World': " + message.contains("World"));
        // Notice: original string is unchanged!
        System.out.println("Still: " + message);
    }
}

public class LengthAndInfo {
    public static void main(String[] args) {
        String text = "Java Programming";
        // length() - returns the number of characters
        int len = text.length();
        System.out.println("Length: " + len);  // 16
        // charAt(index) - returns character at position
        char firstChar = text.charAt(0);      // 'J'
        char lastChar = text.charAt(len - 1); // 'g'
        System.out.println("First: " + firstChar + ", Last: " + lastChar);
        // isEmpty() - checks if string is empty
        String empty = "";
        String notEmpty = "Hello";
        System.out.println("Is empty? " + empty.isEmpty());      // true
        System.out.println("Is notEmpty? " + notEmpty.isEmpty()); // false
        // isBlank() - checks if string is empty or only whitespace (Java 11+)
        String blank = "   ";
        System.out.println("Is blank? " + blank.isBlank());     // true
        System.out.println("Is empty? " + blank.isEmpty());     // false
    }
}

public class ManipulationMethods {
    public static void main(String[] args) {
        String text = "  Hello World  ";
        // toUpperCase() / toLowerCase()
        System.out.println(text.toUpperCase()); // "  HELLO WORLD  "
        System.out.println(text.toLowerCase()); // "  hello world  "
        // trim() - removes leading and trailing whitespace
        String trimmed = text.trim();
        System.out.println("[" + trimmed + "]"); // "[Hello World]"
        // replace(old, new) - replaces all occurrences
        String replaced = text.replace("World", "Java");
        System.out.println(replaced); // "  Hello Java  "
        // replaceAll(regex, replacement) - using regex patterns
        String noSpaces = text.replaceAll("\\s+", "-");
        System.out.println(noSpaces); // "-Hello-World-"
        // substring(start) - from start to end
        String sub1 = "Programming".substring(3);
        System.out.println(sub1); // "gramming"
        // substring(start, end) - from start to end-1
        String sub2 = "Programming".substring(3, 7);
        System.out.println(sub2); // "gram"
        // concat() - joins strings (can also use +)
        String greeting = "Hello".concat(" ").concat("World");
        System.out.println(greeting); // "Hello World"
    }
}

public class SearchingMethods {
    public static void main(String[] args) {
        String text = "Java is awesome and Java is fun";
        // contains(substring) - checks if substring exists
        boolean hasJava = text.contains("Java");
        System.out.println("Contains 'Java': " + hasJava); // true
        // indexOf(substring) - finds first occurrence (returns -1 if not found)
        int firstJava = text.indexOf("Java");
        System.out.println("First 'Java' at: " + firstJava); // 0
        // indexOf(substring, fromIndex) - search from position
        int secondJava = text.indexOf("Java", firstJava + 1);
        System.out.println("Second 'Java' at: " + secondJava); // 20
        // lastIndexOf(substring) - finds last occurrence
        int lastJava = text.lastIndexOf("Java");
        System.out.println("Last 'Java' at: " + lastJava); // 20
        // startsWith(prefix) - checks if starts with
        boolean startsWithJava = text.startsWith("Java");
        System.out.println("Starts with 'Java': " + startsWithJava); // true
        // endsWith(suffix) - checks if ends with
        boolean endsWithFun = text.endsWith("fun");
        System.out.println("Ends with 'fun': " + endsWithFun); // true
        // Example: checking if substring not found
        int notFound = text.indexOf("Python");
        if (notFound == -1) {
            System.out.println("'Python' not found in text");
        }
    }
}

public class ComparisonMethods {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "Hello";
        String str3 = "hello";
        String str4 = "World";
        // equals(other) - checks if content is exactly the same
        System.out.println(str1.equals(str2));  // true
        System.out.println(str1.equals(str3));  // false (case matters!)
        // equalsIgnoreCase(other) - ignores case differences
        System.out.println(str1.equalsIgnoreCase(str3)); // true
        // compareTo(other) - lexicographic comparison
        // Returns: 0 if equal, negative if this < other, positive if this > other
        System.out.println(str1.compareTo(str2)); // 0 (equal)
        System.out.println(str1.compareTo(str4)); // negative (H comes before W)
        System.out.println(str4.compareTo(str1)); // positive (W comes after H)
        // compareToIgnoreCase(other) - case-insensitive comparison
        System.out.println(str1.compareToIgnoreCase(str3)); // 0
        // IMPORTANT: Never use == for string comparison!
        String a = new String("Test");
        String b = new String("Test");
        System.out.println(a == b);        // false (different objects)
        System.out.println(a.equals(b));   // true (same content)
    }
}

public class SplitMethod {
    public static void main(String[] args) {
        // split(delimiter) - breaks string into array
        String fruits = "apple,banana,orange,grape";
        String[] fruitArray = fruits.split(",");
        System.out.println("Number of fruits: " + fruitArray.length);
        for (String fruit : fruitArray) {
            System.out.println("- " + fruit);
        }
        // Split by spaces
        String sentence = "Java is awesome";
        String[] words = sentence.split(" ");
        System.out.println("\nWords: " + words.length);
        for (String word : words) {
            System.out.println(word);
        }
        // Split with limit
        String data = "name:age:city:country";
        String[] parts = data.split(":", 2);
        System.out.println("\nFirst part: " + parts[0]);  // name
        System.out.println("Rest: " + parts[1]);           // age:city:country
        // Split with regex
        String mixed = "one1two2three3four";
        String[] pieces = mixed.split("\\d+"); // split by digits
        System.out.println("\nPieces:");
        for (String piece : pieces) {
            System.out.println(piece);
        }
    }
}

public class EmailValidator {
    public static boolean isValidEmail(String email) {
        // Check if email is null or empty
        if (email == null || email.isEmpty()) {
            return false;
        }
        // Trim whitespace
        email = email.trim();
        // Check if contains @ symbol
        if (!email.contains("@")) {
            return false;
        }
        // Split by @ to get username and domain
        String[] parts = email.split("@");
        if (parts.length != 2) {
            return false;
        }
        String username = parts[0];
        String domain = parts[1];
        // Validate username (not empty)
        if (username.isEmpty()) {
            return false;
        }
        // Validate domain (must contain .)
        if (!domain.contains(".")) {
            return false;
        }
        // Check domain doesn't start or end with .
        if (domain.startsWith(".") || domain.endsWith(".")) {
            return false;
        }
        return true;
    }
    public static void main(String[] args) {
        String[] emails = {
            "user@example.com",
            "invalid.email",
            "@nodomain.com",
            "noatsign.com",
            "user@.com",
            "user@domain.",
            "  valid@email.com  "
        };
        for (String email : emails) {
            boolean valid = isValidEmail(email);
            System.out.println(email + " -> " + (valid ? "VALID" : "INVALID"));
        }
    }
}