Java String Methods Explained — The Complete Beginner's Guide

public class StringBasics {
    public static void main(String[] args) {

        // Creating a String — Java stores this in the String Pool
        String greeting = "Hello, World!";

        // length() — returns the total number of characters including spaces and punctuation
        int totalCharacters = greeting.length();
        System.out.println("Total characters: " + totalCharacters); // 13

        // charAt(index) — returns the single character at that position
        // Remember: indexing starts at 0, not 1
        char firstChar = greeting.charAt(0);  // 'H' is at position 0
        char seventhChar = greeting.charAt(7); // 'W' is at position 7
        System.out.println("First character: " + firstChar);   // H
        System.out.println("Seventh character: " + seventhChar); // W

        // Strings are immutable — this does NOT change the original
        String shoutyGreeting = greeting.toUpperCase();
        System.out.println("Original is unchanged: " + greeting);       // Hello, World!
        System.out.println("New uppercase string: " + shoutyGreeting);  // HELLO, WORLD!
    }
}

public class UsefulStringMethods {
    public static void main(String[] args) {

        String userEmail = "  support@thecodeforge.io  "; // Notice the leading/trailing spaces

        // trim() removes whitespace from both ends — critical for cleaning user input
        String cleanEmail = userEmail.trim();
        System.out.println("Cleaned email: '" + cleanEmail + "'");
        // Output: 'support@thecodeforge.io'

        // contains() — asks 'does this String contain this text?'
        boolean hasAtSymbol = cleanEmail.contains("@");
        System.out.println("Contains '@': " + hasAtSymbol); // true

        // endsWith() — great for validating file types or domains
        boolean isIoEmail = cleanEmail.endsWith(".io");
        System.out.println("Ends with .io: " + isIoEmail); // true

        // indexOf() — find where something first appears. Returns -1 if not found
        int atPosition = cleanEmail.indexOf("@");
        System.out.println("'@' found at index: " + atPosition); // 7

        // substring(start, end) — extract a slice of the String
        // Extract just the domain part: everything after the '@'
        String domain = cleanEmail.substring(atPosition + 1); // from index 8 to end
        System.out.println("Domain: " + domain); // thecodeforge.io

        // Extract just the username: everything before the '@'
        String username = cleanEmail.substring(0, atPosition); // from 0 up to (not including) '@'
        System.out.println("Username: " + username); // support

        // replace() — swap out characters or substrings. Returns a NEW String.
        String updatedDomain = domain.replace(".io", ".com");
        System.out.println("Updated domain: " + updatedDomain); // thecodeforge.com
        System.out.println("Original domain unchanged: " + domain); // thecodeforge.io

        // toLowerCase() / toUpperCase() — useful for case-insensitive comparisons
        String productCode = "PRD-4821-X";
        System.out.println("Lowercase code: " + productCode.toLowerCase()); // prd-4821-x

        // isEmpty() — true if the String has zero characters (length is 0)
        String emptyField = "";
        System.out.println("Is empty: " + emptyField.isEmpty()); // true

        // isBlank() — true if empty OR contains only whitespace (added in Java 11)
        String blankField = "   ";
        System.out.println("Is blank: " + blankField.isBlank()); // true
    }
}

public class StringComparison {
    public static void main(String[] args) {

        String language1 = "Java";          // stored in String Pool
        String language2 = "Java";          // Java reuses the same pool entry
        String language3 = new String("Java"); // forces a NEW object in heap memory

        // == compares memory addresses (references), NOT content
        System.out.println(language1 == language2);  // true  (same pool object)
        System.out.println(language1 == language3);  // false (different object in heap!)

        // .equals() compares the actual text content — use this for String comparison
        System.out.println(language1.equals(language2)); // true
        System.out.println(language1.equals(language3)); // true — same text, right answer!

        // Real-world scenario: validating a password reset answer
        String correctAnswer = "Fluffy";          // stored answer
        String userAnswer = "  fluffy  ";         // user typed with extra spaces and lowercase

        // Wrong approach — this would fail even though the answer is essentially correct
        boolean wrongCheck = correctAnswer.equals(userAnswer);
        System.out.println("Wrong check result: " + wrongCheck); // false

        // Right approach: clean the input first, then compare without case sensitivity
        boolean correctCheck = correctAnswer.equalsIgnoreCase(userAnswer.trim());
        System.out.println("Correct check result: " + correctCheck); // true

        // compareTo() — useful for sorting. Returns 0 if equal, negative if 'less', positive if 'greater'
        String alpha = "Apple";
        String beta = "Banana";
        int comparison = alpha.compareTo(beta);
        System.out.println("compareTo result: " + comparison); // negative number (Apple comes before Banana alphabetically)
    }
}

import java.util.Arrays;

public class SplitJoinFormat {
    public static void main(String[] args) {

        // --- SPLIT ---
        // Imagine reading a CSV line from a file
        String csvLine = "Alice,30,Engineer,London";

        // split() breaks the String at every comma, returns a String array
        String[] fields = csvLine.split(",");

        System.out.println("Number of fields: " + fields.length); // 4
        System.out.println("Name: " + fields[0]);       // Alice
        System.out.println("Age: " + fields[1]);        // 30
        System.out.println("Role: " + fields[2]);       // Engineer
        System.out.println("City: " + fields[3]);       // London

        // --- JOIN ---
        // Now rebuild the CSV line with a different delimiter
        String tsvLine = String.join("\t", fields); // join with a tab character
        System.out.println("Tab-separated: " + tsvLine);
        // Alice    30    Engineer    London

        // Join a fresh set of values directly
        String fullName = String.join(" ", "James", "Arthur", "Morrison");
        System.out.println("Full name: " + fullName); // James Arthur Morrison

        // --- FORMAT ---
        // String.format() — cleaner than messy concatenation with +
        String name = "Maria";
        int orderCount = 5;
        double totalSpend = 149.99;

        // %s = String placeholder, %d = integer placeholder, %.2f = float with 2 decimal places
        String orderSummary = String.format(
            "Customer: %s | Orders: %d | Total Spend: $%.2f",
            name, orderCount, totalSpend
        );
        System.out.println(orderSummary);
        // Customer: Maria | Orders: 5 | Total Spend: $149.99

        // --- REPEAT (Java 11+) ---
        // Repeat a String N times — handy for formatting or testing
        String divider = "-".repeat(30);
        System.out.println(divider); // ------------------------------

        // --- TEXT BLOCK (Java 15+) ---
        // Write multi-line text without escape characters
        String jsonSnippet = """
                {
                  "name": "Maria",
                  "orders": 5
                }
                """;
        System.out.println(jsonSnippet);
    }
}