public class StringUtils {        // Check if string is empty or null    public static boolean isEmpty(String str) {        return str == null || str.isEmpty();    }        // Check if string is blank (whitespace only)    public static boolean isBlank(String str) {        return str == null || str.trim().isEmpty();    }        // Reverse string    public static String reverse(String str) {        if (str == null) return null;        return new StringBuilder(str).reverse().toString();    }        // Count occurrences    public static int countOccurrences(String str, String substring) {        if (str == null || substring == null || substring.isEmpty()) {            return 0;        }        int count = 0;        int index = 0;        while ((index = str.indexOf(substring, index)) != -1) {            count++;            index += substring.length();        }        return count;    }        // Remove whitespace    public static String removeWhitespace(String str) {        if (str == null) return null;        return str.replaceAll("\\s+", "");    }        // Capitalize first letter    public static String capitalize(String str) {        if (isEmpty(str)) return str;        return str.substring(0, 1).toUpperCase() + str.substring(1);    }        // Join strings    public static String join(String delimiter, String... strings) {        if (strings == null || strings.length == 0) return "";        StringBuilder result = new StringBuilder(strings[0]);        for (int i = 1; i < strings.length; i++) {            result.append(delimiter).append(strings[i]);        }        return result.toString();    }        // Truncate string    public static String truncate(String str, int maxLength) {        if (str == null || str.length() <= maxLength) return str;        return str.substring(0, maxLength) + "...";    }}

// String creationString str1 = "Hello";String str2 = new String("World");// ConcatenationString combined = str1 + " " + str2; // "Hello World"String withConcat = str1.concat(" Java"); // "Hello Java"// Lengthint length = str1.length(); // 5// Character accesschar first = str1.charAt(0); // 'H'// SubstringString sub1 = str1.substring(1);     // "ello"String sub2 = str1.substring(1, 4);  // "ell"// Searchboolean contains = str1.contains("ell"); // trueint index = str1.indexOf('l');            // 2int lastIndex = str1.lastIndexOf('l');     // 3boolean startsWith = str1.startsWith("He"); // trueboolean endsWith = str1.endsWith("lo");    // true// ReplaceString replaced = str1.replace('l', 'L');        // "HeLLo"String replacedAll = str1.replaceAll("l", "L"); // "HeLLo"String replacedFirst = str1.replaceFirst("l", "L"); // "HeLlo"// Case conversionString upper = str1.toUpperCase(); // "HELLO"String lower = upper.toLowerCase(); // "hello"// SplitString text = "one,two,three";String[] parts = text.split(","); // ["one", "two", "three"]// TrimString withSpaces = "  Hello  ";String trimmed = withSpaces.trim(); // "Hello"// FormatString formatted = String.format("Name: %s, Age: %d", "Alice", 30);// "Name: Alice, Age: 30"// StringBuilder for efficient concatenationStringBuilder sb = new StringBuilder();sb.append("Hello");sb.append(" ");sb.append("World");String result = sb.toString(); // "Hello World"// String comparisonboolean equals = str1.equals("Hello"); // trueboolean equalsIgnoreCase = str1.equalsIgnoreCase("HELLO"); // trueint compare = str1.compareTo("World"); // negative value// Check emptyboolean isEmpty = str1.isEmpty(); // falseboolean isBlank = "   ".trim().isEmpty(); // true