String Manipulation

String s = "hello";
s = s.toUpperCase(); // creates a NEW String "HELLO"; original unchanged

String a = "hello";          // stored in String pool
String b = "hello";          // same reference from pool
String c = new String("hello"); // new heap object (avoid this)

System.out.println(a == b);  // true  (same pool reference)
System.out.println(a == c);  // false (different objects)
System.out.println(a.equals(c)); // true (same content)

// Bad: creates N intermediate String objects
String result = "";
for (String s : list) {
    result += s; // O(n²) time!
}

// Good: single mutable buffer
StringBuilder sb = new StringBuilder();
for (String s : list) {
    sb.append(s);
}
String result = sb.toString();

// Common builder operations
sb.append("text");
sb.insert(0, "prefix");
sb.delete(2, 5);
sb.reverse();
sb.replace(1, 3, "new");

String s = "  Hello, World!  ";

// Inspection
s.length()                     // 17
s.isEmpty()                    // false
s.isBlank()                    // false (Java 11+)
s.charAt(2)                    // 'H'
s.indexOf('o')                 // 4
s.contains("World")            // true

// Transformation
s.trim()                       // "Hello, World!"
s.strip()                      // Java 11+, handles Unicode whitespace
s.toLowerCase()                // "  hello, world!  "
s.toUpperCase()                // "  HELLO, WORLD!  "
s.replace('l', 'r')            // "  Herro, Worrd!  "
s.replaceAll("[aeiou]", "*")   // regex replace

// Splitting and joining
s.split(",")                   // ["  Hello", " World!  "]
String.join(", ", "a", "b", "c") // "a, b, c"
String.join("-", List.of("x", "y")) // "x-y"

// Substring
s.substring(2, 7)              // "Hello"
s.startsWith("Hello", 2)       // true
s.endsWith("!"  )              // true (after trim)

// Java 11+ methods
"  ".isBlank()                 // true
"a\nb\nc".lines().count()      // 3
"ab".repeat(3)                 // "ababab"
"  hi  ".stripLeading()        // "hi  "
"  hi  ".stripTrailing()       // "  hi"

// String.format
String msg = String.format("Hello, %s! You are %d years old.", name, age);

// Java 15+ Text Blocks
String json = """
        {
            "name": "%s",
            "age": %d
        }
        """.formatted(name, age);

// String to char array and back
char[] chars = s.toCharArray();
String back = new String(chars);

// Reverse a string
String reversed = new StringBuilder(s).reverse().toString();

// Check palindrome
public boolean isPalindrome(String s) {
    int left = 0, right = s.length() - 1;
    while (left < right) {
        if (s.charAt(left++) != s.charAt(right--)) return false;
    }
    return true;
}