1.4 Text Blocks, String Immutability, and StringBuilder APIs

Text Blocks, String Immutability, and StringBuilder APIs

String processing is central to Java development and exam assessments. Java SE 17 includes text blocks (standardized in Java 15 via JEP 378), modern String inspection and transformation methods added across Java 11–17, and high-performance in-place mutation using StringBuilder.

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Text Blocks Syntax and Indentation Algorithms

A text block is a multi-line string literal that begins and ends with three double-quote characters ("""). It eliminates the need for manual escape sequences (\n, \") when authoring embedded HTML, SQL, JSON, or XML snippets.

Delimiter Requirements

The opening """ delimiter may be followed only by optional white space and then a line terminator. You cannot place actual text content on the same line as the opening """:

// COMPILE ERROR: illegal text block open delimiter sequence, missing newline
// String bad = """SELECT * FROM users""";

// Correct syntax:
String query = """
    SELECT id, name
    FROM users
    WHERE status = 'ACTIVE';
    """;

Incidental vs. Essential Whitespace

Java separates indentation into incidental whitespace (boilerplate indentation added by an IDE or developer to keep code aligned) and essential whitespace (indentation intended to remain in the final string). During compilation, the compiler executes a three-step normalization process:

1. Line terminators are normalized to standard newline characters (\n).
2. Incidental whitespace is stripped: The compiler calculates the common minimum leading whitespace across all non-blank lines and the line containing the closing """ delimiter.
3. Escape sequences are interpreted.

String block1 = """
        Red
        Green
        Blue
        """; // Closing delimiter indented 8 spaces; all lines have 8 leading spaces stripped
// Result: "Red\nGreen\nBlue\n"

String block2 = """
        Red
        Green
        Blue
"""; // Closing delimiter at column 0 (0 spaces)
// Result: "        Red\n        Green\n        Blue\n"

Text Block Escape Sequences

Inside text blocks, double quotes (") and double pairs ("") do not require escaping. Escaping is only required for three consecutive quotes (\"\"\"). Java introduces two specific escape sequences for text blocks:

• \ (Line continuation): Suppresses the line break, joining two lines into a single logical line.
• \s (Explicit space): Translates to an ASCII space character (\u0020) and preserves trailing whitespace that would otherwise be trimmed by the text block compiler algorithm.

String singleLine = """
    SELECT id, name \
    FROM employees \
    WHERE active = true;
    """;
// Result: "SELECT id, name FROM employees WHERE active = true;\n"

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String Immutability and the String Constant Pool

String objects in Java are immutable. Once created, their internal byte arrays cannot be modified. Any method that appears to modify a String (such as concat(), replace(), or toLowerCase()) creates and returns a brand-new String object.

The String Constant Pool and intern()

The JVM maintains a specialized memory region inside the heap called the String Constant Pool to conserve memory:

• String Literals: Automatically interned into the pool. Identical literals share the same object reference.
• new String("abc"): Explicitly bypasses the pool, allocating a new, distinct String object on the heap.
• intern(): Searches the pool for a string with equal character content. If found, returns the pooled reference; if not found, adds the string to the pool and returns its reference.

String s1 = "Java";
String s2 = "Java";
String s3 = new String("Java");
String s4 = s3.intern();

System.out.println(s1 == s2); // true (both reference the same pooled literal)
System.out.println(s1 == s3); // false (s3 is a separate heap instance)
System.out.println(s1 == s4); // true (s4 refers to the pooled instance returned by intern())

Compile-Time Constant Concatenation

Expressions composed purely of compile-time constants (literals or final variables) are evaluated at compile time and pooled. Expressions involving non-final variables are evaluated at runtime, producing new heap instances:

String base = "Java";
final String finalBase = "Java";

String test1 = base + " 17";        // Runtime concatenation -> new heap object
String test2 = finalBase + " 17";   // Compile-time constant expression -> interned literal
String literal = "Java 17";

System.out.println(test1 == literal); // false
System.out.println(test2 == literal); // true

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Modern String Methods (Java 11 through Java 17)

Modern Java versions introduced powerful methods on java.lang.String:

String text = "  \u2000Hello Java\u2000  ";
System.out.println(text.trim().length());  // trim() fails to remove Unicode whitespace \u2000
System.out.println(text.strip().length()); // strip() removes full Unicode whitespace

"abc ".repeat(3); // "abc abc abc "
"line1\nline2".lines().count(); // 2

String result = "hello"
    .transform(String::toUpperCase)
    .transform(s -> s + " WORLD"); // "HELLO WORLD"

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StringBuilder vs. StringBuffer

StringBuilder (introduced in Java 5) and StringBuffer (legacy Java 1.0) represent mutable sequences of characters:

• StringBuilder: Not thread-safe (methods are unsynchronized). Offers maximum performance for single-threaded usage.
• StringBuffer: Thread-safe (methods are synchronized). Carries locking overhead; rarely needed in modern code.

Internal Capacity Mechanics

A StringBuilder maintains an internal buffer with an initial capacity:

• new StringBuilder(): Default capacity of 16 characters.
• new StringBuilder(int capacity): Explicit capacity set to specified integer.
• new StringBuilder(String str): Capacity initialized to str.length() + 16.

When appends exceed capacity, the buffer automatically expands to (oldCapacity * 2) + 2.

Mutation Methods and Method Chaining

All mutating methods return a reference to this (StringBuilder), enabling fluent method chaining:

StringBuilder sb = new StringBuilder("Java");
sb.append(" 17")             // "Java 17"
  .insert(0, "Modern ")      // "Modern Java 17"
  .replace(0, 6, "Fast")     // "Fast Java 17"
  .delete(4, 9)              // "Fast 17" (deletes indices 4 through 8 inclusive)
  .reverse();                // "71 tsaF"

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Core String Methods You Must Trace by Hand

Most String questions on the exam are about exact return values and index boundaries rather than modern additions. Indexes are zero-based, and every end index is exclusive.

Other methods to recognize: startsWith, endsWith, contains, toUpperCase, toLowerCase, trim, isEmpty (length 0), isBlank (only whitespace), concat, and chars() (an IntStream). Because strings are immutable, a statement such as t.concat(" 17"); that ignores the return value leaves t unchanged.

More StringBuilder methods

StringBuilder also offers deleteCharAt(int), setCharAt(int, char) (returns void, so it cannot be chained further), setLength(int), indexOf(String), charAt(int), and length(). For example, new StringBuilder("abcdef").deleteCharAt(1).insert(2, "-") produces "ac-def", and setLength(2) on "hello" truncates it to "he".