StringBuilder vs StringBuffer & Buffer Capacity Expansion

1Concept

Repeated String concatenation with `+` in loops creates intermediate throwaway objects. `StringBuilder` provides a mutable character array that appends characters in-place without heap allocations. `StringBuffer` is synchronized (thread-safe) but slower; `StringBuilder` is unsynchronized and preferred in 99% of single-threaded code.

2Architecture Diagram

Buffer Capacity Growth:
Initial Capacity: 16 chars ---> When exceeded: newCapacity = (oldCapacity * 2) + 2

3Code Example

public class StringBuilderDemo {
    public static void main(String[] args) {
        // Specifying initial capacity eliminates array resizing
        StringBuilder sb = new StringBuilder(64);
        sb.append("SELECT * FROM users ");
        sb.append("WHERE status = '").append("ACTIVE").append("' ");
        sb.append("ORDER BY id DESC;");

        String query = sb.toString();
        System.out.println("Generated SQL Query: " + query);
        System.out.println("Buffer Length: " + sb.length() + " | Allocated Capacity: " + sb.capacity());
    }
}

4Expected Output

Generated SQL Query: SELECT * FROM users WHERE status = 'ACTIVE' ORDER BY id DESC;
Buffer Length: 60 | Allocated Capacity: 64

5Key Takeaways

• ✓Always initialize StringBuilder with an estimated capacity to prevent multiple internal array reallocations.
• ✓Modern javac automatically converts simple `str1 + str2` concatenations to `invokedynamic` (StringConcatFactory).
• ✓Use StringBuffer ONLY when multiple threads concurrently mutate the same buffer.