StringBuilder Class

StringBuilder Class

1. StringBuilder vs StringBuffer (beginner — one difference!)

Same API, same mutable magic — only difference: threads! StringBuffer = synchronized (thread-safe, slower). StringBuilder = NOT synchronized (faster, single-thread default!). Beginners: default to StringBuilder (loops, single thread).

2. Code — Drop-In Replacement (10× faster loop!)

// SLOW (String garbage!): 10,000 new objects!
String s = "";
for (int i = 0; i < 10000; i++) s += i; // O(n²) — exam's slow example!

// FAST (one object!): same result, ~100× faster!
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 10000; i++) sb.append(i);
String fast = sb.toString(); // convert once at end!
// StringBuffer version identical: new StringBuffer() + append (slower, safe threads)

3. Comparison Table (exam favourite!)

	String	StringBuffer	StringBuilder
Mutable?	No (new each edit!)	Yes (same object)	Yes (same object)
Threads?	Safe (immutable!)	Safe (synchronized)	NOT safe (fast!)
Speed (loop)	Slowest	Fast	Fastest
Use	fixed text	shared multi-thread text	single-thread building (default!)

4. Traps

• StringBuilder methods identical to StringBuffer (append/insert/delete/reverse — next topic!).
• Need String back? .toString() (both!).
• Multi-thread shared builder? Use StringBuffer (or lock!).

5. Exam Points

• Three-way table + loop program — 7 marks, very frequent.
• When to use which — 3 marks, frequent.