StringBuffer vs StringBuilder: Key Differences | Generated by AI
1. Memory Overhead
- StringBuffer: Slightly higher memory overhead due to synchronization metadata and lock objects
- StringBuilder: Lower memory footprint, no synchronization structures needed
2. Method Invocation Cost
- StringBuffer: Each method call has monitor acquisition/release cost (even in single-threaded scenarios)
- StringBuilder: Direct method invocation without lock overhead
3. JVM Optimizations
- StringBuffer: Limited JIT compiler optimizations due to synchronization barriers
- StringBuilder: More aggressive optimizations possible (method inlining, escape analysis, stack allocation)
4. Inheritance Hierarchy
Both inherit from AbstractStringBuilder (package-private class), which contains the actual implementation:
Object
└── AbstractStringBuilder (contains char array and logic)
├── StringBuffer (adds synchronized wrapper)
└── StringBuilder (no synchronization)
5. Default Capacity
Both have the same default initial capacity:
- Default: 16 characters
- Grows dynamically:
(oldCapacity * 2) + 2when exceeded
6. API Compatibility
- Identical methods: append(), insert(), delete(), reverse(), substring(), charAt(), length(), capacity(), etc.
- Same return types: Both return
thisfor method chaining - Interchangeable: Can replace one with another by just changing the class name (if thread safety isn’t required)
7. Serialization
- StringBuffer: Implements
Serializable - StringBuilder: Implements
Serializable - Both can be serialized, but the underlying char array is serialized
8. Use in Collections/Frameworks
- Neither should be used as HashMap keys (both are mutable)
- StringBuilder preferred in toString() implementations
- StringBuffer used in older thread-safe utility classes
9. Compiler Behavior
// String concatenation in loops
String s = "";
for(int i = 0; i < 100; i++) {
s += i; // Compiler may use StringBuilder internally (Java 9+)
}
Modern Java compilers automatically use StringBuilder for string concatenation, not StringBuffer.
10. Benchmarking Context
Performance difference becomes significant with:
- High iteration counts (1000+ operations)
- Concurrent access patterns
- Large string manipulations
- Performance-critical applications
11. Backwards Compatibility
- StringBuffer: Cannot be removed due to legacy code dependencies
- StringBuilder: Recommended for all new code since Java 5
12. Common Mistake
// WRONG: Sharing StringBuilder between threads
StringBuilder sb = new StringBuilder();
thread1.run(() -> sb.append("A"));
thread2.run(() -> sb.append("B")); // Race condition!
// CORRECT: Use StringBuffer or external synchronization
StringBuffer sbf = new StringBuffer();
thread1.run(() -> sbf.append("A"));
thread2.run(() -> sbf.append("B")); // Thread-safe
Summary Table
| Aspect | StringBuffer | StringBuilder |
|---|---|---|
| Speed | Slower | Faster |
| Thread-Safe | Yes | No |
| Since | Java 1.0 | Java 5 |
| Synchronization | All methods | None |
| Use Case | Multi-threaded | Single-threaded |
| JVM Optimization | Limited | Aggressive |
| Memory | Higher | Lower |
| Default Choice | No | Yes |
Bottom Line: Unless you have explicit multi-threaded requirements, always use StringBuilder. It’s faster, more efficient, and the modern standard.