StringBuilder vs StringBuffer

Both StringBuilder and StringBuffer are classes in Java used to create mutable strings.
They allow modification of string content without creating new objects like the String class.

The key difference between them is thread safety.

Simple Definition

StringBuilder: Mutable string class that is not thread-safe, but faster.
StringBuffer: Mutable string class that is thread-safe (synchronized).

Example: StringBuilder

StringBuilder sb = new StringBuilder("Java");

sb.append(" Programming");

System.out.println(sb); // Java Programming

Example: StringBuffer

StringBuffer sb = new StringBuffer("Java");

sb.append(" Programming");
 
System.out.println(sb); //  Java Programming

Key Differences

Feature	StringBuilder	StringBuffer
Thread Safety	Not thread-safe	Thread-safe
Synchronization	Not synchronized	Synchronized
Performance	Faster	Slower
Introduced	Java 5	Java 1.0
Use Case	Single-threaded applications	Multi-threaded applications

Performance Difference

Because StringBuffer methods are synchronized, only one thread can access them at a time.

This makes it:

Safe for multi-threaded environments
Slightly slower than StringBuilder

StringBuilder does not use synchronization, so it performs faster in single-threaded scenarios.

When To Use

Use StringBuilder when:

Application is single-threaded
Performance is important
Frequent string modifications are required

Use StringBuffer when:

Application is multi-threaded
Thread safety is required
Multiple threads may modify the same string

Summary

Both classes create mutable strings
StringBuilder is faster but not thread-safe
StringBuffer is thread-safe but slightly slower
Choose based on thread safety requirements