Javaeasy

String vs StringBuilder vs StringBuffer — when to use which?

Answer

`String` is immutable, so repeated concatenation creates many objects. `StringBuilder` is mutable and fast for building strings in a single thread; `StringBuffer` is similar but synchronized (rarely needed today).

StringBuilder sb = new StringBuilder();
for (int i = 0; i < 3; i++) {
  sb.append(i).append(",");
}
System.out.println(sb.toString());

Advanced answer

Deep dive

`String` is immutable: concatenation creates new objects.
`StringBuilder` is mutable and not synchronized: fastest for building strings in a single thread (or when the builder is not shared).
`StringBuffer` is like `StringBuilder` but synchronized: safer if one instance is shared across threads, but slower.

Practical guidance

Use `String` for fixed literals and small, non-loop concatenations.
Use `StringBuilder` in loops and when building large strings.
Prefer `StringBuilder` over `StringBuffer` unless you truly share the same instance across threads.

Example

StringBuilder sb = new StringBuilder();
for (int i = 0; i < 3; i++) sb.append(i).append(",");
String out = sb.toString();

Common pitfalls

`s += x` in a loop (many temporary Strings).
Assuming `StringBuilder` is thread-safe.
Not sizing builders when you know approximate length (`new StringBuilder(capacity)`).