java.lang · java.base · since Java 1.5

StringBuilder

declaration

public final class StringBuilder
        implements Appendable, CharSequence, Serializable

Mutable string buffer for efficient incremental construction. The cure for quadratic loop concatenation; not thread-safe (its synchronized twin StringBuffer is legacy).

Presize with the expected capacity to avoid regrowth copies

Key methods

`StringBuilder append(…)`	Overloaded for every type; returns this for chaining.
`StringBuilder insert(int offset, …)`	Insert at position.
`StringBuilder deleteCharAt(int) / delete(int, int)`	Remove characters.
`StringBuilder reverse()`	In-place reversal.
`void setLength(int)`	Truncate or pad — setLength(0) resets for reuse.
`String toString()`	Snapshot the current contents.

Example

Java

StringBuilder sb = new StringBuilder(lines.size() * 32);   // presize when known
for (String line : lines) sb.append(line).append('\n');
String doc = sb.toString();