StringBuffer

StringBuffer is a class in the Java standard library that represents a mutable sequence of characters. Unlike String, which is immutable, a StringBuffer can be modified in place, avoiding the creation of intermediate objects during repeated concatenation. StringBuffer is part of java.lang and is designed to be thread-safe through synchronized methods, making it suitable for use in multi-threaded contexts where a shared string-like buffer is updated by multiple threads.

Typical operations include appending, inserting, deleting, and replacing characters. Common methods are append, insert, delete, deleteCharAt, setCharAt, reverse, setLength, and capacity-related methods such as capacity and ensureCapacity. The class also implements CharSequence and Appendable, and provides conversion to String via toString.

A key distinction from the related class StringBuilder is synchronization. StringBuffer methods are synchronized, which enforces safe access from multiple threads but incurs overhead. For single-threaded code, StringBuilder offers similar functionality with better performance. If thread safety is required only for specific sequences, external synchronization around a StringBuilder may be preferred.

StringBuffer has several constructors: a no-argument constructor that creates an empty buffer with a default capacity, a constructor that accepts an initial capacity, and a constructor that initializes the buffer with the contents of a given string. Internally, StringBuffer maintains a character array and a count of characters; it grows the array when needed, typically by expanding capacity.

In practice, StringBuffer is used when a mutable string is built incrementally in a multi-threaded environment, while StringBuilder is chosen for non-concurrent use cases. The toString method returns an immutable String containing the current contents.