Understand immutable strings and learn when to use mutable StringBuilder and StringBuffer.
Handling text is a fundamental part of programming, and in Java, this is primarily done through the `String` class. A crucial characteristic of Java's `String` objects is that they are immutable. This means that once a string object is created, its contents cannot be altered. Any method that appears to modify a string, like `concat()` or `toUpperCase()`, actually creates and returns a new `String` object with the modified content, leaving the original string unchanged. This immutability provides benefits like thread safety and security. However, it can be inefficient if you need to perform many string modifications in a loop, as each modification creates a new object, leading to memory overhead. To address this, Java provides two mutable string classes: `StringBuilder` and `StringBuffer`. Both classes allow you to modify the sequence of characters they contain without creating new objects each time. The main difference between them lies in thread safety. `StringBuffer` is thread-safe, meaning its methods are synchronized, and it can be safely used by multiple threads simultaneously. This synchronization, however, adds a performance overhead. `StringBuilder`, introduced in Java 5, is a non-thread-safe alternative. It provides the same API as `StringBuffer` but is faster because it doesn't have the overhead of synchronization. The general rule is: use `String` for text that won't change, use `StringBuilder` for mutable strings in a single-threaded environment (the most common case), and use `StringBuffer` only when you need a mutable string in a multi-threaded environment.