What is String, why is it immutable? What happens during concatenation?

In Java, the String class represents an immutable sequence of characters. Immutability means that once a String object is created, its contents cannot be changed. This is done for security, ease of use in multithreaded environments, and optimization (e.g., string pool).

When concatenating strings, for example using the + operator, a new String object is actually created that contains the combined text. The old objects remain unchanged. This can lead to excessive object creation and reduced performance when many concatenation operations are performed.

Example:

String a = "Hello";
String b = a + " World"; // creates a new String object with the content "Hello World"

For efficient concatenation in loops, it is recommended to use StringBuilder or StringBuffer classes, which allow modifying the content without creating new objects.