Explain the concept of strings that cannot be changed after their creation.

In Java, strings (objects of the String class) are immutable. This means that once a string object is created, its contents cannot be changed.

If you perform operations that seem to modify a string (for example, concatenation), a new String object with the new content is actually created, and the old one remains unchanged.

This ensures safety when working with strings in a multithreaded environment and allows strings to be used as keys in collections without the risk of modification.

Example:

String s = "Hello";
String t = s + " World"; // a new string object is created
// s remains "Hello"
// t contains "Hello World"