How is immutability of String type objects implemented in Java?

In Java, String objects are immutable due to the following implementation features:

• Final class: the String class is declared as final, preventing inheritance and behavior modification.
• Final fields: the internal character array (char[] value) is declared as final, and cannot be changed after object creation.
• No methods that modify state: all methods that seem to modify a string (e.g., concat, replace) actually create and return a new String object.
• Thread safety: immutability ensures thread safety without additional synchronization.

Example:

String s = "hello";
String t = s.toUpperCase(); // creates a new object, s remains "hello"

Thus, immutability is achieved through control over internal state and the absence of methods that modify it after creation.