TLDR: When a variable, method, or class is declared `final` in programming languages like Java, it indicates immutability or restriction in behavior. For variables, `final` prevents reassignment after initialization. For methods, it disallows overriding in subclasses, and for classes, it prevents inheritance entirely. Declaring elements as `final` enhances reliability, security, and predictability in software development.

https://en.wikipedia.org/wiki/Final_(Java)

In Java, a `final` variable must be initialized once and cannot be changed afterward, making it ideal for defining constants. For example, declaring `final double PI = 3.14159;` ensures that the value of `PI` remains constant throughout the program. Methods marked `final` cannot be overridden by subclasses, preserving the original implementation. This is particularly useful when the method contains sensitive or critical logic that should remain unaltered.

https://docs.oracle.com/javase/tutorial/java/IandI/final.html

A `final` class, such as `String` in Java, cannot be extended, preventing the creation of subclasses. This restriction enhances security and encapsulation, ensuring that the class's behavior remains consistent and cannot be inadvertently modified. While powerful, the `final` keyword must be used judiciously, as it can limit flexibility and extensibility in cases where customization is necessary.

https://docs.oracle.com/javase/specs/jls/se20/html/jls-8.html