What is the purpose of final keyword and when to use it?

The purpose of the final keyword is to make things unchanged in application lifeline. We can make a class, method or variable unchanged by writing "final" keyword before the method, class, and variable. When the final is written before the variable, it remains unchanged (you can’t change it). A function which use final keyword cannot be overridden. On the other hand, if we make a class final we cannot extend it. When we have to make these unchanged conditions, we prefer to use the final keyword.