The final modifier can be applied to four Java constructs:
variables: a final variable can be set once and only once.
fields: a final field can also be set only once, by the constructor of the class which defines it.
methods: a final method cannot be overridden nor hidden.
classes: a final class cannot be extended.
Notice how using final is an entirely negative act. The final keyword works by subtracting, limiting default language mechanisms: the ability to override a method, to set a variable or a field. The motivations behind using final fall into three broad categories: correctness, robustness, and finally performance.
