Use Final Liberally

Summary

Consider making final any variable or parameter that does not change.

Details

Making parameters and variables that are assigned once final makes a method easier to understand because it constrains the things that could possibly happen within the code.

It would be reasonable to make all parameters and assign-once variables final, but this needs to be weighed against the noise created by inserting the final keyword everywhere.

For short methods, whether the benefit outweighs the cost is arguable, but if a method is large and unwieldy then the case for making things final is much stronger.

Each team should agree a policy for making final variables.

At a minimum, everything should be made final within large methods. This may also be extended to shorter methods at the team's discretion. A blanket policy has the advantage of being easy to automate/understand. A more nuanced policy is harder to communicate.

When working with legacy code, making parameters and variables final is also a useful first step in gaining understanding of the method before re-factoring. Methods that have proved difficult to express in smaller chunks will also become easier to understand when single assignment variables are final.