Local final variables do not work in Groovy

I noticed that something was really strange in Grails project I worked on. When I found out what is going on I couldn’t believe my eyes. Groovy do not respect local final variables. Yes, I really mean that, do not respect, not at all.

First of all, I want to remind how it works in java:

public class FinalTest {
    public static void main(String[] args) {
        final int var = 1;
        var = 2;
        assert 1 == var;
    }
}

This code above can not be compiled. Java says:

chriss-MBP:final_test andrej$ javac FinalTest.java 
FinalTest.java:4: error: cannot assign a value to final variable var
        var = 2;
        ^
1 error

Ok, now let’s look at the similar script in Groovy. I called it final_test.groovy, that’s it’s content:

final def var = 1
var = 2
assert 1 == var

Let’s run it:

chriss-MBP:final_test andrej$ groovy final_test.groovy 
Caught: Assertion failed: 

assert 1 == var
         |  |
         |  2
         false

Assertion failed: 

assert 1 == var
         |  |
         |  2
         false

	at final_test.run(final_test.groovy:3)

Script was successfully compiled, no compile time errors. At runtime Groovy ignored final keyword and just assigned value 2 to the variable var.

For testing I was using Java version 1.7.0_79 for MacOS and Groovy 2.4.4. The test examples are in repository. You can test it in your environment if you do not think that my experiment is good enough. :)

Maybe in the future they will fix this problem with final variables: GROOVY-1628

Be careful with that, do not rely on Groovy.

Happy coding everybody! :)