Question: Possible ways to refactor the code which had Java interface solely used to define lots of constants.. You can now imagine how this class is used to access these consts.

Answer: 
This is known as a [constant interface anti-pattern](http://en.wikipedia.org/wiki/Constant_interface). Although the previous link provides a way to fix this (using a class and static imports), I think there is a better way of refactoring this. Follow the [suggestion here](http://www.odi.ch/prog/design/newbies.php#40) to fix this. Overall it is better to move the constants to the appropriate classes/abstractions rather than using one utility constant class. For e.g Calendar class defines only the constants that are relevant to its operations. Also as *CoolBeans* suggested try converting those String constants to enums where applicable.
You could attack the source files with a shell script that does the following:

```
 for all .java files:
   if (content matches " class ? imports Singleton {"):
      replace "imports Singleton" with "" 
      append "import static Singleton.*;\n" after package declaration

```

This is far from perfect (it just ignores cases where a class imports Singleton and other interfaces...) but it could be a practical strategy - and maybe it's OK to solve 80% with a quick script and correct the remaining 20% manually (IDE will report errors).