interfaces are used in java
to impose or to provide some functionality to the user.
means if want to force user to do something which will be helpful for user only or to stenderzie the code for all we will develop interfaces.
so anything we will write inside the interface is to provide flexibility for the users at the same time to maintain uniqueness among the code we have the things inside the interface which user can user for flexibility but can't modify to achieve the uniqueness in the code architecture.
so because of this only by default member variables are final in an interface...
you can use but can't be able to change the value.....
interfaces are something like propretry softwares you can use the things but can't change the things.