having problem deciding whether interface, abstract class, or concrete class shld b used...

i'm not used to having interface class, where the variables need to be constant (cannot b changed). den, abstract class is not permitted. it's really hard to find a reason to use interface..

but after discussing n doing some practical work, i somehow thot of a way to describe it: use interface when u want to force subclasses to implement methods with the same names. n well, there can b multiple interfaces in java to be implemented, but not class. haha.. tis is how i see it for now.. am i thinking in the right track?