An interface allows you to guarantee that certain methods exist and return the required types. When the compiler knows this, it can use that assumption to work with unknown classes as if they had certain known behavior. For example, the comparable interface guarantees that an implementing class will be able to compareTo() some similar object and will return an int.
This means that you can compare anything that implements this interface - so you can sort anything that is Comparable instead of writing one method to sort Strings and another to sort Integers and another to sort LabelledBoxesOfBooks
