Interfaces can hold Constant Definitions
Interfaces can hold fields, provided are static and final.
An interface can be a natural place to define a collection of related constants, perhaps simulating a C-like enumeration type:
public interface Direction {
public final static int NORTH = 0 ;
public final static int EAST = 1 ;
public final static int SOUTH = 2 ;
public final static int WEST = 4 ;
}
Use constants by, eg, Direction.NORTH.
Sometimes a class will implement such an interface, just so it can access the included constants without using the Direction prefix.