Interface

Interface can also used to model common features like Abstract, but it used for a weak is-a relationship, also known as an is-kind-of relationship, indicates that an object possesses a certain property. Abstract can be used to describe a strong is-a relationship.
Interface has no contructor, variables, and concrete methods. It only contains constants and abstract methods. This is also point that interface differs from the Abstract class.
Interface may empty, referred to as a marker interface. It contains no constants or methods, used to denote that a class posses certain desirable properties. A class may implement several interface to circumvent the single-inheritance restriction in java.