IS-A Relationship
in OOP this sort of relationship refers to inheritance, both class and interface types. In the hierarchy tree, traversing up to determine whether the relation corresponds to IS-A.
Assume the situation simply; Class A implements Interface B, then we can say Class A IS-A Interface B, this is to traverse up, and the reverse is not true because it is traverse down.
public interface Creature{}
public interface Animal extends Creature{}
public interface Movable{}
public class Dog extends Animal implements Movable{}
Correct
- Dog IS-A Animal,
- Dog IS-A Movable,
- Animal IS-A Creature.
Incorrect
- Creature IS-A Animal (Traverse down doesn’t count),
- Movable IS-A Creature (There is no relation defined in between),
- Animal IS-A Dog (Traverse down doesn’t count),
- Animal IS-A Movable (There is not traverse up relation, there is only connection via Dog, but since we cannot traverse down, this doesn’t count either)
HAS-A Relationship
This is simply to define an instance of an object in a different object and instantiation doesn’t matter. This type of relation is also known as composition. Assume that we have Car and Engine Objects. A car object having an Engine instance defined.
public class Engine {}
public class Car {
Engine engine;
}