- Is A (Inheritence)
- Has A (Composition)
- Works With (Collaboration)
Is A (Inheritence)
In object-oriented programming, the concept of IS-A is a totally based on Inheritance, which can be of two types Class Inheritance or Interface Inheritance.
It is just like saying “A is a B type of thing”. For example, Apple is a Fruit, Car is a Vehicle, etc. Inheritance is uni-directional. For example, House is a Building. But Building is not a House.
It is a key point to note that you can easily identify an IS-A relationship. Wherever you see an extends keyword or implements keyword in a class declaration, then this class is said to have IS-A relationship.
Has A (Composition)
Composition (HAS-A) simply mean the use of instance variables that are references to other objects. For example Maruti has Engine, or House has Bathroom.
package relationships;
class Car {
// Methods implementation and class/Instance members
private String color;
private int maxSpeed;
public void carInfo() {
System.out.println("Car Color= " + color + " Max Speed= " + maxSpeed);
}
public void setColor(String color) {
this.color = color;
}
public void setMaxSpeed(int maxSpeed) {
this.maxSpeed = maxSpeed;
}
}
class Maruti extends Car {
//Maruti extends Car and thus inherits all methods from Car (except final and static)
//Maruti can also define all its specific functionality
public void MarutiStartDemo() {
Engine MarutiEngine = new Engine();
MarutiEngine.start();
}
}
As shown above, Car class has a couple of instance variable and few methods. Maruti is a specific type of Car which extends Car class means Maruti IS-A Car.
Maruti class uses Engine object’s start() method via composition. We can say that Maruti class HAS-A Engine.