Aggregation: has-a (weak)

1. Aggregation is a weak has-a relationship.
2. One class uses/contains another, but both can exist independently. The contained object can outlive the container or be shared among multiple containers.
3. UML Notation: o-- or Hollow diamond (◇) at the container class. Container-Class o-- Contained-Class
4. Example: Car has-a Engine.

Car o-- Engine

Following the code, for more details:

// Car.java
package oops.aggregation;

public class Car{
    Engine engineObj;

    Car(Engine engineObj){
        // Engine is passed from outside the Car
        this.engineObj = engineObj;
    }

    public static void main(String[] args){
        /*
         * Note: that Engine is created outside class, 
         * And it is passed in during car-object creation.
         */
        Engine engineObj = new Engine(2);
        Car carObj = new Car(engineObj);
        carObj.engineObj.drive();
        carObj.engineObj.drive();
        carObj.engineObj.drive();
    }
}

//Engine.java
package oops.aggregation;

public class Engine {
    private float fuel = 0;
    
    Engine(float fuel){
        this.fuel = fuel;
    }

    void drive(){
        if(this.fuel <= 0){
            System.out.println("Not Enough Fuel");
            return;
        }
        System.out.println("Drove one unit");
        this.fuel--;
    }
}