Java in Practice: Coding and UML Aggregation v/s Composition

Aggregation [ 'HAS-A' Relationship ]

class StereoSystem {
    private boolean state ;
    StereoSystem() {}
    StereoSystem(boolean state) {
        this.state = state ;
        System.out.println("Stereo System State: " + (state == true ? "On!" : "Off!")) ;
    }
}

class Car {
    private StereoSystem s ;
    Car() {}
    Car(String name, StereoSystem s) {
        this.s = s ;
    }
    public static void main(String[] args) {
        StereoSystem ss = new StereoSystem(true) ;  // true(System is ON.) or false (System is OFF)
        Car c = new Car("BMW", ss) ;
    }
}


In UML Aggregation is Expressed as

Composition [ 'Contains' Relationship ]

import java.util.Date ;

class Piston {
    private Date pistonDate ;
    Piston() {
        pistonDate = new Date() ;
        System.out.println("Manufactured Date :: " + pistonDate) ;
    }
}

class Engine {
    private Piston piston ;
    Engine() {
        piston = new Piston() ;
    }
    public static void main(String[] args) {
        Engine engine = new Engine() ;
    }
}

In UML Composition is Expressed as 

I originally posted this here: http://www.coderanch.com/t/443002/java/java/Java-Coding-UML-Aggregation-Composition


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