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Siva Nookala - 12 Apr 2016
As discussed in Java Class, we can have classes as part of another class. i.e. we can have a user defined type inside another user defined type. e.g., room having fan and lights, car having engine and tyres. This way of including classes inside other classes is also referred as class composition or has a relation. That is similar to car is composed of engine, tyres and seats or room has a fan etc.,.

There is no limit on the level of composition. An apartment contains multiple flats, a flat contains multiple rooms, rooms contain fans and lights and fans contain motor, wings etc.,.
Similar to primitive data types, the dot(.) operator can also be used to access the variables of user defined types (non primitive types). Since there is no limit on the level to which classes can composed, there is also no limit on the number of levels to which the dot operators can be used.
The below program shows how classes can be composed of other classes and how those objects can be accessed.
Class Composition
class ClassComposition
{
    public static void main(String arg[])
    {
        Room hall = new Room("Hall"); // LINE A
        hall.area = 200.0;
        hall.paintColor = "Voilet";
        hall.flooring = "Marble";
        
        hall.ceilingFan = new Fan("Bajaj"); // LINE B
        hall.ceilingFan.speed = 500;     // LINE B1
        hall.ceilingFan.numberOfSpeeds = 5;
        
        hall.tube = new Light("Hall tube light"); // LINE C
        hall.tube.color = "White";
        hall.tube.watts = 40;
        hall.tube.tube = true;    // LINE C1
        
        Light bl = new Light("Yellow bed light"); // LINE D
        hall.bedLight = bl;
        hall.bedLight.color = "Yellow";
        hall.bedLight.watts = 50;
        hall.bedLight.tube = false;
        
        System.out.println("Hall fan speed : " + hall.ceilingFan.speed);
        System.out.println("Bed Light color : " + hall.bedLight.color);    
    }
}

class Fan
{
    String name;
    double speed;
    String color;
    int numberOfSpeeds;

    Fan(String name)
    {
        this.name = name;
    }
}

class Light
{
    String name;
    String color;
    int watts;
    boolean tube;

    Light(String name)
    {
        this.name = name;
    }
}

class Room
{
    String name;
    double area;
    String paintColor;
    String flooring;
    Fan ceilingFan;
    Light tube;
    Light bedLight;

    Room(String name)
    {
        this.name = name;
    }
}
OUTPUT

Hall fan speed : 500.0
Bed Light color : Yellow

DESCRIPTION

Here we have defined 3 classes - Fan, Light and Room. At LINE A, we have created an object of Room class, at LINE B we have created an object of Fan class, at LINE C and LINE D we have created two objects of Light class - one is a tube light and the other is bed light. The variables of the composed class Fan can be accessed using dot operator. At LINE B1 we have accessed hall.ceilingFan.speed and assigned it a speed of 500. Here hall is object of Room class, ceilingFan is object of Fan class and speed variable in the ceilingFan object is changed.
If we observe carefully in LINE C, we have tube variable in Room class and also tube variable in Light class. So hall.tube refers to the Light object in Room class, where as hall.tube.tube refers to a boolean datatype. This hall.tube.tube tells if the Light object is a tube or not. Although both of them have the same variable name, the scope is different. hall.tube is in the scope of Room object and hall.tube.tube is in the scope of Light object inside the Room object.
Also note that we have created two references to the Bed light object. The reference bl and hall.bedLight point to the same object. Either bl.watts is assigned to 50 or hall.bedLight.color is assigned "Yellow", both of them point to the same bed light object created at LINE D. Refer to Java Objects References and Class References And Objects In Java for more details about the references and objects.

THINGS TO TRY
  • Add a tableFan variable to the Room class and initialize data to it.
  • Try setting the value of hall.ceilingFan.speed to 200 before LINE B. Since we have not created the Fan object, the hall.ceilingFan contains null, as the object references are by default initialized to null. We will get a NullPointerException when we try to access a reference which is null.
  • Create one more constructor for Light class which takes name, watts, color and whether tube or not. Initialize the member variables in the constructor using this keyword. Also change the code in the main method to use the newly created constructor. Observe how much duplicate code is reduced.

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