Referencing Subclass objects with Subclass vs Superclass reference

Prerequisite : Inheritance
In Java, all non-static methods are based on the runtime type of the underlying object rather than the type of the reference that points to that object. Therefore, it doesn’t matter which type you use in the declaration of the object, the behavior will be the same.

How to Refer a subclass object

There are two approaches to refer a subclass object. Both have some advantages/disadvantages over the other. The declaration affect is seen on methods that are visible at compile-time.

1. First approach (Referencing using Superclass reference): A reference variable of a superclass can be used to a refer any subclass object derived from that superclass. If the methods are present in SuperClass, but overridden by SubClass, it will be the overridden method that will be executed.
2. Second approach (Referencing using subclass reference) : A subclass reference can be used to refer its object.

Consider an example explaining both the approaches.

seat height of first bicycle is 25 No of gears are 3 speed of bicycle is 100 seat height is 25 No of gears are 4 speed of bicycle is 200 seat height is 20

Explanation of above program :

The object of MountainBike class is created which is referred by using subclass reference ‘mb1’. Using this reference we will have access both parts(methods and variables) of the object defined by the superclass or subclass. See below image for clear understanding.

MountainBike mb1 = new MountainBike(3, 100, 25);

Now we again create object of MountainBike class but this time it is referred by using superclass Bicycle reference ‘mb2’. Using this reference we will have access only to those parts(methods and variables) of the object defined by the superclass.

Bicycle mb2 = new MountainBike(4, 200, 20);

Since the reference ‘mb1’ have access to field ‘seatHeight’, so we print this on console.

System.out.println("seat height of first bicycle is " + mb1.seatHeight);

System.out.println(mb1.toString()); System.out.println(mb2.toString());

System.out.println("seat height of second bicycle is " + mb2.seatHeight);

mb2.setHeight(21);

Use of type casting In above example, we have seen that by using reference ‘mb2’ of type Bicycle, we are unable to call subclass specific methods or access subclass fields. This problem can be solved using type casting in java. For example, we can declare another reference say ‘mb3’ of type MountainBike and assign it to ‘mb2’ using typecasting.

// declaring MountainBike reference MountainBike mb3; // assigning mb3 to mb2 using typecasting. mb3 = (MountainBike)mb2;

System.out.println("seat height of second bicycle is " + mb3.seatHeight); mb3.setHeight(21);

When to go for first approach (Referencing using superclass reference)