+ 4

What is special about objects created this way?

Lets say I have a superclass A, and a subclass B that inherits from A. Now what is the purpose of creating an object this way? A test = new B (); Or in other words superclass = new subclass? Grateful for any answers!

29th Jan 2018, 7:19 PM
Alexander E
Alexander E - avatar
3 Antworten
+ 5
I think you must to check Casting and Downcasting in Java Sololearn https://www.sololearn.com/learn/Java/2168/ https://www.sololearn.com/learn/Java/2169/
29th Jan 2018, 7:31 PM
Daniel
Daniel - avatar
+ 3
There are some good uses for this - Take the Java Collections API. I might create a method that takes in a Set and does some operation. I write public void operation(Set mySet) { // code } If I had instead used a HashSet I would have access to any methods in Hashsets that Set doesn't, but that now restricts me to only using HashSet. If, later on, I wanted to switch to a TreeSet, it would be more difficult. However with the Set interface used as the type, I can currently pass in any Set I want. Coding to an interface or parent/base class keeps the code more general, and further separated from implementation. There is also a 'strategy pattern' that takes advantage of this. I used it in one of my codes, which is commented: https://code.sololearn.com/cop6od6LxalY/?ref=app for example, look at the Token interface and classes that implement it (the code is actually executed near the top of the code)
29th Jan 2018, 7:48 PM
Dan Walker
Dan Walker - avatar
0
I hope this code helps explaining : public class Animal { void sound() { System.out.println("Animal sound"); } } public class dogs extends Animal { void sound() { System.out.println("Woof-Woof"); } } public class cats extends Animal { void sound() { System.out.println("Meao-Meao"); } } class MyClass { public static void main(String[ ] args) { Animal animal= new Animal(); animal.sound(); Animal dog= new dogs(); dog.sound(); Animal cat= new cats(); cat.sound(); } }
30th Jan 2018, 7:20 AM
Taha ALOMERI
Taha ALOMERI - avatar