I wrote an article about that: Abstract classes and interfaces Summarizing: When we talk about abstract classes we are defining characteristics of an object type; specifying what an object is. When we talk about an interface and define capabilities that we promise to provide, we are talking about establishing a contract about what the object … Read more
I will give you an example first: public interface LoginAuth{ public String encryptPassword(String pass); public void checkDBforUser(); } Suppose you have 3 databases in your application. Then each and every implementation for that database needs to define the above 2 methods: public class DBMySQL implements LoginAuth{ // Needs to implement both methods } public class … Read more
There’s a lot more to abstract classes than default method implementations (such as private state), but as of Java 8, whenever you have the choice of either, you should go with the defender (aka. default) method in the interface. The constraint on the default method is that it can be implemented only in the terms … Read more
What you’ll see sometimes is the following: class Abstract1: “””Some description that tells you it’s abstract, often listing the methods you’re expected to supply.””” def aMethod(self): raise NotImplementedError(“Should have implemented this”) Because Python doesn’t have (and doesn’t need) a formal Interface contract, the Java-style distinction between abstraction and interface doesn’t exist. If someone goes through … Read more
Because “abstract” means: “Implements no functionality”, and “static” means: “There is functionality even if you don’t have an object instance”. And that’s a logical contradiction.
Yes, an abstract class can have a constructor. Consider this: abstract class Product { int multiplyBy; public Product( int multiplyBy ) { this.multiplyBy = multiplyBy; } public int mutiply(int val) { return multiplyBy * val; } } class TimesTwo extends Product { public TimesTwo() { super(2); } } class TimesWhat extends Product { public TimesWhat(int … Read more
To expand on the answer by bradtgmurray, you may want to make one exception to the pure virtual method list of your interface by adding a virtual destructor. This allows you to pass pointer ownership to another party without exposing the concrete derived class. The destructor doesn’t have to do anything, because the interface doesn’t … Read more
How about an analogy: when I was in the Air Force, I went to pilot training and became a USAF (US Air Force) pilot. At that point I wasn’t qualified to fly anything, and had to attend aircraft type training. Once I qualified, I was a pilot (Abstract class) and a C-141 pilot (concrete class). … Read more
Interfaces An interface is a contract: The person writing the interface says, “hey, I accept things looking that way“, and the person using the interface says “OK, the class I write looks that way“. An interface is an empty shell. There are only the signatures of the methods, which implies that the methods do not … Read more