The reason =0 is used is that Bjarne Stroustrup didn’t think he could get another keyword, such as “pure” past the C++ community at the time the feature was being implemented. This is described in his book, The Design & Evolution of C++, section 13.2.3: The curious =0 syntax was chosen … because at the … Read more
When To Use Interfaces An interface allows somebody to start from scratch to implement your interface or implement your interface in some other code whose original or primary purpose was quite different from your interface. To them, your interface is only incidental, something that have to add on to the their code to be able … Read more
The following suggestion let’s you test abstract classes without creating a “real” subclass – the Mock is the subclass. use Mockito.mock(My.class, Mockito.CALLS_REAL_METHODS), then mock any abstract methods that are invoked. Example: public abstract class My { public Result methodUnderTest() { … } protected abstract void methodIDontCareAbout(); } public class MyTest { @Test public void shouldFailOnNullIdentifiers() … Read more
Interface is used when you only want to declare which methods and members a class MUST have. Anyone implementing the interface will have to declare and implement the methods listed by the interface. If you also want to have a default implementation, use abstract class. Any class extending the abstract class will have to implement … Read more
Typically, Objective-C class are abstract by convention only—if the author documents a class as abstract, just don’t use it without subclassing it. There is no compile-time enforcement that prevents instantiation of an abstract class, however. In fact, there is nothing to stop a user from providing implementations of abstract methods via a category (i.e. at … Read more
An abstract class is a class which cannot be instantiated. An abstract class is used by creating an inheriting subclass that can be instantiated. An abstract class does a few things for the inheriting subclass: Define methods which can be used by the inheriting subclass. Define abstract methods which the inheriting subclass must implement. Provide … 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.
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
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
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