Abstract classes and interfaces in Java serve fundamentally different purposes. Learn the differences between these Java language elements and how to use them in your programs.

Exception in thread "main" java.lang.ClassCastException: class Superclass cannot be cast to class Subclass (Superclass and Subclass are in unnamed module of loader 'app') at BadDowncast.main ...

Abstract classes contrast with concrete classes, which are the default type. A concrete class has no abstracted methods and can be instantiated and used in code. Abstract classes can also be compared ...

Basically ComponentA depends on ComponentB. To enable unit-testing class ComponentAImpl, the implementation of interface ComponentB must be injected into ComponentAImpl. The Java code below shows the ...