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What is the main reason for using abstract classes?
abstract, you allow the implementation to change or for new implementations to be added without affecting the client code. Typically one uses an abstract class to provide some incomplete functionality that will be fleshed out by concrete subclasses.
How many abstract methods should an abstract class have?
A class which contains 0 or more abstract methods is known as abstract class. If it contains at least one abstract method, it must be declared abstract. And yes, you can declare abstract class without defining an abstract method in it.
Is it necessary to define all methods of abstract class?
Yes, you must implement all abstract methods.
Can a class can be abstract even if all of its methods are concrete?
You can’t have an abstract method in a concrete class. If you want an abstract method, the class must also be abstract. Otherwise users could instantiate the class and try to implement its methods. However, if your class is abstract, it may have some methods that are abstract and others that are concrete.
When should we use abstract class?
An abstract class is used if you want to provide a common, implemented functionality among all the implementations of the component. Abstract classes will allow you to partially implement your class, whereas interfaces would have no implementation for any members whatsoever.
Can we use final in abstract class?
Yes, there may be “final” methods in “abstract” class. But, any “abstract” method in the class can’t be declared final. It will give “illegal combination of modifiers: abstract and final” error. Here is the working example of the implementation.
How are abstract methods defined in a class?
If a class defines any abstract methods (or, as we will see, inherits any abstract methods and doesn’t override them), that class must be defined using the abstract keyword in its access modifiers.
How are abstract classes different from concrete classes?
When a class is abstract, an instance of it cannot be created; one can only create instances of (concrete) subclasses. A concrete class is a class that is not declared abstract (and therefore has no abstract methods and implements all inherited abstract methods). For instance:
Can a class be defined with the keyword abstract?
Generally, abstract classes can specify all the standard class components: constructors, methods, and instance variables. Again, a class must be defined with the keyword abstract (as is the case above) if any of its methods is defined with the keyword abstract (as is the case above).
Can a subclass override an abstract class?
We can treat an abstract class as a superclass and extend it; its subclasses can override some or all of its inherited abstract methods. If through this overriding a subclass contains no more abstract methods, that class is concrete (and we can construct objects directly from it).