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How does abstract modifier work?
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Abstract means no concrete, it isn’t directly used. It depends on an implementation to be used. The abstract key word can be used before class (top-level/inner) or method.
Abstract class
• If we make a class abstract, it is not directly instantiated. It have to have a child class to be used.
• It is only be used through inheritance.
• The legacy or common functionality are kept in abstract class.
• It is also used for up casting and dynamic polymorphism.
• It is mostly used in factory design pattern.
• An abstract class can implement and extends another interface and class.
Abstract method
• An abstract doesn’t have any body.
• It has to have an override non-abstract method.
• If a class has an abstract method, then the class has to be abstract.
• An abstract method can only be set visibility modifier either public or protected.
Abstract inner class
• An inner class can be abstract but it is not commonly used and is not recommended also.
• A tight coupling utility is implemented as inner class but if the utility is too complex and we need to segregate into different inner subclasses, then we should go for abstract inner class. The scenario is very rare and it may not come in web applications but may come very big swing application.
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