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Abstraction

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As per dictionary, abstraction is the quality of dealing with ideas rather than events. Consider a real-life example of a man driving a car. The man only knows that pressing the accelerators will increase the speed of car, but he does not know about how on pressing the accelerator the speed is actually increasing, he does not know about the inner mechanism of the car or the implementation of accelerator in the car. This is what abstraction is.

Abstract Classes and Methods
In OOP, abstraction is a process of hiding the implementation details from the user, and only the functionality will be provided to the user. In Java, abstraction is achieved by using abstract classes and interfaces.

• An abstract class is a class that is declared with abstract keyword.
• An abstract method is a method that is declared without an implementation.
• An abstract class may or may not have all abstract methods. Some of them can be concrete methods.
• A method defined abstract must be redefined in the subclass, thus making overriding compulsory OR either make subclass itself abstract.
• Any class that contains one or more abstract methods must also be declared with abstract keyword.
• There can be no object of an abstract class. That is, an abstract class can not be directly instantiated with the new operator.
• An abstract class can have parametrized constructors and default constructor is always present in an abstract class.

Why Abstraction?
There are situations in which we will want to define a superclass that declares the structure of a given abstraction without providing a complete implementation of every method. That is, sometimes we will want to create a superclass that only defines a generalization form that will be shared by all of its subclasses, leaving it to each subclass to fill in the details.

Review: Abstraction Definition

Which statement is NOT true about abstract classes?   All methods in an abstract class must be abstract methods (no concrete methods allowed).   An abstract method is a method that is declared without an implementation.   Any class that contains one or more abstract methods must also be declared with abstract keyword.   There can be no object of an abstract class.

Result:

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Slide 13.b: Multiple inheritance Slide 14.2: Abstraction (cont.): An example Home         Print version

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A man comes to Mrs. Smith’s door and says,           “There’s been an accident at the brewery.           Your husband fell into a vat of beer and drowned.”           Mrs. Smith wails, “Oh, the poor man! He never had a chance!”           The man says, “I don’t know about that.           He got out three times to go to the bathroom.”