Abstraction is one of the Objective oriented programming concepts of Java. It means hiding the information which is repeated multiple times and not needed for the user.
Example: It is like we use smartphones every day but it is not necessary to know how it works to use.
In Java, abstraction can be implemented by abstract classes and methods. Abstract classes can be implemented just by adding an abstract before the class. We can use abstract classes only by child class for creating objects in inheritance. The abstract method does not have variables or code inside a function that will be provided by the child class.
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You can watch this video on the Java tutorial: