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An abstract class may or may not define an abstract method. But a concrete class can’t define an abstract method.


Cover of OCA Java SE 8 Programmer I Certification Guide


Abstract method basically says, that there is no implementation of the method and it needs to be implemented in a subclass. However if you had an abstract method in a non-abstract class, you could instantiate the class and get an object, that would have an unimplemented method, which you would be unable to call.