a subclass of a non abstract class must be non abstract

Is it mandatory to have one abstract method in abstract class?
An abstract class is not required to have an abstract method in it.
But any class that has an abstract method in it or that does not provide an implementation for any abstract methods declared in its superclasses or implemented interfaces must be declared as an abstract class.
Can a subclass of a non-abstract class be abstract?
A subclass can be declared abstract regardless of whether the superclass was declared abstract.
A class cannot be declared abstract and final at the same time.
This restriction makes sense because abstract classes need to be subclassed to be useful and final forbids subclasses.
Abstract classes require subclasses to further define attributes necessary for individual instantiation.
Subclasses that extend from the abstract class all have that abstract class's attributes, as well as their own attributes that are specific to each subclass.

Can an abstract parent class have non-abstract concrete subclasses?

Of course.
Declaring a class abstract only means that you don't allow it to be instantiated on its own.
Declaring a method abstract means that subclasses have to provide an implementation for that method.
The two are separate concepts, though obviously you can't have an abstract method in a non-abstract class.

Abstract classes cannot be instantiated, but they can be subclassed. When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, then the subclass must also be declared abstract .

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PDF	Abstract Classes and Interfaces abstract class. In a nonabstract (a.k.a. concrete) subclass extended from an abstract super-class

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PDF	Chapter 11 Abstract Classes and Interfaces class ▻ If a subclass of an abstract superclass does not implement all the abstract methods, the subclass must be declared abstract ▻ In other words, in a

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An abstract method cannot be placed in a non-abstract class If a subclass of an abstract superclass does not implement all the abstract methods, the subclass must be declared abstract A classes that contains abstract methods must be abstract A subclass can be abstract even if its superclass is concrete

PDF	Inheritance II: Abstract classes, methods - MIT OpenCourseWare 21 jan 2012 · every subclass must use, for consistency: e g , getObjectID() class and a default is available: non-abstract method public final int objectID() {

PDF	Lecture Notes Chapter Abstract Classes & Interfaces abstract methods cannot be contained in non-abstract classes if a subclass of an abstract class does not implement all the abstract methods, that subclass must

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