Why do abstract classes in Java have constructors

Question

Why does an abstract class in Java have a constructor   What is it constructing  as we can t instantiate an abstract class   Any thoughts

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Implementation wise you will often see inside super   statement in subclasses constructors  something like     public class A extends AbstractB     public A            super String constructorArgForB

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Because another class could extend it  and the child class needs to invoke a superclass constructor

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Two reasons for this   1  Abstract classes have constructors and those constructors are always invoked when a concrete subclass is instantiated  We know that when we are going to instantiate a class  we always use constructor of that class  Now every constructor invokes the constructor of its super class with an implicit call to super     2  We know constructor are also used to initialize fields of a class  We also know that abstract classes may contain fields and sometimes they need to be initialized somehow by using constructor

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All the classes including the abstract classes can have constructors Abstract class constructors will be called when its concrete subclass will be instantiated

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Because abstract classes have state  fields  and somethimes they need to be initialized somehow

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A constructor in Java doesn t actually  build  the object  it is used to initialize fields   Imagine that your abstract class has fields x and y  and that you always want them to be initialized in a certain way  no matter what actual concrete subclass is eventually created  So you create a constructor and initialize these fields   Now  if you have two different subclasses of your abstract class  when you instantiate them their constructors will be called  and then the parent constructor will be called and the fields will be initialized   If you don t do anything  the default constructor of the parent will be called  However  you can use the super keyword to invoke specific constructor on the parent class

User · Answer

I guess root of this question is that people believe that a call to a constructor creates the object  That is not the case  Java nowhere claims that a constructor call creates an object  It just does what we want constructor to do  like initialising some fields  that s all  So an abstract class s constructor being called doesn t mean that its object is created

[java] Why do abstract classes in Java have constructors?

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