Why do abstract classes in Java have constructors?

问题:

This question already has an answer here:

• Can an abstract class have a constructor? 21 answers

Why does an abstract class in Java have a constructor?

What is it constructing, as we can't instantiate an abstract class?

Any thoughts?

回答1:

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.

回答2:

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

回答3:

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.

回答4:

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

回答5:

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

回答6:

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.

回答7:

Implementation wise you will often see inside super() statement in subclasses constructors, something like:

public class A extends AbstractB{

  public A(...){
     super(String constructorArgForB, ...);
     ...
  }
}