Public constructor in abstract classes

[Kalyan Anand]

AFAIK There is no way to create an instance of abstract class. Then why did the Java implementors allow providing a public constructor in an abstract class ?

public abstract class MyAbstractClass
{
 public MyAbstractClass() 
 {
   // Instead they could have forced us to 
   // use a protected MyAbstractClass() here
 }
}

[Carl Wauters]

The reason why you still can write constructors in an abstract class: think about inheritance. A subclass constructor always calls the default constructor of the superclass, even when that superclass in abstract.(unless you explicitly call another one)

[Jesper de Jong]

Yes Carl, but that doesn't solve the question. If the constructor of the abstract superclass is protected, the concrete subclass can also access it.

Indeed, it doesn't make much sense to have public constructors in an abstract class. You can't use them directly, since you can't create an instance of an abstract class, and such a constructor doesn't need to be public if a subclass needs to access it.

Also, a subclass does not always access the no-args constructor (what you call 'default constructor'). In the constructor of a subclass you can explicitly invoke a different constructor of the superclass by using the super(...) syntax:

class Super {
    protected Super(String name) {
        // ...
    }
}
 
class Sub {
    public Sub(String name) {
        super(name);
    }
}