Can an abstract class have a constructor?

Question

Can an abstract class have a constructor?

If so, how can it be used and for what purposes?

Answer 1

Although there are many good answers, I would like to give my 2 cents.

Constructor DOES NOT BUILD THE OBJECT. It is used to initialize an object.

Yes, an Abstract class always has a constructor. If you do not define your own constructor, the compiler will give a default constructor to the Abstract class. Above holds true for all classes - nested, abstract, anonymous, etc.

An abstract class (unlike interface) can have non-final non-static fields which need initialization. You can write your own constructor in the abstract class to do that. But, in that case, there won't be any default constructor.

public abstract class Abs{
    int i;
    int j;
    public Abs(int i,int j){
        this.i = i;
        this.j = j;
        System.out.println(i+" "+j);
    }
}

Be careful while extending above abstract class, you have to explicitly call super from each constructor. The first line of any constructor calls to super(). if you do not explicitly call super(), Java will do that for you. Below code will not compile:

public class Imp extends Abs{

public Imp(int i, int j,int k, int l){
    System.out.println("2 arg");
}
}

You have to use it like below example:

public class Imp extends Abs{

public Imp(int i, int j,int k, int l){
    super(i,j);
    System.out.println("2 arg");
}
}

Answer 2

Of Course, abstract class can have a constructor.Generally class constructor is used to initialise fields.So, an abstract class constructor is used to initialise fields of the abstract class. You would provide a constructor for an abstract class if you want to initialise certain fields of the abstract class before the instantiation of a child-class takes place. An abstract class constructor can also be used to execute code that is relevant for every child class. This prevents code duplication.

We cannot create an instance of an abstract class,But we can create instances of classes those are derived from the abstract class. So, when an instance of derived class is created, the parent abstract class constructor is automatically called.

Reference :This Article

Answer 3

Yes it can have a constructor and it is defined and behaves just like any other class's constructor. Except that abstract classes can't be directly instantiated, only extended, so the use is therefore always from a subclass's constructor.

Answer 4

Yes..It is like any other class. It can have a constructor and it is called after creating object for the base class.

Answer 5

Yes it can, abstract classes constructors are generally used for super calls for initialization events common to all the subclasses

Answer 6

Yes, an Abstract Class can have a Constructor. You Can Overload as many Constructor as you want in an Abstract Class. These Contractors Can be used to Initialized the initial state of the Objects Extending the Abstract Class. As we know we can't make an object of an Abstract Class because Objects are Created by the "new" keywords and not by the constructors...they are there for only initializing the state of the subclass Objects.