why abstract class can have constructors

[debabrata das]

why we can have/write constructor of an abstract class, if we can't instantiate. why it is so? It is unnecessary.
check this out.........

public abstract class javaRanch{

public javaRanch(){

}

javaRanch(int i){

}

}

[Paul Sturrock]

Not really an advanced question. Please try to CarefullyChooseOneForum before posting.

Abstract classes, unlike Interfaces, can be used to suggest the structure of the code that extends it. So suppose I write an abstract class that has one imutable property and I expect all classes that inherit from this to supply this property I could define a constructor that takes this property and define a no-args constructor as private.

[Larry Frissell]

debabrata das asked

why we can have/write constructor of an abstract class, if we can't instantiate.

Abstract class's constructor can be called if it is followed by the body for the anonymous class, see notes in
JavaNotes

[Jesper de Jong]

Originally posted by debabrata das:
why we can have/write constructor of an abstract class, if we can't instantiate. why it is so? It is unnecessary.

It is not unnecessary.

You can regard an abstract class as an incomplete class: some parts of it are implemented, but other parts are left for a concrete subclass to implement. When you create an instance of the subclass, then the constructor of the abstract superclass is also called.

Example:

public abstract class Animal
{
    private String name;
 
    public Animal(String name) {
        this.name = name;
    }
 
    public abstract void makeNoise();
}
 
public class Dog extends Animal
{
    public Dog(String name) {
        super(name);
    }
 
    public void makeNoise() {
        System.out.println("Woof Woof!");
    }
}

In this example, the constructor of class Animal initializes the field 'name' in the Animal class. So, you see that the constructor is not unnecessary.