Why do we need constructors and private members in the abstract class?

Question

Why do we need constructors and private members in the abstract class? It is not like we are ever going to create an instance of that class.

Answer 1

Inheritance

You will create instances, just instances of a derived class. Those derived classes will still need to call constructors, and can still call members of the abstract class - which may in turn use private members.

Here's an example (not a terribly useful one, but just to show the basic idea...)

public abstract class NamedObject
{
    private final String name = name;

    protected NamedObject(String name)
    {
        this.name = name;
    }

    public String getName()
    {
        return name;
    }
}

public class Computer extends NamedObject
{
    private final int processorSpeed;

    public Computer(String name, int processorSpeed)
    {
        super(name); // See, the constructor is useful
        this.processorSpeed = processorSpeed;
    }

    public String toString()
    {
        return getName() + " (" + processorSpeed + ")";
    }
}

I can't say I write abstract classes that often, generally preferring composition to inheritance, but when I do create them I certainly use constructors and private members.

Answer 2

Abstract classes provide a partial implementation of some interface. It's perfectly reasonable to consider that you might want to provide part of that implementation and disallow client code (concrete subclasses) from accessing the specifics - i.e. an extension of the principle of encapsulation.

Marking some members as private forces the inheriting class to call protected methods to access that partial implementation; providing a constructor allows for subclasses to initialise the parent's encapsulated state during their own construction.