What is the point of abstract classes, when you could just do an interface

Question

I recently began to wonder, why would I ever use an abstract class, when i could just do an interface with the exact same functionality. Consider the following:

abstract class abstractClass {

    public int a;

        public void printText(String str) {
        System.out.println(str);
    }
}

If I had a subclass extending this, I would be sure to have an integer a and a method named printText.

interface Interface{

    public int a = 0;

    public default void printtext(String str) {
        System.out.println(str);
    }
}

If I had a class implement this, I would be sure to have an integer a and a method printText.

So, the only difference I see is that with the interface, any class I create would be able to implement as many other interfaces as I would like, but with the extension of the abstract class, I would not be allowed to extend more classes. Why would I ever go for the abstract class? I hope this question isn't too broad, although I can see why some would consider it that.

The `a` in your interface is a constant, not an instance variable. Interfaces cannot declare instance variables. — khelwood, Jun 07 '18 at 11:03
@chrylis It's a perfectly valid interface, but `a` is implicitly public and static. — Michael, Jun 07 '18 at 11:10
@Michael I should have been more specific, yes; it will compile, but it does not do the same thing at all (note that in addition to public and static, it is also *final*). — chrylis -cautiouslyoptimistic-, Jun 07 '18 at 12:00

T.J. Crowder · Accepted Answer · 2018-06-07T11:31:06.413

1

A few points to consider:

Interfaces didn't have default methods until Java 8.
That a in your interface is implicitly final. Interfaces can't define public mutable fields.
Interfaces can't define private (or protected, or package) fields.
Interfaces can't have protected or package methods; they couldn't have private methods until Java 9.

Abstract classes don't have any of those issues. So when you need to do any of those things (other than #1, now that default methods exist), you reach for an abstract class (perhaps in addition to an interface that it implements).

edited Jun 07 '18 at 11:31

answered Jun 07 '18 at 11:06

T.J. Crowder

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since Java 9, interfaces can have private methods. – Anton Balaniuc Jun 07 '18 at 11:19
@AntonBalaniuc - Doh! I'd seen that, too, but managed to forget. Thanks. But still not protected or package, right? (They'd need a new keyword for package, since leaving off `public` wouldn't do it...) – T.J. Crowder Jun 07 '18 at 11:30

What is the point of abstract classes, when you could just do an interface

1 Answers1