Java - Is every interface an abstract class?

Question

In Java every method in an interface is implicitly abstract. But when at least one method in a class is abstract, that means that this class is abstract. Does that mean, that every interface is an abstract class?

score 1 · Answer 1 · answered Jun 23 '13 at 13:15

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I guess in some sort of way, yes. An abstract class can be used as an interface.

The issue is that a class can only extends on other class. A class can implements any number of interfaces.

So while abstract classes are similar to interfaces they are very different in reality.

answered Jun 23 '13 at 13:15

Boris the Spider

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This is heavily explained and detailed in the possible dup question. – Luiggi Mendoza Jun 23 '13 at 13:15
@LuiggiMendoza ah yes, sorry. Hadn't noticed that one - voted to close as dup. – Boris the Spider Jun 23 '13 at 13:16

score 1 · Answer 2 · answered Jun 23 '13 at 13:15

Does that mean, that every interface is an abstract class?

No. An interface (in Java 7 and earlier) can have no method implementations and can define no instance fields. An abstract class can do both. These are just some of the substantive differences.

Suresh Atta · Answer 3 · 2013-06-23T13:18:51.040

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But when at least one method in a class is abstract.

But its not a class,Its interface.

Instead you can say it's an abstract interface.

And finally :An interface is abstract by definition.

edited Jun 23 '13 at 13:18

answered Jun 23 '13 at 13:15

Suresh Atta

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score -2 · Answer 4 · answered Jun 23 '13 at 13:15

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Yes when i decompiled a interface it had the modifier public abstract interface, and by definition they must be abstract to contain abstract methods.

answered Jun 23 '13 at 13:15

Popgalop

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abstract interface is not the same as an abstract class. – Simon Forsberg Jun 23 '13 at 13:16

Java - Is every interface an abstract class?

4 Answers4