what is a abstract method on a interface in java

Question

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Why would one declare a Java interface method as abstract?

I found the following code in one of our ejb interfaces. Does anyone know what the abstract does in the interface? If you do please also explain why it might be needed or provide a reference to read about it =)

@Local
public interface IDomasOrderProcessor {

    public abstract void executeOrderLines(List<OrderLine> lines);
    public abstract void setupJob(List<OrderLine> lines);
    public abstract void setupJob(OrderLine line);
}

Dev · Accepted Answer · 2012-02-14T13:37:32.953

50

abstract is redundant in this case. All methods defined on an interface are public and abstract by definition.

Excerpt Java Language Specification section 9.4

Every method declaration in the body of an interface is implicitly abstract, so its body is always represented by a semicolon, not a block.

Every method declaration in the body of an interface is implicitly public.

edited Feb 14 '12 at 13:37

answered Feb 14 '12 at 13:28

Dev

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So is "public", in this case, incidentally. Which is to say that you can drop both and it will do the same thing. It's possible that the code was originally an abstract class that got refactored. – Calum Feb 14 '12 at 13:29
1

Indeed, `public` as well. I added that to the answer, as my first was somewhat terse. – Dev Feb 14 '12 at 13:32
Since Java 8 interfaces can have methods with implementation. These are called default methods. Default methods can be added to an interface without affecting the classes that already implement the interface. – Stephan van Hoof Aug 09 '23 at 07:47

score 17 · Answer 2 · answered Feb 14 '12 at 13:29

17

Both public and abstract modifiers are implicit in interfaces and should be avoided.

answered Feb 14 '12 at 13:29

Tomasz Nurkiewicz

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score 6 · Answer 3 · answered Feb 14 '12 at 13:30

6

A method in an interface is public and abstract by definition. I have heard some people say they feel that explicitly declaring them like that makes it clearer, but to me it seems like extra noise.

answered Feb 14 '12 at 13:30

matt freake

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It's certainly noise. (I had actually been programming for years in Java before I realised that the `public` was redundant.) – Tom Hawtin - tackline Feb 14 '12 at 13:33

score 3 · Answer 4 · answered Feb 14 '12 at 13:41

3

As per this document all the methods of interface is public and abstract, so there is no mean to define explicitly abstract method inside the interface.

answered Feb 14 '12 at 13:41

subodh

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what is a abstract method on a interface in java

4 Answers4

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