I guess you're mixing up here some slightly different things. Don't confuse interface in general software design and interfaces in the sense of the Java keyword "interface". Although they are related you have to be careful what exactly articles or papers on the internet talk about!
In Java there's no distinction between interface and abstract interface, as Campbell already pointed out. Every JAVA interface is implicitly abstract (meaning the keywords "interface" and "abstract").
In the broader sense of software design interfaces are usually an abstract concept for something, too. So the abstract is also redundant here.
If you don't have any specific piece of information I'd say interface and abstract interface are the same because an interface is implicitly abstract. I hope this answers your question.
These are the non-access modifiers an interface(Outer) can have
abstract (Not recommended)
If use this abstract non-access modifier compiler will not give error and you can run it successfully BUT As
Marco Ehrentreich wrote:Every JAVA interface is implicitly abstract
Muhammad Khojaye wrote:JLS says: Every interface is implicitly abstract. This modifier is obsolete and should not be used in new programs.
It is fruitless to use abstract keyword with an interface because use of abstract is to make a class or method abstract which can be non- abstract(may have body defined) but here an interface is already abstract so you don't have to use abstract keyword.
This is the same case with access modifiers of methods(though it is abstract, static and default last two since 1.8 and later) in an interface which are declared public implicitly so you don't have to declare it with public keyword. So there is no difference, if you want to use abstract keyword before an interface ignoring what JLS rule says else If want to follow JLS then say as
Campbell Ritchie wrote:What's an abstract interface? I have never heard of it.
These are the non-access modifiers an interface(inner in nested interface) can have
abstract (Not recommended, as above said)
static (I don't know its use or recommended or not but you can use static keyword for an interface which is in another interface)
Being Java programmer.
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