Java abstract interface

Question

Consider an example  which compiles in java   public abstract interface Interface       public void interfacing        public abstract boolean interfacing boolean really       Why is it necessary for an interface to be  declared  abstract  Is there other rules that applies with an abstract interface     Finally  If abstract is obsolete  why is it included in Java  Is there a history for abstract interface

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Be aware that in Spring it has non academic meaning. The abstract interface is a warning to the developer not to use it for @Autowired. I hope that spring/eclipse @Autowired will look at this attribute and warn/fail about usages of such.

A real example: @Service proxy under @Transnational to a @Repository need to use same basic methods however they should use different interfaces that extends this abstract interface due to @Autowired. (I call this XXXSpec interface)

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It isn t necessary  It s a quirk of the language

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It s not necessary  it s optional  just as public on interface methods   See the JLS on this   http   java sun com docs books jls second edition html interfaces doc html     9 1 1 1 abstract Interfaces Every interface is implicitly abstract    This modifier is obsolete and should not be used in new programs    And     9 4 Abstract Method Declarations                 For compatibility with older versions of the Java platform  it is   permitted but discouraged  as a matter of style  to redundantly   specify the abstract modifier for methods declared in interfaces       It is permitted  but strongly discouraged as a matter of style  to   redundantly specify the public modifier for interface methods

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Well  Abstract Interface  is a Lexical construct  http   en wikipedia org wiki Lexical analysis   It is required by the compiler  you could also write interface    Well don t get too much into Lexical construct of the language as they might have put it there to resolve some compilation ambiguity which is termed as special cases during compiling process or for some backward compatibility  try to focus on core  Lexical construct   The essence of  interface is to capture some abstract concept  idea thought higher order thinking etc  whose implementation may vary     that is  there may be multiple implementation   An Interface is a pure abstract data type that represents the features of the Object it is capturing or representing   Features can be represented by space or by time  When they are represented by space  memory storage  it means that your concrete class will implement a field and  method methods that will operate on that field or by time which means that the task of implementing the feature is purely computational  requires more cpu clocks for processing  so you have a trade off between space and time for feature implementation   If your concrete class does not implement all features it again becomes abstract because you have a implementation of your thought or idea or abstractness but it is not complete   you specify it by abstract class    A concrete class will be a class set of classes which  will fully capture the abstractness you are trying to capture class XYZ   So the Pattern is   Interface--- gt Abstract class Abstract classes depends -- gt Concrete class

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It is not necessary to declare the interface abstract   Just like declaring all those methods public  which they already are if the interface is public  or abstract  which they already are in an interface  is redundant   No one is stopping you  though   Other things you can explicitly state  but don t need to    call super   on the first line of a constructor extends Object implement inherited interfaces      Is there other rules that applies with an abstract interface    An interface is already  abstract   Applying that keyword again makes absolutely no difference

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Why is it necessary for an interface to be  declared  abstract    It s not   public abstract interface Interface                                            ---- gt  Neither this         public void interfacing        public abstract boolean interfacing boolean really                                                        ---- gt  nor this  are necessary      Interfaces and their methods are implicitly abstract and adding that modifier makes no difference      Is there other rules that applies with an abstract interface    No  same rules apply  The method must be implemented by any  concrete  implementing class      If abstract is obsolete  why is it included in Java  Is there a history for abstract interface    Interesting question  I dug up the first edition of JLS  and even there it says  This modifier is obsolete and should not be used in new Java programs    Okay  digging even further    After hitting numerous broken links  I managed to find a copy of the original Oak 0 2 Specification  or  manual    Quite interesting read I must say  and only 38 pages in total   -   Under Section 5  Interfaces  it provides the following example   public interface Storing       void freezeDry Stream s    0      void reconstitute Stream s    0      And in the margin it says     In the future  the    0  part of declaring methods in interfaces may go away    Assuming  0 got replaced by the abstract keyword  I suspect that abstract was at some point mandatory for interface methods     Related article  Java  Abstract interfaces and abstract interface methods

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It s not necessary  as interfaces are by default abstract as all the methods in an interface are abstract

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An abstract Interface is not as redundant as everyone seems to be saying  in theory at least    An Interface can be extended  just as a Class can   If you design an Interface hierarchy for your application you may well have a  Base  Interface  you extend other Interfaces from but do not want as an Object in itself   Example   public abstract interface MyBaseInterface       public String getName       public interface MyBoat extends MyBaseInterface       public String getMastSize       public interface MyDog extends MyBaseInterface       public long tinsOfFoodPerDay        You do not want a Class to implement the MyBaseInterface  only the other two  MMyDog and MyBoat  but both interfaces share the MyBaseInterface interface  so have a  name  property   I know its kinda academic  but I thought some might find it interesting   -   It is really just a  marker  in this case  to signal to implementors of the interface it wasn t designed to be implemented on its own   I should point out a compiler  At least the sun ora 1 6 I tried it with  compiles a class that implements an abstract interface

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Every interface is implicitly abstract    This modifier is obsolete and should not be used in new programs     The Java Language Specification - 9 1 1 1 abstract Interfaces   Also note that interface member methods are implicitly public abstract   The Java Language Specification - 9 2 Interface Members   Why are those modifiers implicit  There is no other modifier  not even the  no modifier -modifier  that would be useful here  so you don t explicitly have to type it

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