Why can t I declare static methods in an interface

Question

The topic says the most of it - what is the reason for the fact that static methods can t be declared in an interface   public interface ITest       public static String test        The code above gives me the following error  in Eclipse  at least    Illegal modifier for the interface method ITest test    only public  amp  abstract are permitted

User · Answer

The reason why you can t have a static method in an interface lies in the way Java resolves static references  Java will not bother looking for an instance of a class when attempting to execute a static method  This is because static methods are not instance dependent and hence can be executed straight from the class file  Given that all methods in an interface are abstract  the VM would have to look for a particular implementation of the interface in order to find the code behind the static method so that it could be executed  This then contradicts how static method resolution works and would introduce an inconsistency into the language

User · Answer

An interface is used for polymorphism  which applies to Objects  not types  Therefore  as already noted  it makes no sense to have an static interface member

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I ll answer your question with an example  Suppose we had a Math class with a static method add  You would call this method like so   Math add 2  3     If Math were an interface instead of a class  it could not have any defined functions  As such  saying something like Math add 2  3  makes no sense

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There s a very nice and concise answer to your question here   It struck me as such a nicely straightforward way of explaining it that I want to link it from here

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Static methods are not instance methods  There s no instance context  therefore to implement it from the interface makes little sense

User · Answer

Perhaps a code example would help  I m going to use C   but you should be able to follow along   Lets pretend we have an interface called IPayable  public interface IPayable       public Pay double amount       Now  we have two concrete classes that implement this interface   public class BusinessAccount   IPayable       public void Pay double amount                  Logic          public class CustomerAccount   IPayable       public void Pay double amount                  Logic           Now  lets pretend we have a collection of various accounts  to do this we will use a generic list of the type IPayable  List lt IPayable gt  accountsToPay   new List lt IPayable gt     accountsToPay add new CustomerAccount     accountsToPay add new BusinessAccount       Now  we want to pay  50 00 to all those accounts   foreach  IPayable account in accountsToPay        account Pay 50 00       So now you see how interfaces are incredibly useful    They are used on instantiated objects only  Not on static classes    If you had made pay static  when looping through the IPayable s in accountsToPay there would be no way to figure out if it should call pay on BusinessAcount or CustomerAccount

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With Java 8  interfaces can now have static methods    For example  Comparator has a static naturalOrder   method   The requirement that interfaces cannot have implementations has also been relaxed  Interfaces can now declare  default  method implementations  which are like normal implementations with one exception  if you inherit both a default implementation from an interface and a normal implementation from a superclass  the superclass s implementation will always take priority

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Now Java8 allows us to define even Static Methods in Interface   interface X       static void foo            System out println  foo             class Y implements X                public class Z      public static void main String   args          X foo             Y foo       won t compile because foo   is a Static Method of X and not Y          Note  Methods in Interface are still public abstract by default if we don t explicitly use the keywords default static to make them Default methods and Static methods resp

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Java 8 Had changed the world you can have static methods in interface but it forces you to provide implementation for that   public interface StaticMethodInterface   public static int testStaticMethod         return 0            Illegal combination of modifiers for the interface method    testStaticMethod  only one of abstract  default  or static permitted         param i     return        public static abstract int testStaticMethod float i    default int testNonStaticMethod         return 1            Without implementation          param i     return     int testNonStaticMethod float i

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The reason lies in the design-principle  that java does not allow multiple inheritance  The problem with multiple inheritance can be illustrated by the following example   public class A      public method x           public class B      public method x           public class C extends A  B           Now what happens if you call C x    Will be A x   or B x   executed  Every language with multiple inheritance has to solve this problem   Interfaces allow in Java some sort of restricted multiple inheritance  To avoid the problem above  they are not allowed to have methods  If we look at the same problem with interfaces and static methods   public interface A      public static method x           public interface B      public static method x           public class C implements A  B           Same problem here  what happen if you call C x

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Illegal combination of modifiers   static and abstract  If a member of a class is declared as static  it can be used with its class name which is confined to that class  without creating an object   If a member of a class is declared as abstract  you need to declare the class as abstract and you need to provide the implementation of the abstract member in its inherited class  Sub-Class    You need to provide an implementation to the abstract member of a class in sub-class where you are going to change the behaviour of static method  also declared as abstract which is a confined to the base class  which is not correct

User · Answer

Since static methods can not be inherited   So no use placing it in the interface  Interface is basically a contract which all its subscribers have to follow   Placing a static method in interface will force the subscribers to implement it   which now becomes contradictory to the fact that static methods can not be inherited

User · Answer

There are a few issues at play here   The first is the issue of declaring a static method without defining it   This is the difference between  public interface Foo     public static int bar        and  public interface Foo     public static int bar                     The first is impossible for the reasons that Espo mentions  you don t know which implementing class is the correct definition   Java could allow the latter  and in fact  starting in Java 8  it does

User · Answer

It seems the static method in the interface might be supported in Java 8  well  my solution is just define them in the inner class   interface Foo                  class fn           public static void func1                                               The same technique can also be used in annotations   public  interface Foo       String value         class fn           public static String getValue Object obj                Foo foo   obj getClass   getAnnotation Foo class               return foo    null   null   foo value                        The inner class should always be accessed in the form of Interface fn    instead of Class fn     then  you can get rid of ambiguous problem

[java] Why can't I declare static methods in an interface?

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