Implementing two interfaces in a class with same method Which interface method is overridden

Question

Two interfaces with same method names and signatures  But implemented by a single class then how the compiler will identify the which method is for which interface   Ex   interface A    int f       interface B    int f       class Test implements A  B       public static void main String    args  throws Exception              Override   public int f         from which interface A or B     return 0

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Try implementing the interface as anonymous   public class MyClass extends MySuperClass implements MyInterface   MyInterface myInterface   new MyInterface        Overrided method from interface     override public void method1               Overrided method from superclass    override public void method1

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This was marked as a duplicate to this question https   stackoverflow com questions 24401064 understanding-and-solving-the-diamond-problems-in-java  You need Java 8 to get a multiple inheritance problem  but it is still not a diamon problem as such   interface A       default void hi     System out println  A         interface B       default void hi     System out println  B         class AB implements A  B      won t compile    new AB   hi       won t compile    As JB Nizet comments you can fix this my overriding   class AB implements A  B       public void hi     A super hi          However  you don t have a problem with  interface D extends A      interface E extends A      interface F extends A       default void hi     System out println  F         class DE implement D  E      new DE   hi       prints A  class DEF implement D  E  F      new DEF   hi       prints F as it is closer in the heirarchy than A

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There is nothing to identify  Interfaces only proscribe a method name and signature  If both interfaces have a method of exactly the same name and signature  the implementing class can implement both interface methods with a single concrete method   However  if the semantic contracts of the two interface method are contradicting  you ve pretty much lost  you cannot implement both interfaces in a single class then

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Well if they are both the same it doesn t matter  It implements both of them with a single concrete method per interface method

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If a type implements two interfaces  and each interface define a method that has identical signature  then in effect there is only one method  and they are not distinguishable  If  say  the two methods have conflicting return types  then it will be a compilation error  This is the general rule of inheritance  method overriding  hiding  and declarations  and applies also to possible conflicts not only between 2 inherited interface methods  but also an interface and a super class method  or even just conflicts due to type erasure of generics     Compatibility example  Here s an example where you have an interface Gift  which has a present   method  as in  presenting gifts   and also an interface Guest  which also has a present   method  as in  the guest is present and not absent    Presentable johnny is both a Gift and a Guest   public class InterfaceTest       interface Gift    void present          interface Guest   void present           interface Presentable extends Gift  Guest          public static void main String   args            Presentable johnny   new Presentable                  Override public void present                     System out println  Heeeereee s Johnny                                        johnny present                            Heeeereee s Johnny                Gift  johnny  present                   Heeeereee s Johnny               Guest  johnny  present                  Heeeereee s Johnny              Gift johnnyAsGift    Gift  johnny          johnnyAsGift present                      Heeeereee s Johnny              Guest johnnyAsGuest    Guest  johnny          johnnyAsGuest present                     Heeeereee s Johnny               The above snippet compiles and runs   Note that there is only one  Override necessary     This is because Gift present   and Guest present   are   Override-equivalent   JLS 8 4 2    Thus  johnny only has one implementation of present    and it doesn t matter how you treat johnny  whether as a Gift or as a Guest  there is only one method to invoke     Incompatibility example  Here s an example where the two inherited methods are NOT  Override-equivalent    public class InterfaceTest       interface Gift    void present          interface Guest   boolean present           interface Presentable extends Gift  Guest        DOES NOT COMPILE            types InterfaceTest Guest and InterfaceTest Gift are incompatible          both define present    but with unrelated return types      This further reiterates that inheriting members from an interface must obey the general rule of member declarations  Here we have Gift and Guest define present   with incompatible return types  one void the other boolean  For the same reason that you can t an void present   and a boolean present   in one type  this example results in a compilation error     Summary  You can inherit methods that are  Override-equivalent  subject to the usual requirements of method overriding and hiding  Since they ARE  Override-equivalent  effectively there is only one method to implement  and thus there s nothing to distinguish select from    The compiler does not have to identify which method is for which interface  because once they are determined to be  Override-equivalent  they re the same method   Resolving potential incompatibilities may be a tricky task  but that s another issue altogether   References   JLS 8 4 2 Method Signature JLS 8 4 8 Inheritance  Overriding  and Hiding JLS 8 4 8 3 Requirements in Overriding and Hiding JLS 8 4 8 4 Inheriting Methods with Override-Equivalent Signatures    It is possible for a class to inherit multiple methods with override-equivalent signatures

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As in interface we are just declaring methods concrete class which implements these both interfaces understands is that there is only one method as you described both have same name in return type   so there should not be an issue with it You will be able to define that method in concrete class   But when two interface have a method with the same name but different return type and you implement two methods in concrete class   Please look at below code   public interface InterfaceA     public void print        public interface InterfaceB     public int print       public class ClassAB implements InterfaceA  InterfaceB     public void print           System out println  Inside InterfaceA          public int print           System out println  Inside InterfaceB        return 5          when compiler gets method  public void print    it first looks in InterfaceA and it gets it But still it gives compile time error that return type is not compatible with method of InterfaceB   So it goes haywire for compiler   In this way  you will not be able to implement two interface having a method of same name but different return type

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As far as the compiler is concerned  those two methods are identical  There will be one implementation of both   This isn t a problem if the two methods are effectively identical  in that they should have the same implementation  If they are contractually different  as per the documentation for each interface   you ll be in trouble

[java] Implementing two interfaces in a class with same method. Which interface method is overridden?

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