Why an abstract class implementing an interface can miss the declaration implementation of one of the interface s methods

Question

A curious thing happens in Java when you use an abstract class to implement an interface  some of the interface s methods can be completely missing  i e  neither an abstract declaration or an actual implementation is present   but the compiler does not complain   For example  given the interface   public interface IAnything     void m1      void m2      void m3        the following abstract class gets merrily compiled without a warning or an error   public abstract class AbstractThing implements IAnything     public void m1        public void m3          Can you explain why

User · Answer

Abstract classes are not required to implement the methods  So even though it implements an interface  the abstract methods of the interface can remain abstract  If you try to implement an interface in a concrete class  i e  not abstract  and you do not implement the abstract methods the compiler will tell you  Either implement the abstract methods or declare the class as abstract

User · Answer

Perfectly fine  You can t instantiate abstract classes   but abstract classes can be used to house common implementations for m1   and m3    So if m2   implementation is different for each implementation but m1 and m3 are not  You could create different concrete IAnything implementations with just the different m2 implementation and derive from AbstractThing -- honoring the DRY principle  Validating if the interface is completely implemented for an abstract class is futile     Update  Interestingly  I find that C  enforces this as a compile error  You are forced to copy the method signatures and prefix them with  abstract public  in the abstract base class in this scenario    something new everyday

User · Answer

Interface means a class that has no implementation of its method  but with just declaration  Other hand  abstract class is a class that can have implementation of some method along with some method with just declaration  no implementation  When we implement an interface to an abstract class  its means that the abstract class inherited all the methods of the interface  As  it is not important to implement all the method in abstract class however it comes to abstract class  by inheritance too   so the abstract class can left some of the method in interface without implementation here  But  when this abstract class will inherited by some concrete class  they must have to implements all those unimplemented method there in abstract class

User · Answer

When an Abstract Class Implements an Interface      In the section on Interfaces  it was noted that a class that   implements an interface must implement all of the interface s methods    It is possible  however  to define a class that does not implement all   of the interface s methods  provided that the class is declared to be   abstract  For example    abstract class X implements Y             implements all but one method of Y           class XX extends X             implements the remaining method in Y          In this case  class X must be abstract because it does not fully   implement Y  but class XX does  in fact  implement Y    Reference  http   docs oracle com javase tutorial java IandI abstract html

User · Answer

That s because if a class is abstract  then by definition you are required to create subclasses of it to instantiate   The subclasses will be required  by the compiler  to implement any interface methods that the abstract class left out   Following your example code  try making a subclass of AbstractThing without implementing the m2 method and see what errors the compiler gives you   It will force you to implement this method

User · Answer

Perfectly fine  You can t instantiate abstract classes   but abstract classes can be used to house common implementations for m1   and m3    So if m2   implementation is different for each implementation but m1 and m3 are not  You could create different concrete IAnything implementations with just the different m2 implementation and derive from AbstractThing -- honoring the DRY principle  Validating if the interface is completely implemented for an abstract class is futile     Update  Interestingly  I find that C  enforces this as a compile error  You are forced to copy the method signatures and prefix them with  abstract public  in the abstract base class in this scenario    something new everyday

User · Answer

That s because if a class is abstract  then by definition you are required to create subclasses of it to instantiate   The subclasses will be required  by the compiler  to implement any interface methods that the abstract class left out   Following your example code  try making a subclass of AbstractThing without implementing the m2 method and see what errors the compiler gives you   It will force you to implement this method

User · Answer

Given the interface   public interface IAnything     int i    void m1      void m2      void m3        This is how Java actually sees it   public interface IAnything     public static final int i    public abstract void m1      public abstract void m2      public abstract void m3        So you can leave some  or all  of these abstract methods unimplemented  just as you would do in the case of abstract classes extending another abstract class   When you implement an interface  the rule that all interface methods must be implemented in the derived class  applies only to concrete class implementation  i e   which isn t abstract itself    If you indeed plan on creating an abstract class out of it  then there is no rule that says you ve to implement all the interface methods  note that in such a case it is mandatory to declare the derived class as abstract

User · Answer

Perfectly fine  You can t instantiate abstract classes   but abstract classes can be used to house common implementations for m1   and m3    So if m2   implementation is different for each implementation but m1 and m3 are not  You could create different concrete IAnything implementations with just the different m2 implementation and derive from AbstractThing -- honoring the DRY principle  Validating if the interface is completely implemented for an abstract class is futile     Update  Interestingly  I find that C  enforces this as a compile error  You are forced to copy the method signatures and prefix them with  abstract public  in the abstract base class in this scenario    something new everyday

User · Answer

When an Abstract Class Implements an Interface      In the section on Interfaces  it was noted that a class that   implements an interface must implement all of the interface s methods    It is possible  however  to define a class that does not implement all   of the interface s methods  provided that the class is declared to be   abstract  For example    abstract class X implements Y             implements all but one method of Y           class XX extends X             implements the remaining method in Y          In this case  class X must be abstract because it does not fully   implement Y  but class XX does  in fact  implement Y    Reference  http   docs oracle com javase tutorial java IandI abstract html

User · Answer

Given the interface   public interface IAnything     int i    void m1      void m2      void m3        This is how Java actually sees it   public interface IAnything     public static final int i    public abstract void m1      public abstract void m2      public abstract void m3        So you can leave some  or all  of these abstract methods unimplemented  just as you would do in the case of abstract classes extending another abstract class   When you implement an interface  the rule that all interface methods must be implemented in the derived class  applies only to concrete class implementation  i e   which isn t abstract itself    If you indeed plan on creating an abstract class out of it  then there is no rule that says you ve to implement all the interface methods  note that in such a case it is mandatory to declare the derived class as abstract

User · Answer

Abstract classes are not required to implement the methods  So even though it implements an interface  the abstract methods of the interface can remain abstract  If you try to implement an interface in a concrete class  i e  not abstract  and you do not implement the abstract methods the compiler will tell you  Either implement the abstract methods or declare the class as abstract

User · Answer

That s fine  To understand the above  you have to understand the nature of abstract classes first  They are similar to interfaces in that respect  This is what Oracle say about this here      Abstract classes are similar to interfaces  You cannot instantiate them  and they may contain a mix of methods declared with or without an implementation    So you have to think about what happens when an interface extends another interface  For example        Filename  Sports java public interface Sports      public void setHomeTeam String name      public void setVisitingTeam String name        Filename  Football java public interface Football extends Sports      public void homeTeamScored int points      public void visitingTeamScored int points      public void endOfQuarter int quarter           as you can see  this also compiles perfectly fine  Simply because  just like an abstract class  an interface can NOT be instantiated  So  it is not required to explicitly mention the methods from its  parent   However  ALL the parent method signatures DO implicitly become a part of the extending interface or implementing abstract class  So  once a proper class  one that can be instantiated  extends the above  it WILL be required to ensure that every single abstract method is implemented    Hope that helps    and Allahu  alam

User · Answer

Interface means a class that has no implementation of its method  but with just declaration  Other hand  abstract class is a class that can have implementation of some method along with some method with just declaration  no implementation  When we implement an interface to an abstract class  its means that the abstract class inherited all the methods of the interface  As  it is not important to implement all the method in abstract class however it comes to abstract class  by inheritance too   so the abstract class can left some of the method in interface without implementation here  But  when this abstract class will inherited by some concrete class  they must have to implements all those unimplemented method there in abstract class

User · Answer

Perfectly fine  You can t instantiate abstract classes   but abstract classes can be used to house common implementations for m1   and m3    So if m2   implementation is different for each implementation but m1 and m3 are not  You could create different concrete IAnything implementations with just the different m2 implementation and derive from AbstractThing -- honoring the DRY principle  Validating if the interface is completely implemented for an abstract class is futile     Update  Interestingly  I find that C  enforces this as a compile error  You are forced to copy the method signatures and prefix them with  abstract public  in the abstract base class in this scenario    something new everyday

User · Answer

Abstract classes are not required to implement the methods  So even though it implements an interface  the abstract methods of the interface can remain abstract  If you try to implement an interface in a concrete class  i e  not abstract  and you do not implement the abstract methods the compiler will tell you  Either implement the abstract methods or declare the class as abstract

User · Answer

That s because if a class is abstract  then by definition you are required to create subclasses of it to instantiate   The subclasses will be required  by the compiler  to implement any interface methods that the abstract class left out   Following your example code  try making a subclass of AbstractThing without implementing the m2 method and see what errors the compiler gives you   It will force you to implement this method

User · Answer

That s fine  To understand the above  you have to understand the nature of abstract classes first  They are similar to interfaces in that respect  This is what Oracle say about this here      Abstract classes are similar to interfaces  You cannot instantiate them  and they may contain a mix of methods declared with or without an implementation    So you have to think about what happens when an interface extends another interface  For example        Filename  Sports java public interface Sports      public void setHomeTeam String name      public void setVisitingTeam String name        Filename  Football java public interface Football extends Sports      public void homeTeamScored int points      public void visitingTeamScored int points      public void endOfQuarter int quarter           as you can see  this also compiles perfectly fine  Simply because  just like an abstract class  an interface can NOT be instantiated  So  it is not required to explicitly mention the methods from its  parent   However  ALL the parent method signatures DO implicitly become a part of the extending interface or implementing abstract class  So  once a proper class  one that can be instantiated  extends the above  it WILL be required to ensure that every single abstract method is implemented    Hope that helps    and Allahu  alam

User · Answer

Abstract classes are not required to implement the methods  So even though it implements an interface  the abstract methods of the interface can remain abstract  If you try to implement an interface in a concrete class  i e  not abstract  and you do not implement the abstract methods the compiler will tell you  Either implement the abstract methods or declare the class as abstract

User · Answer

That s because if a class is abstract  then by definition you are required to create subclasses of it to instantiate   The subclasses will be required  by the compiler  to implement any interface methods that the abstract class left out   Following your example code  try making a subclass of AbstractThing without implementing the m2 method and see what errors the compiler gives you   It will force you to implement this method

[java] Why an abstract class implementing an interface can miss the declaration/implementation of one of the interface's methods?

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