Should methods in a Java interface be declared with or without a public access modifier

Question

Should methods in a Java interface be declared with or without the public access modifier   Technically it doesn t matter  of course  A class method that implements an interface is always public  But what is a better convention   Java itself is not consistent in this  See for instance Collection vs  Comparable  or Future vs  ScriptEngine

User · Answer

People will learn your interface from code completion in their IDE or in Javadoc  not from reading the source   So there s no point in putting  public  in the source - nobody s reading the source

User · Answer

Despite the fact that this question has been asked long time ago but I feel a comprehensive description would clarify why there is no need to use public abstract before methods and public static final before constants of an interface   First of all Interfaces are used to specify common methods for a set of unrelated classes for which every class will have a unique implementation  Therefore it is not possible to specify the access modifier as private since it cannot be accessed by other classes to be overridden   Second  Although one can initiate objects of an interface type but an interface is realized by the classes which implement it and not inherited  And since an interface might be implemented  realized  by different unrelated classes which are not in the same package therefore protected access modifier is not valid as well  So for the access modifier we are only left with public choice   Third  an interface does not have any data implementation including the instance variables and methods  If there is logical reason to insert implemented methods or instance variables in an interface then it must be a superclass in an inheritance hierarchy and not an interface  Considering this fact  since no method can be implemented in an interface therefore all the methods in interface must be abstract    Fourth  Interface can only include constant as its data members which means they must be final and of course final constants are declared as static to keep only one instance of them  Therefore static final also is a must for interface constants   So in conclusion although using public abstract before methods and public static final before constants of an interface is valid but since there is no other options it is considered redundant and not used

User · Answer

I always write what I would use if there was no interface and I was writing a direct implementation  i e   I would use public

User · Answer

The JLS makes this clear      It is permitted  but discouraged as a matter of style  to redundantly specify the public and or abstract modifier for a method declared in an interface

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The public modifier should be omitted in Java interfaces  in my opinion    Since it does not add any extra information  it just draws attention away from the important stuff   Most style-guides will recommend that you leave it out  but of course  the most important thing is to be consistent across your codebase  and especially for each interface  The following example could easily confuse someone  who is not 100  fluent in Java   public interface Foo    public void MakeFoo      void PerformBar

User · Answer

It s totally subjective   I omit the redundant public modifier as it seems like clutter   As mentioned by others - consistency is the key to this decision   It s interesting to note that the C  language designers decided to enforce this   Declaring an interface method as public in C  is actually a compile error   Consistency is probably not important across languages though  so I guess this is not really directly relevant to Java

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With the introduction of private  static  default modifiers for interface methods in Java 8 9  things get more complicated and I tend to think that full declarations are more readable  needs Java 9 to compile    public interface MyInterface          minimal     int CONST00   0      void method00        static void method01          default void method02          private static void method03          private void method04             full     public static final int CONST10   0      public abstract void method10        public static void method11          public default void method12          private static void method13          private void method14

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I used declare methods with the public modifier  because it makes the code more readable  especially with syntax highlighting  In our latest project though  we used Checkstyle which shows a warning with the default configuration for public modifiers on interface methods  so I switched to ommitting them   So I m not really sure what s best  but one thing I really don t like is using public abstract on interface methods  Eclipse does this sometimes when refactoring with  Extract Interface

User · Answer

I would avoid to put modifiers that are applied by default  As pointed out  it can lead to inconsistency and confusion   The worst I saw is an interface with methods declared abstract

User · Answer

I prefer skipping it  I read somewhere that interfaces are by default  public and abstract    To my surprise the book - Head First Design Patterns  is using public with interface declaration and interface methods    that made me rethink once again and I landed up on this post    Anyways  I think redundant information should be ignored

User · Answer

I disagree with the popular answer  that having public implies that there are other options and so it shouldn t be there  The fact is that now with Java 9 and beyond there ARE other options   I think instead Java should enforce require  public  to be specified  Why   Because the absence of a modifier means  package  access everywhere else  and having this as a special case is what leads to the confusion   If you simply made it a compile error with a clear message  e g   Package access is not allowed in an interface    we would get rid of the apparent ambiguity that having the option to leave out  public  introduces   Note the current wording at  https   docs oracle com javase specs jls se9 html jls-9 html jls-9 4      A method in the body of an interface may be declared public or   private    6 6   If no access modifier is given  the method is implicitly public  It is permitted  but discouraged as a matter of   style  to redundantly specify the public modifier for a method   declaration in an interface     See that  private  IS allowed now   I think that last sentence should have been removed from the JLS   It is unfortunate that the  implicitly public  behaviour was ever allowed as it will now likely remain for backward compatibilty and lead to the confusion that the absence of the access modifier means  public  in interfaces and  package  elsewhere

User · Answer

The reason for methods in interfaces being by default public and abstract seems quite logical and obvious to me   A method in an interface it is by default abstract to force the implementing class to provide an implementation and is public by default so the implementing class has access to do so   Adding those modifiers in your code is redundant and useless and can only lead to the conclusion that you lack knowledge and or understanding of Java fundamentals

[java] Should methods in a Java interface be declared with or without a public access modifier?

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