Attributes member variables in interfaces

Question

I wish to know is there any way in which I can make it compulsory for the implementer class to declare the objects handles primitives as they do with methods  for e g    public interface Rectangle           int height   0      int width   0       public int getHeight        public int getWidth        public void setHeight int height       public void setWidth int width                       public class Tile implements Rectangle       Override     public int getHeight             return 0              Override     public int getWidth             return 0              Override     public void setHeight int height                Override     public void setWidth int width                  In the above method how can we compel Tile class to declare height and width attributes using the interface  For some reason I wish to do it with interface only   I initially thought of using it with inheritance  But thing is I have to deal with 3 classes     Rectangle Tile JLabel      nbsp    class Tile extends JLabel implements Rectangle      would work    but  class Tile extends JLabel extends Rectangle      woud not

User · Answer

Fields in interfaces are implicitly public static final   Also methods are implicitly public  so you can drop the public keyword   Even if you use an abstract class instead of an interface  I strongly suggest making all non-constant  public static final of a primitive or immutable object reference  private  More generally  prefer composition to inheritance  - a Tile is-not-a Rectangle  of course  you can play word games with  is-a  and  has-a

User · Answer

You can only do this with an abstract class  not with an interface   Declare Rectangle as an abstract class instead of an interface and declare the methods that must be implemented by the sub-class as public abstract   Then class Tile extends class Rectangle and must implement the abstract methods from Rectangle

User · Answer

Java 8 introduced default methods for interfaces using which you can body to the methods  According to OOPs interfaces should act as contract between two systems parties    But still i found a way to achieve storing properties in the interface  I admit it is kinda ugly implementation      import java util Map     import java util WeakHashMap   interface Rectangle    class Storage       private static final Map lt Rectangle  Integer gt  heightMap   new WeakHashMap lt  gt         private static final Map lt Rectangle  Integer gt  widthMap   new WeakHashMap lt  gt        default public int getHeight         return Storage heightMap get this      default public int getWidth         return Storage widthMap get this      default public void setHeight int height        Storage heightMap put this  height      default public void setWidth int width        Storage widthMap put this  width         This interface is ugly  For storing simple property it needed two hashmaps and each hashmap by default creates 16 entries by default  Additionally when real object is dereferenced JVM additionally need to remove this weak reference

User · Answer

The point of an interface is to specify the public API   An interface has no state   Any variables that you create are really constants  so be careful about making mutable objects in interfaces    Basically an interface says here are all of the methods that a class that implements it must support  It probably would have been better if the creators of Java had not allowed constants in interfaces  but too late to get rid of that now  and there are some cases where constants are sensible in interfaces    Because you are just specifying what methods have to be implemented there is no idea of state  no instance variables    If you want to require that every class has a certain variable you need to use an abstract class   Finally  you should  generally speaking  not use public variables  so the idea of putting variables into an interface is a bad idea to begin with     Short answer - you can t do what you want because it is  wrong  in Java   Edit   class Tile      implements Rectangle        private int height      private int width         Override     public int getHeight             return height              Override     public int getWidth             return width              Override     public void setHeight int h            height   h              Override     public void setWidth int w             width   w              an alternative version would be   abstract class AbstractRectangle      implements Rectangle        private int height      private int width         Override     public int getHeight             return height              Override     public int getWidth             return width              Override     public void setHeight int h            height   h              Override     public void setWidth int w             width   w             class Tile      extends AbstractRectangle

User · Answer

Something important has been said by Tom      if you use the has-a concept  you avoid the issue     Indeed  if instead of using extends and implements you define two attributes  one of type rectangle  one of type JLabel in your Tile class  then you can define a Rectangle to be either an interface or a class    Furthermore  I would normally encourage the use of interfaces in connection with has-a  but I guess it would be an overkill in your situation  However  you are the only one that can decide on this point  tradeoff flexibility over-engineering

User · Answer

In Java you can t  Interface has to do with methods and signature  it does not have to do with the internal state of an object -- that is an implementation question  And this makes sense too -- I mean  simply because certain attributes exist  it does not mean that they have to be used by the implementing class  getHeight could actually point to the width variable  assuming that the implementer is a sadist     As a note -- this is not true of all languages  ActionScript allows for declaration of pseudo attributes  and I believe C  does too

User · Answer

Interfaces cannot require instance variables to be defined -- only methods    Variables can be defined in interfaces  but they do not behave as might be expected  they are treated as final static    Happy coding

[java] Attributes / member variables in interfaces?

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