returning a Void object

Question

What is the correct way to return a Void type  when it isn t a primitive  Eg  I currently use null as below   interface B lt E gt   E method       class A implements B lt Void gt        public Void method               do something         return null

User · Answer

There is no generic type which will tell the compiler that a method returns nothing.

I believe the convention is to use Object when inheriting as a type parameter

OR

Propagate the type parameter up and then let users of your class instantiate using Object and assigning the object to a variable typed using a type-wildcard ?:

interface B<E>{ E method(); }

class A<T> implements B<T>{

    public T method(){
        // do something
        return null;
    }
}

A<?> a = new A<Object>();

User · Answer

Java 8 has introduced a new class  Optional lt T gt   that can be used in such cases  To use it  you d modify your code slightly as follows   interface B lt E gt   Optional lt E gt  method       class A implements B lt Void gt        public Optional lt Void gt  method               do something         return Optional empty              This allows you to ensure that you always get a non-null return value from your method  even when there isn t anything to return  That s especially powerful when used in conjunction with tools that detect when null can or can t be returned  e g  the Eclipse  NonNull and  Nullable annotations

User · Answer

If you just don t need anything as your type  you can use void  This can be used for implementing functions  or actions  You could then do something like this   interface Action lt T gt        public T execute       abstract class VoidAction implements Action lt Void gt        public Void execute             executeInternal            return null             abstract void executeInternal        Or you could omit the abstract class  and do the return null in every action that doesn t require a return value yourself   You could then use those actions like this   Given a method  private static  lt T gt  T executeAction Action lt T gt  action        return action execute        you can call it like  String result   executeAction new Action lt String gt           Override     public String execute               code here         return  Return me                or  for the void action  note that you re not assigning the result to anything   executeAction new VoidAction          Override     public void executeInternal               code here

User · Answer

It is possible to create instances of Void if you change the security manager  so something like this  static Void getVoid   throws SecurityException  InstantiationException          IllegalAccessException  InvocationTargetException       class BadSecurityManager extends SecurityManager                 Override         public void checkPermission Permission perm                    Override         public void checkPackageAccess String pkg                 System setSecurityManager badManager   new BadSecurityManager         Constructor lt   gt  constructor   Void class getDeclaredConstructors   0       if  constructor isAccessible              constructor setAccessible true             return  Void  constructor newInstance       Obviously this is not all that practical or safe  however  it will return an instance of Void if you are able to change the security manager

User · Answer

The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the Java keyword void     So any of the following would suffice    parameterizing with Object and returning new Object   or null parameterizing with Void and returning null parameterizing with a NullObject of yours   You can t make this method void  and anything else returns something  Since that something is ignored  you can return anything

User · Answer

Just for the sake of it  there is of course the possibility to create Void instance using reflection   interface B lt E gt   E method       class A implements B lt Void gt        public Void method               do something          try               Constructor lt Void gt  voidConstructor   Void class getDeclaredConstructor                voidConstructor setAccessible true               return voidConstructor newInstance              catch  Exception ex                   Rethrow  or return null  or whatever                      You probably won t do that in production

[java] returning a Void object

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