java custom annotation: make an attribute optional

I defined my own custom annotation

@Target(value={ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyCustomAnnotation  {
    Class<?> myType();
}

how, if at all, can I make the attribute optional

You can provide a default value for the attribute:

@Target(value={ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyCustomAnnotation  {
    Class<?> myType() default Object.class;
}

Found it. It can't be optional, but a default can be declared like this:

@Target(value={ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyCustomAnnotation  {
    Class<?> myType() default String.class;
}

If no default can make sense as "empty" value then that is a problem.

For Optional attribute you need to provide default value for that attribute you can provide default value using "default" keyword.

Note : For only one attribute you can use attribute name as value. If you use your attribute name as value you can directly pass value like this @MyCustomAnnotation(true) instead of @MyCustomAnnotation(myType = true).

See this example for more details