Question

Function abstraction:

  public abstract class Function<X, Y> {     
      abstract Y apply(X x);      
  }

max method implementation

public static <V extends Comparable<V>> Function<List<V>, V> max() {

    return new Function<List<V>, V>() {
        @Override
        public V apply(List<V> list) {
            return Collections.max(list);
        }
    };
}

And usage (how it should look like)

Date result = max().apply(datesList);

But I get this error and don’t understand why it requires Object

incompatible types; inferred type argument(s) java.lang.Object do not conform to bounds of type variable(s) V
found   : <V>project.Function<java.util.List<V>,V>
required: java.lang.Object

I have read big amount of similar QA but didn’t get how to fix this.
Thanks.

Answer 1

Java has a very limited type inference. If you write this:

Date result = max().apply(datesList);

it is not sophisticated enough to infer the type parameter of the max() method, V, so it takes java.lang.Object instead. You could try this:

Function<List<Date>, Date> fn = max();
Date result = fn.apply(dates);

Or, if you want to write it in one line, you could do the following, assuming that your max() method is defined in a class named Example:

Date result = Example.<Date>max().apply(dates);