Java- Generic Type not found?

Question

i'm writing code for homework and I'm getting this error

GenericSet.java:101: error: method map in class GenericSet<T> cannot be applied
to given types;
            E ans = map(item);
                    ^
  required: LMap<T,E>
  found: T
  reason: cannot infer type-variable(s) E
    (argument mismatch; T cannot be converted to LMap<T,E>)
  where T,E are type-variables:
    T extends Object declared in class GenericSet
    E extends Object declared in method <E>map(LMap<T,E>)
Note: GenericSet.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

from what I understand, the interface I'm being given passes in a function, and I have to use the a generic input(T) to get a generic output (E) which will be stored in a new custom generic object that I made. It looks like it's finding T fine but not E. Could you guys tell me what I'm doing wrong?

Here's my code:

public <E> ExtendedSet<E> map(LMap<T, E> map) {
        GenericSet<E> finalVal = new GenericSet();
        for (T item: this.myList) {
            E ans = map(item);
            finalVal.addThis(ans);
        }
        return finalVal;
    }

Note: the object GenericSet implements ExtendedSet Note2: The interface method im given to implement looks like this:

@FunctionalInterface
public interface LMap<T, E> {
    /**
      *Maps an element of type T to type E
      *@param element the source element to map from
      *@return E the destination element to map to
      */
    E map(T element);
}

You are trying to pass an instance of T (item) to a method that accepts an instance of LMap<T, E>. Look at your error more closely: required: LMap<T,E> :: found: T — Vince
– Vince, Commented Apr 9, 2015 at 2:32

Mark Peters · Accepted Answer · 2015-04-09 02:33:17Z

1

You want to use

E ans = map.map(item);

A functional interface's method still needs to be invoked via the instance. By omitting the instance, you're calling this.map(item), which takes an LMap<T, E>.

edited Apr 9, 2015 at 2:33

answered Apr 9, 2015 at 2:32

Mark Peters

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3 Comments

Joseph hooper Over a year ago

so is map an object that contains the method map?

Mark Peters Over a year ago

@Josephhooper: Exactly. It's an implementation of LMap<T, E>, just like any other interface.

Vince Over a year ago

@Josephhooper Don't forget to mark this answer as "accepted answer" by clicking on the checkmark next to the voting arrows! :)

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