CODEWITHSUNDEEP
javagenericsinheritance
Jameshobbs
Jameshobbs

Reputation: 524

The method add(T) in the type List<T> is not applicable for the arguments

I searched stack overflow for this error, but none quite had the same design as I have. Suggestions for terminology to aid in finding a similar topic like the sample code below would be appreciated.

Here is a simple test case that demonstrates the error:

import java.util.List;

public class SimpleTest {

    abstract class AbsTask<T>
    {

    }

    abstract class AbsQueue<T extends AbsTask<?>>
    {
        private List<T> lst;

        public void addSpecialItem()
        {
            lst.add(new SpecialItem()); // Error occurs here
        }



    }

    class SpecialItem extends AbsTask<Void>
    {

    }

}

I am trying to add a method to my abstract class AbsQueue called addSpecialItem, which will insert the SpecialItem class into the list generic list T which is essentially a list of AbsTask.

Here is the error: The method add(T) in the type List<T> is not applicable for the arguments (SimpleTest.SpecialItem)

I can resolve this error if I type case the add line as follows:

lst.add((T)new SpecialItem());

Is there a way of handling this without type casting new SpecialItem() to T?

Upvotes: 2

Views: 10951

Answers (3)

Christopher Perry
Christopher Perry

Reputation: 39225

Your abstract class must be instantiated to define what T is. Try this:

public class SimpleTest {

  static abstract class AbsTask<T> { }

  static class AbsQueue<T extends AbsTask<?>> {
    private List<T> lst;

    public void addSpecialItem(T item) {
      lst.add(item); 
    }
  }

  static class Test {

    public void main() {
      AbsQueue<SpecialItem> queue = new AbsQueue<SpecialItem>();
      queue.addSpecialItem(new SpecialItem());
    }
  }

  static class SpecialItem extends AbsTask<String> {

  }

}

Upvotes: 2

Dave L.
Dave L.

Reputation: 9781

At that line of code lst.add(new SpecialItem()); the compiler does not yet know what T is.

Upvotes: 0

Louis Wasserman
Louis Wasserman

Reputation: 198023

A List<T> is supposed to be a list that can only include elements of type T, but the code you've written doesn't ensure that SpecialItem is a subtype of T.

It's not clear what you actually want, but I think what you want is a List<AbsTask<?>>, not a List<T> for some specific T that extends AbsTask<?>.

Upvotes: 0

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