Generic method doesn't compile - the method is not applicable for the arguments

Question

Here is my code. The compiler refuses to compile it :

private transient List<? extends Map<String, Object>> donnees;

// ...

public <M extends Map<String, Object>> void addDonnee(M nouvelleDonnee) {
    getDonnees().add(nouvelleDonnee);
}

public List<? extends Map<String, Object>> getDonnees() {
    // ...

    return donnees;
}

Why do I get this error ?

The method add(capture#4-of ? extends Map<String,Object>) in the type List<capture#4-of ? extends Map<String,Object>> is not applicable for the arguments (M)

EDIT Here how I solve my problem :

private transient List<Map<String, Object>> donnees;

// ...

public void addDonnee(Map<String, Object> nouvelleDonnee) {
    getDonnees().add(nouvelleDonnee);
}

public List<Map<String, Object>> getDonnees() {
    // ...

    return donnees;
}

And now the compiler is happy ! :)

Check the answer of [Jack](http://stackoverflow.com/questions/1684121/generics-error-not-applicable-for-the-arguments) — GokcenG, Aug 22 '13 at 11:52

score 3 · Answer 1 · answered Aug 22 '13 at 11:51

The getDonnees method returns a list of "something" that implements Map<String,Object>. It could be a list of TreeMaps, or of HashMaps for example; there's no way to know.

Since you don't know the exact component type of the list, you can't add anything to it. For example, you could be trying to add a HashMap to a list of TreeMap.

The two methods should use the same parameter somehow, for example one declared with the class:

class SomeClass<M extends Map<String, Object>> {

  public void addDonnee(M nouvelleDonnee) {
    getDonnees().add(nouvelleDonnee);
  }

  public List<M> getDonnees() {
    // ...
    return donnees;
  }
}

score 3 · Accepted Answer · edited Aug 22 '13 at 12:40

3

Both ? and M are subclasses, and may not be castable into each other. Try declaring donnees:

private transient List<Map<String, Object>> donnees;

edited Aug 22 '13 at 12:40

Duncan Jones

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answered Aug 22 '13 at 11:54

András Hummer

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score 1 · Answer 3 · answered Aug 22 '13 at 11:51

1

You cannot insert anything into a List defined as List<? extends Foo>.

This is discussed in a decent level of detail in the JDK documentation (see the bottom of the page).

answered Aug 22 '13 at 11:51

Duncan Jones

67,400
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Do you mean I must create a class like this `FooList extends ArrayList` ? – Stephan Aug 22 '13 at 11:53
The solution from [Dukelink](http://stackoverflow.com/a/18379657/474189) may work for you? – Duncan Jones Aug 22 '13 at 11:54

score 1 · Answer 4 · answered Aug 22 '13 at 11:52

Java is complaining because it doesn't know that the types of donnees, addDonnee and getDonnees are the same. The one could be a List<HashMap<String, Object>>, while the others can be a List<TreeMap<String, Object>>, which are not compatible (you can't add a HashMap to a List<TreeMap> and vice versa).

You can add a generic parameter to the class instead:

public class SomeClass<T extends Map<String, Object>>
{
  private transient List<T> donnees;

  public void addDonnee(T nouvelleDonnee) {
      getDonnees().add(nouvelleDonnee);
  }

  public List<T> getDonnees() {
      return donnees;
  }
}

Generic method doesn't compile - the method is not applicable for the arguments

4 Answers4