Question

I didn’t manage to compile a set of classes with nested interfaces.

Let’s assume there are two interfaces:

public interface IntA
{   
    public Map<String, ? extends IntB> getName();
    public void setName(Map<String, ? extends IntB> name);
}

public interface IntB
{
    long getId();
    void setId(long id);
}

and two corresponding implementation classes

public class ImplA implements IntA,Serializable
{
    private Map<String, ImplB> name;
    public Map<String, ? extends IntB> getName(){return name;}
    public void setName(Map<String, ? extends IntB> name)
                  {this.name = (Map<String, ImplB>)name;}
}

public class ImplB implements IntB,Serializable
{
    private long id;
    public long getId() {return id;}
    public void setId(long id) {this.id = id;}
}

How can i use the setter of ImplA?

public static void create(Random rnd, String code)
{
  ImplB b = new ImplB();
  b.setId(1);

  ImplA a = new ImplA();
  a.getName().put("key", b);
  a.getName().put("key", (IntB)b);
}

results in:

put(java.lang.String,capture#50 of ? extends IntB) in
java.util.Map<java.lang.String,capture#50 of ? extends .IntB>
cannot be applied to (java.lang.String,ImplB)

put(java.lang.String,capture#845 of ? extends IntB) in
java.util.Map<java.lang.String,capture#845 of ? extends IntB>
cannot be applied to (java.lang.String,IntB)

Thank you

Thor

Answer 1

Your getter is returning a more general type. Imagine if it the actual return value was a reference to a Map<String, SomeOtherImplB> – you wouldn’t want to be able to put an ImplB there.

If you want to be able to put a value into a Map, you either need the value type to be specified exactly, or specified with a “super” constraint rather than an “extends” constraint. For example:

// This is fine
Map<String, ? super Apple> map = getMapFromSomewhere();
map.put("Foo", new Apple());

// This isn't - it might be a Map<String, Orange>!
Map<String, ? extends Fruit> map = getMapFromSomewhere();
map.put("Foo", new Apple());

So that’s why it’s not working at the moment. As for the right way to work around that… it’s hard to know exactly what you’re trying to do. Maybe you want to change it to:

public interface IntA
{   
    public Map<String, IntB> getName();
    public void setName(Map<String, IntB> name);
}

… or the alternative is to make IntA generic:

public interface IntA<T extends IntB> 
{   
    public Map<String, T> getName();
    public void setName(Map<String, T> name);
}

So in this case, your ImplA class would implement IntA<ImplB>.