Question

Can someone explain to me why the following code does not work?

public class TestGeneric {

    EntityManager entityManager = new EntityManager();

    public <T extends Textable & Entity> void doAction(T obj) {
        entityManager.getAliasForEntityClass(obj.getClass());
    }
}

class EntityManager {

    public String getAliasForEntityClass(Class<? extends Entity> clazz) {
        return clazz.getCanonicalName();
    }
}

interface Entity {
    Long getId();
}

interface Textable {
    String getText();
}

I’m getting the following error:

The method getAliasForEntityClass(Class<? extends Entity>) in the type EntityManager is not applicable for the arguments 
 (Class<capture#1-of ? extends Textable>)

If I wrote in this way, I did not have any error:

public class TestGeneric {

    EntityManager entityManager = new EntityManager();

    public <T extends Entity & Textable> void doAction(T obj) {
        entityManager.getAliasForEntityClass(obj.getClass());
    }
}

class EntityManager {

    public String getAliasForEntityClass(Class<? extends Entity> clazz) {
        return clazz.getCanonicalName();
    }
}

interface Entity {
    Long getId();
}

interface Textable {
    String getText();
}

Template <T extends Entity & Textable> and <T extends Textable & Entity> means: T must implements Entity and Textable interfaces. Why position of interface so important in this example?

Answer 1

The reason is that the erasure of T is the leftmost type in the bound. From the getClass() documentation:

The actual result type is Class<? extends |X|> where |X| is the
erasure of the static type of the expression on which getClass is
called.

The JLS specifies that “the erasure of a type variable is the erasure of its leftmost bound” (see here). So the type of obj.getClass() is Class<? extends Textable> in the first case and Class<? extends Entity> in the other.