List<? extends MyType>

Question

I have a Java question about generics. I declared a generic list:

List<? extends MyType> listOfMyType;

Then in some method I try instantiate and add items to that list:

listOfMyType = new ArrayList<MyType>();
listOfMyType.add(myTypeInstance); 

Where myTypeInstance is just an object of type MyType; it won't compile. It says:

The method add(capture#3-of ? extends MyType) in the type List<capture#3-of ? extends MyType> is not applicable for the arguments (MyType)

Any idea?

Answer 1

You cannot do a "put" with extends . Look at Generics - Get and Put rule.

Answer 2

Consider:

class MySubType extends MyType {
}

List<MySubType> subtypeList = new ArrayList<MySubType>();
List<? extends MyType> list = subtypeList;
list.add(new MyType());
MySubType sub = subtypeList.get(0);

sub now contains a MyType which is very wrong.

Answer 3

You shouldn't need to use the wildcard capture syntax in your case, simply declaring

List<MyType> listOfMytype;

should be enough. If you want to know exactly why, the Java Generics Tutorial has more than you would ever want to know about the esoteric craziness of Java Generics. Page 20 addresses your specific case.

As for why add with the wildcard capture does not work, it is because the compiler can't determine exactly what subclass of MyType the list will be in every case, so the compiler emits an error.

Answer 4

There is a similar thread here: How can elements be added to a wildcard generic collection?

To get an idea of how generics works check out this example:

    List<SubFoo> sfoo = new ArrayList<SubFoo>();
    List<Foo> foo;
    List<? extends Foo> tmp;

    tmp = sfoo;
    foo = (List<Foo>) tmp;

The thing is, that wasn't designed for local/member variables, but for function signatures, that's why it's so ass-backwards.

Answer 5

I dont know if this will really help you, but this is something I had to use while calling a generic method of Spring Framework and wanting to return also a generic list:

public <T> List<T> findAll(String tableName,Class<?> table) {
    String sql = "SELECT * FROM "+ tableName ;
    List<?> entities = getSimpleJdbcTemplate().query(sql,
            ParameterizedBeanPropertyRowMapper.newInstance(table));
            return (List<T>) entities;
}

Seems the parametrization needs you to use the ? sign in the list to receive the results and then cast the list to the expected return type.

Iam still dazzled by generics...