Question

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Is `List<Dog>` a subclass of `List<Animal>`? Why aren’t Java’s generics implicitly polymorphic?

I need help understanding whats going on.

I have an interface

public interface Foo {

  Map<String, ParameterValue> getParameters();
}

public interface ParameterValue { }

And an implementation of Foo.

class FooImpl implements Foo {
   Map<String, ParameterValueImpl> parameters = new HashMap<String, ParameterValueImpl>();
   //ParameterValueImpl an implementation of ParameterValue

@Override
    public Map<String, ParameterValue> getParameters() {
        return ((Map<String,ParameterValue>) parameters);
    }   

}

I get the following error

[ERROR] Failed to execute goal
failure [ERROR]
src/main/java/domainJPA/model/FooImpl.java:[92,45]
inconvertible types [ERROR] found   :
java.util.Map<java.lang.String,domainJPA.model.ParameterValueImpl>
[ERROR] required:
java.util.Map<java.lang.String,domain.model.ParameterValue>

I have to use the ParameterValue in the interface of Foo, and in FooImpl I must just the implementation of ParameterValue (ParameterValueImpl) because its an @Entity and jpa requires it.

How can I make this code compile?

EDIT

If I change the code to ? extends ParameterValue as suggested in one of the answers, then I cannot use the Foo interface.

The following code results in compilation error: The method put(String, capture#8-of ? extends ParameterValue) in the type Map<String,capture#8-of ? extends ParameterValue> is not applicable for the arguments (String, ParameterValueImpl)

foo.getParameters().put("test", new ParameterValueImpl);

Answer 1

Think about it — if the return value from getParameters() is actually a Map<String, ParameterValueImpl>, then you have the ability to add a value to the map that derives from ParameterValue, but not ParameterValueImpl. You would be breaking type safety by doing that.

You have two options to correct this:

1. In getParameters(), copy the map into a new map of the correct type.
2. Use Map<String, ParameterValue> as the type for FooImpl.parameters.