Question

I have an interface with a method

public List< Object > getLeftBusinessObjects( List< Object > businessObjectIds, Object owningSystemId );

My implementation is:

public List<Object> getLeftBusinessObjects(List< Object > businessObjectIds, Object owningSystemId)
{    
    List<MyObject> myObjs= Helper.getMyObjects(businessObjectIds);
    return myObjs;
}

Doing so I get the error:

Type mismatch: cannot convert from List<MyObject> to List<Object>

I have to double cast this way

return (List<Object>) (Object) myObjs;

to avoid the error.

Can someone explain me why? Why do I have to cast if all objects have per dafault Object as superclass?

Answer 1

A List<MyObject> is not a List<Object>.,because otherwise you would be able to put an Object into a List<MyObject>

try this

public List<? extends Object> getLeftBusinessObjects(List< Object > businessObjectIds, Object owningSystemId)
{    
    List<MyObject> myObjs= Helper.getMyObjects(businessObjectIds);
    return myObjs;
}