Isn't List> same as List

Question

public List<Object> query(){

        List<Object> listToReturn = new ArrayList<Object>();  
        List<String> listOfString = new ArrayList<String>();
        List<List<String>> listOfListOfString = new ArrayList<List<String>>();

        listOfListOfString.add(listOfString);
        listToReturn.add(listOfListOfString);

        return listToReturn;
    }

Instead of above method if I write :

public List<Object> query(){

        List<Object> listToReturn = new ArrayList<Object>();    
        List<String> listOfString = new ArrayList<String>();
        List<List<String>> listOfListOfString = new ArrayList<List<String>>();

        listOfListOfString.add(listOfString);

        return listOfListOfString;

    }

I get a compile-time error, Type mismatch: cannot convert from List<List<List<String>>> to List<Object>

I am a bit confused.

+1: Good question. I don't know why. Probably somewhere described in the JLS. — Martijn Courteaux, Jan 18 '14 at 13:20
concrete parameterized type are different! http://www.angelikalanger.com/GenericsFAQ/FAQSections/ParameterizedTypes.html#FAQ101 — MariuszS, Jan 18 '14 at 13:21

score 4 · Answer 1 · edited May 23 '17 at 11:49

Is List<Object> a supertype of List<String>?

No, different instantiations of the same generic type for different concrete type arguments have no type relationship. FAQ102

The error Type mismatch: cannot convert from List<List<List<String>>> to List<Object> means that both objects has different concrete parameterized type.

What is a concrete parameterized type?

An instantiation of a generic type where all type arguments are concrete types rather than wildcards. FAQ101

More:

Source: Java Generics FAQs - Generic And Parameterized Types

JB Nizet · Accepted Answer · 2014-01-18T17:22:42.853

If you could do that, the caller, which gets a List<Object>, would be able to add any kind of Object to the list, although it's been declared as a List<List<String>>. It would thus ruin the type-safety of the list:

List<List<String>> listOfListOfString = ...;
List<Object> listOfObjects = listOfListOfString;
listOfObjects.add(new Banana()); 
// now your list of lists of strings contains a banana: oops!

You can do the following, however, which would prevent the caller from adding anything to the list though:

List<?> query() { // equivalent of List<? extends Object>
    ...
    return listOfListOfString;
}

2 Answers2