Why List<String> is not acceptable as List<Object>? [duplicate]

Question

Consider below method doSomething(List<Object>) which accepts List<Object> as parameter.

private void doSomething(List<Object> list) {
    // do something
}

Now consider below code snippet which tries to call doSomething() where I try to pass List<String> to doSomething()

List<Object> objectList;
List<String> stringList;

doSomething(stringList); // compilation error incompatible types
doSomething(objectList); // works fine 

Even below code throws compilation error

objectList = stringList;  // compilation error incompatible types

My question is why List<String> can not be passed to a method which accepts List<Object>?

Answer 1

Because while String extends Object, List<String> does not extend List<Object>

Update:
In general, if Foo is a subtype (subclass or subinterface) of Bar, and G is some generic type declaration, it is not the case that G<Foo> is a subtype of G<Bar>.

This is because collections do change. In your case, If List<String> was a subtype of List<Object>, then types other than String can be added to it when the list is referenced using its supertype, as follows:

List<String> stringList = new ArrayList<String>;
List<Object> objectList = stringList;// this does compile only if List<String> where subtypes of List<Object>
objectList.add(new Object());
String s = stringList.get(0);// attempt to assign an Object to a String :O

and the Java compiler has to prevent these cases.

More elaboration on this Java Tutorial page.

Answer 2

You could put an object of a wrong type into the list IF this worked:

private void doSomething(List<Object> list) {
    list.add(new Integer(123)); // Should be fine, it's an object
}

List<String> stringList = new ArrayList<String>();
doSomething(stringList); // If this worked....
String s = stringList.get(0); // ... you'd receive a ClassCastException here

Answer 3

This generic question in Java may look confusing to any one who is not very familiar with Generics as in first glance it looks like String is object so List<String> can be used where List<Object> is required but this is not true. It will result in compilation error.

It does make sense if you go one step further because List<Object> can store anything including String, Integer etc but List<String> can only store Strings.

Also have a look at: Why not inherit from List<T>?