What you cannot do List<W> l1 = new ArrayList<String>(); while this works when calling m-CodePudding

I am really not able to understand that why I get compilation error when in first case while works fine in second case.

public class GenericsTest3 {

    public static <W> void main(String[] args) {
        List<W> l1 = new ArrayList<String>();       // compilation error: Type mismatch: cannot convert from ArrayList<String> to List<W>
        doSomething1(new ArrayList<String>());      // works fine
    }
    
    public static <L> L doSomething1(List<L> list) {
        list.get(0);
        list.add(list.get(0));
        return list.get(1);
    }
    
}

In my understand in both the cases List is defined of type parameter T/W, so why parameterized type new ArrayList<String>() fails in one case while passes in other case.

CodePudding user response：

List<W> l1 = new ArrayList<String>();

Issue here is, that you declare a variable l1 for a List of type W items, but then you assign an (Array)List of type String to it. This is only acceptable, if W is also String.

You can just use W as type argument for the ArrayList or remove type argument from it.

List<W> l1 = new ArrayList<>();

Regarding your next question:

doSomething1(new ArrayList<String>());

Here you simply create a new ArrayList of type String and pass it as argument to doSomething1. At doSomething1, type argument L will therefore be String.

CodePudding user response：

case 1 :

List<W> l1 = new ArrayList<String>();

You are telling java that create a generic list and in the same declaration you are specifying the data type as String. So compiler is confused what to do.

Case 2 :

<L> L doSomething1(List<L> list) method

In this case java knows that generic list is input . All the action done on List interface. So compiles.