Raw and Generic List

Question

This is a code just to know what is wrong and what is right.

public class JavaApplication5 {


public static void main(String[] args) {

  List l=new ArrayList<String>();//Line 1

  List<Object> x=new ArrayList<String>();//Line 2


}

 }

In the above line 1 is working fine but line 2 gives me compilation error. Can you tell me why?

Are not List and List<Object> equivalent? Either both should be wrong or both should be correct.

You are using different types in List on left side you have Object and on the right side you are instancing it as a Strings. — wedo, Apr 09 '17 at 07:35
Possible duplicate of [Is List a subclass of List? Why aren't Java's generics implicitly polymorphic?](http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicitly-p) — QBrute, Apr 09 '17 at 07:38
Raw types bypass generic type checks, deferring the catch of unsafe code to runtime. So `List` becomes `List` at runtime, whereas `List — beatngu13, Apr 09 '17 at 08:13

score 1 · Answer 1 · answered Apr 09 '17 at 07:35

You have to set the same Type in the both place :

List<Object> x = new ArrayList<String>();
//----^-------------------------^--------

So you have to option to solve your problem :

One don't set any type in your ArrayList

List<Object> x = new ArrayList<>();

Or set the same type :

List<Object> x = new ArrayList<Object>();
//or
List<String> x = new ArrayList<String>();

score 0 · Answer 2 · answered Apr 09 '17 at 07:36

Neither posted option. For a List of String(s), since Java 7, you can use the diamond operator - like,

List<String> x = new ArrayList<>();

However, the older

List<String> x = new ArrayList<String>();

is still legal.

Your first option is a raw type, and your second option (if it were legal) makes a List that can contain any type of Object (not just String).

Raw and Generic List

2 Answers2