Creating an instance of a generic class of type <?>

Question

I am participating in SCJP / OCPJP and I came across a sample question that seems strange to me.

The sample code instantiated two common collections:

List<?> list = new ArrayList<?>(); List<? extends Object> list2 = new ArrayList<? extends Object>(); 

The “correct” answer to the question was that this code would compile, but adding to any collection would result in a runtime error.

When I try to compile such code, I just get errors. The Java tutorial doesn't even show this type of code; instead, it usually uses wildcards as part of the upcast.

 Collection<?> c = new ArrayList<String>(); 

Are two common collections above even legal code? The second one in my logic will only prohibit interfaces. The first looks completely useless. Why use a common element that is not trying to control?

Answer 1

Check out this great Java Java tutorial . More specifically, the wildcard section contains the answer to your question, and I quote

 Collection<?> c = new ArrayList<String>(); c.add( new Object() ); 

Since we do not know what the type of element c means, we cannot add objects to it. The add() method accepts arguments of type E , the type of the item in the collection. When is the actual type parameter ? , it means some unknown type. Any parameter that we pass to add will have to be a subtype of this unknown type. Since we do not know what type, that is, we can not convey anything. The one exception is null , which is a member of each type.

Answer 2

If you want to declare Type at runtime, you can do something like this:

 public class Clazz1<T> { private final List<T> list = new ArrayList<T>(); private List<T> getList() { return list; } /** * @param args */ public static void main(String[] args) { Clazz1<Integer> clazzInt = new Clazz1<Integer>(); clazzInt.getList().add(2); System.out.println(clazzInt.getList()); Clazz1<String> clazzString = new Clazz1<String>(); clazzString.getList().add("test"); System.out.println(clazzString.getList()); } } 

Answer 3

I answered this a bit earlier in this answer. ? cannot be used in an instance. I'm not sure why he says the code will compile, none of the java compilers I used would allow this. You can do what is shown above:

 List<?> list = new ArrayList(); 

This will compile and run, but you will not be able to:

 list.add("hello world"); //This wouldn't compile 

Answer 4

new creates a specific instance of the object. A particular instance can have only one type, including any generics. Knowing this, wildcards cannot work with new .