Java: Generic types inside Raw Type object or why my parametrization doesn't work

Question

I can't grasp why my interface parametrization doesn't work. Let's look at code below:

public interface IType {
  public List<String> getAllItems();
}

......
public void function(IType item) {
  for (String str : item.getAllItems()) { //DOESN'T WORK! Incompoatible types. Required String, Found: Object

  } 
}

Why does it return List<Object> instead of List<String>?

Generics are compile time. In run-time it does not behave as you think. Cast it to `String`. — Anirban Nag 'tintinmj', Dec 19 '13 at 13:26
Yes I can cast the list-reference. But actually the question is WHY? — VB_, Dec 19 '13 at 13:27
@stonedsquirrel it works. But `for (String str : item.getAllItems())` doesn't — VB_, Dec 19 '13 at 13:33
What do you mean it returns? Your function does not return anything, it is void. If you mean you see the type in a debugger, then as tintinmj says - there are no generics during runtime. For backward compatibility, java uses type erasure - it drops all the knowledge of generics after checking them during compilation. — Jirka, Dec 19 '13 at 13:34
That's hard to believe. What's the actual compiler error? Also check your imports and make sure it's `java.util.List` and not some other `List`. — André Stannek, Dec 19 '13 at 13:35
This looks much like a understanding problem of generics, as your interface is without generics, so will its methods at runtime... gotta find back the question that answer this now. — Jonathan Drapeau, Dec 19 '13 at 14:06
@JonathanDrapeau That's just not true. You're thinking of raw types. This won't apply to an un-parameterized interface type. — Sotirios Delimanolis, Dec 19 '13 at 14:28
@VolodymyrBakhmatiuk Again: What's the actual compiler error? — André Stannek, Dec 19 '13 at 15:08
@stonedsquirrel Incompoatible types. Required String, Found: Object — VB_, Dec 19 '13 at 15:10
@VolodymyrBakhmatiuk This definitely shouldn't happen. The error must be in some part of the code you didn't post. Did you check the imports as I mentioned? — André Stannek, Dec 19 '13 at 15:14
Are you sure the `IType` interface doesn't have a type parameter? — Sotirios Delimanolis, Dec 19 '13 at 15:18

score 5 · Accepted Answer · edited May 23 '17 at 12:19

I'm going to make an assumption that your IType is actually a parameterized (and you've just confirmed it) type like so

public interface IType<E> {
    public List<String> getAllItems();
}

In this case, if you declare a variable (or parameter) as

IType item;

you are using a Raw Type. With a variable of a raw type, all generic types in methods or fields accessed on that variable are erased. So

public List<String> getAllItems();

becomes

public List getAllItems();

and so the List Iterator will return references of type Object.

public void function(IType item) {
    for (String str : item.getAllItems()) { // DOESN'T WORK! Incompoatible
                                            // types. Required String,
                                            // Found: Object
    }
}

Combining Raw Types and Generic Methods

Java: Generic types inside Raw Type object or why my parametrization doesn't work

1 Answers1