Passing a map to a function that specifies a map argument with unbounded wild cards

Question

I am basically trying to do a check of a Map in Java. My code is as follows.

private boolean isValid(Map<?,?> map) {
 return (null != map && map.size() > 0);
}

Why can't I use this method as follows?

Map<String, String> m = new HashMap<>();
isValid(m);

In Eclipse, I get the following error message:

The method isValid(Map<?,?>) in the type MyClazz is not applicable for the arguments (Map<String,String>)

As I am writing this question, I tried the following and it "works".

isValid( (Map<?,?>) m);

Why is this? Thumbing through the SO posts, I got a "similar" problem stating the problem of wildcard capture. But it doesn't seem like that's the problem here (without casting).

Which Eclipse version you using? This should compile (although not very good code), and it does compile with Eclipse Mars 4.5.1 (just tested). — Tunaki, Jan 14 '16 at 15:52
@Leo: That'd be working with raw types, and raw types are bad. — Makoto, Jan 14 '16 at 15:56
i'm on Eclipse `Version: Mars.1 Release (4.5.1), Build id: 20150924-1200` with JavaSE-1.8 set on the build path and compiler compliance level set to 1.8. — Jane Wayne, Jan 14 '16 at 16:02
Are some of you saying that this is not a Java issue per say, but one with Eclipse? — Jane Wayne, Jan 14 '16 at 16:04
@Makoto the function isValid() that Jane is using does not need this information. It just tests if the Map is empty — Leo, Jan 14 '16 at 16:07
@Leo: This is why the wildcards are being used. If you can at all avoid using raw types, you should. — Makoto, Jan 14 '16 at 16:08
learning something new everyday. thanks @Makoto -- http://stackoverflow.com/questions/2770321/what-is-a-raw-type-and-why-shouldnt-we-use-it — Leo, Jan 14 '16 at 16:31

score 0 · Answer 1 · answered Apr 19 '16 at 00:51

This error does not occur with the example you've provided in either Java 8's javac or Eclipse Mars. You might try refreshing your project or cleaning the project (Project > Clean...), but otherwise your example does not replicate your actual problem.

One possibility is you're importing some other Map than java.util.Map.

This example compiles without issue, both with javac and Eclipse.

$ cat Demo.java 
import java.util.*;

class Demo {
  // isEmpty() is a better way to check if a Map is empty rather
  // than .size() > 0, which is O(n) for some Map implementations.
  public static boolean nonEmpty(Map<?,?> map) {
    return map != null && !map.isEmpty();
  }

  public static boolean nonZero(Map<?,?> map) {
    return null != map && map.size() > 0;
  }

  public static void main(String[] args) {
    nonEmpty(new HashMap<String, String>());
    nonZero(new HashMap<String, String>());
  }
}

$ javac Demo.java; echo $?
0

Passing a map to a function that specifies a map argument with unbounded wild cards

1 Answers1