Solved

Type safe collection subtract

Posted on 2011-09-16
6
1,365 Views
Last Modified: 2012-05-12
Collection<String> result = CollectionUtils.subtract(List1, List2);
I get the following warning for above code:
Type safety: The expression of type Collection needs unchecked conversion to conform to Collection<String>

How can I make this line of code type safe?
What would be the correct syntax to call an object's static function, and give that object a specific template type?

FYI:
List1 and List2 are of type Set<String>
Set<String> List1 = new HashSet<String>();
Set<String> List2 = new HashSet<String>();
0
Comment
Question by:Axter
  • 3
  • 2
6 Comments
 
LVL 86

Expert Comment

by:CEHJ
ID: 36548549
They might be suggesting the below but i'd need to see api docs for 'CollectionUtils'
Collection<String> result = (Collection<String>)CollectionUtils.subtract(List1, List2);

Open in new window

0
 
LVL 86

Expert Comment

by:CEHJ
ID: 36548564
For your purposes, you might as well do the following probably
Set<String> result = (Set<String>)CollectionUtils.subtract(List1, List2);

Open in new window

0
 
LVL 4

Accepted Solution

by:
stachenov earned 500 total points
ID: 36548625
The problem here is that you're trying to call an old-style non-generic method passing it a new-style generic collection. For compatibility reasons it is allowed, but since type checking is impossible, it results in a warning. If there were a type-safe way to write that, it would probably be an error rather than a warning.

Still, instead of suppressing that warning everywhere, you may wish to isolate that problem inside a separate wrapper method in one of your classes, like this:
 
@SuppressWarnings("unchecked")
    public static <T> Collection<T> mySubtract(Collection<T> a, Collection<T> b) {
        return subtract(a, b);
    }

Open in new window

Now, you may call the new mySubtract anywhere you want without having to suppress that annoying warning in any place but this function.
0
Courses: Start Training Online With Pros, Today

Brush up on the basics or master the advanced techniques required to earn essential industry certifications, with Courses. Enroll in a course and start learning today. Training topics range from Android App Dev to the Xen Virtualization Platform.

 
LVL 30

Author Comment

by:Axter
ID: 36550185
CEHJ,
That neither one worked to remove the warning.
0
 
LVL 30

Author Closing Comment

by:Axter
ID: 36550202
Thanks.
The following version worked form me.
      @SuppressWarnings("unchecked")
      public static <T> Collection<T> subtract(Collection<T> a, Collection<T> b)
      {
            return CollectionUtils.subtract(a, b);
      }

And I changed the calling code to the following:
Collection<String> result = subtract(List1, List2);
0
 
LVL 86

Expert Comment

by:CEHJ
ID: 36550245
Personally i wouldn't worry about warnings. In a sense, it's better not to hide them - it simply makes your code (or theirs) seem stronger than it really is, which is misleading
0

Featured Post

Live: Real-Time Solutions, Start Here

Receive instant 1:1 support from technology experts, using our real-time conversation and whiteboard interface. Your first 5 minutes are always free.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

After being asked a question last year, I went into one of my moods where I did some research and code just for the fun and learning of it all.  Subsequently, from this journey, I put together this article on "Range Searching Using Visual Basic.NET …
This was posted to the Netbeans forum a Feb, 2010 and I also sent it to Verisign. Who didn't help much in my struggles to get my application signed. ------------------------- Start The idea here is to target your cell phones with the correct…
This video teaches viewers about errors in exception handling.
This tutorial will introduce the viewer to VisualVM for the Java platform application. This video explains an example program and covers the Overview, Monitor, and Heap Dump tabs.

786 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question