Can't edit files in Eclipse even though not read-only

This is such an amateur question I'm embarrassed I even have to ask.

In Eclipse, sometimes I can't edit a file. I can open it, but it behaves as it if it's a read-only file—typing has no effect. It only happens sometimes though, and that's what's driving me crazy.

The only perspectives I use are Java and Perforce, and normally I check out a file via Perforce, like so:

Once checked out, it looks like this:

But I can't edit this file. Meanwhile, you see the file directly underneath, "VWAPParameterManagerImpl.java"? I can edit that one just fine, checking it out using the same steps. Furthermore I was able to edit "VWAP.java" just yesterday@ I have also witnessed this happening with non-java files. In Windows Explorer, I checked the file's attributes and verifed it wasn't read-only (if that means anything; I didn't know what else to check).

So, why can't I edit files in Eclipse sometimes, and what can I do make such files writable?

Obviously I can open the file up in gvim or another program, but I'd like to edit within Eclipse. I'm primarily a C++ developer and accustomed to a UNIX environment, so this Java/Windows thing is really tripping me up. Thanks in advance to anyone who might be able to advise...

This is file permissions issue. Perforce is somehow messing with that.

Unlock the file from File Properties and then give Write permissions to yourself.