why i cannot checkout another git branch?

Question

$ git branch -a
* master
  remotes/origin/HEAD -> origin/master
  remotes/origin/lab_master
  remotes/origin/master

$ git checkout lab_master
error: Your local changes to the following files would be overwritten by checkou                                                                                                                           t:
        **project.properties**
Please, commit your changes or stash them before you can switch branches.
Aborting

why i just failed to checkout lab_master branch?

another question: why i cannot compare current file with the file in another branch?

$ git diff project.properties -b lab_master
fatal: bad flag '-b' used after filename

Answer 1

Git protects you from switching to another branch, because that would override some changes you applied to the file project.properties. You can either throw the changes away by using git checkout -f lab_master or stash them first via git stash (and git stash pop after you checked out the other branch.) If you are sure, you want to keep the changes, you can also simply commit them.

Answer 2

You need to either commit your changes or stash them, http://git-scm.com/book/en/Git-Tools-Stashing