code formating with checkstyle

[hugy718]

Use checkstyle to enforce a consistent code formatting.

• inconsistent code formatting between legacy and new codes, and between Intellij users and vscode users.
• Developers are enticed to change formatting, for example indentations, to remove IDE warning or simply because of different coding habits. But, the bad thing is that, during code review, formatting changes confuse the Github diff highlighter and obscure what needs to be reviewed.
• So my plan is:
  • We provide an checkstyle xml file (I intend to use the google_check.xml in this repo, so all developers uses one consistent settings to format their codes.
  • One time effort to remove all checkstyle detected problems on formatting, naming, etc. And all of us uses that as the new base of development and fix all checkstyle warning before submitting new PRs.

[Zrealshadow]

I use the eclipse-formatter.xml which is the default formatter in vscode. If we set a unified formatter file, we can create a github CI to check the new PR's code format automatically

I agree with that we should fix all warning before submitting new PR.