Before you start contributing, please follow the instructions below to setup a code style for an IDE of your choice.
Open the Preferences window, and then navigate to Java -> Code Style -> Formatter.
Click Import and then select the config/eclipse-format.xml file.
Next navigate to Java -> Code Style -> Organize Imports.
Click Import and select the config/src/main/resources/eclipse.importorder file.
Open the Preferences window (or Settings depending on your edition), navigate to Plugins and install the Eclipse Code Formatter Plugin from the Marketplace.
Restart your IDE, open the Preferences (or Settings) window again and navigate to Other Settings -> Eclipse Code Formatter.
Select Use the Eclipse Code Formatter, then change the Eclipse Java Formatter Config File to point to the eclipse-format.xml
file in the config directory.
Make sure the Optimize Imports box is ticked, and select the eclipse.importorder file as the import order config file.
First download the Checkstyle for Java plugin
then right click on config/eclipse-format.xml and Set the *Checkstyle Configuration File*.
Full instruction can be found here