This is a smoothing issue when rendering a Swing-based GUI. Anti-aliasing is enabled by default using the Oracle JDK, and for some reason (which I never wanted to explore) anti-aliasing was not included in OpenJDK.
You can enable anti-aliasing support using the current OpenJDK installation by adding this instruction to your ~/.bash_profile:
export _JAVA_OPTIONS="-Dawt.useSystemAAFontSettings=on"
Good luck