By default Swing uses the native window decorations, and I've tried to make a change in native decoration in different ways but I found out it is a native service, so I referred to another way, it sounds more logical, because if you are going to make change on window decoration, it must been applied in all platforms, so I tried to find a way to set the window decorations the current LookAndFeel (It doesn't work for System LookAnd Feel as I said before)
and write this code before you create or show your main JFrame,
JFrame.setDefaultLookAndFeelDecorated(true)
please read its comment carefully, after that you can do whatever you are looking for by defining your own theme, See the OcceanTheme source code and read some books about
defining a new look and feel.
To remove the default decoration buttons such as Minimize, Maximize and Close buttons, as Babak said, there are some properties in JFrame, JDialog or JInternalFrame, such as setResizable