Set the JPanel size to fill parent with little margin

Question

I have two Jpanel (JpanelLeft and JpanelLeftContent) how can i make the JpanelLeftContent fill parent size with a little margin on the lef

Answer 1

..how can i make the JpanelLeftContent look like a rounded rectangle as shown in the picture.

See TextBubbleBorder for a start.

Obviously you'd need to remove the little v at the bottom, & shove the bottom border further down. The code is not comprehensively tested, and will require further tweaks and fixes. 'Batteries not included'.