Explicit Java Rounded Error

I am working on a GUI in Java and I am trying to give rounded corners to my borders.

JPanel recentProjectsPanel = new JPanel();
    recentProjectsPanel.setLayout(new BorderLayout());
    recentProjectsPanel.setBackground(Color.red);
    recentProjectsPanel.setBorder(new LineBorder(Color.green, 15, true));

This is the result that I get (ignore the colors):

Why am I getting these red stripes in the corners? Any way to fix this?