The content pane.
One method for ensuring that the graphical interface looks they way you intend is to prevent the user from resizing it. This can be done with the frame's method:
public void setResizable(boolean resizable)
If the parameter is false then the user can not resize the frame. But this does not solve all problems. When a frame contains several logical grouping of components it is usually difficult to lay them out correctly using a single layout manager. You would like to group related components together, and then to lay out the groups in the content pane. This is what panels are for.