Strut
Empty space that holds size pixels along orientation and reaches for as much as it is offered across it - the strut of Box.createHorizontalStrut and Box.createVerticalStrut. A horizontal strut holds a width and takes whatever height it is given, a vertical one holds a height and takes whatever width.
In a BoxPanel or a ToolBar laid out along orientation it is a fixed gap between two items, spanning the box across that axis. In one laid out across orientation it holds the box to size along the other axis and takes a share of the leftover extent the way Glue does.
Parameters
orientation
the axis the strut occupies (an Orientation SwingConstants value)
size
the size along orientation in pixels
modifier
the SwingModifier applied to the underlying component