Screens (composites, controls, shells, ...) are filled/composed by using one of several predefined panes. 
Panes are created by using the central pane factory class.

The following coding creates the screen shown below. 

void createShellContents(Shell parent) {
	IGridLayoutPane gridPane = PaneFactory.createGridLayoutPane(parent, 2);

		Button b1 =
			gridPane.addPushButton(
				" <------ Long (b1) ------> ");

		Button b2 =
			gridPane.addPushButton(
				"Right side (b2)",
				gridPane.createGridLayoutData(
					IGridLayoutData.FILL, 
					IGridLayoutData.FILL,
					1, 2));

		Button b3 =
			gridPane.addPushButton(
				"Centered (b3)",
				gridPane.createGridLayoutData(
					IGridLayoutData.CENTER, 
					IGridLayoutData.CENTER,
					true, true));
}

Technical details

Pane interface structure

Pane class hierarchy