Java Tutorial/SWT/Custom Layout
17. Creating Your Own Layouts
A layout must extend the org.eclipse.swt.widgets.Layout class. org.eclipse.swt.widgets.Layout has only two abstract methods:
The computeSize method computes and returns the minimum size of the specified composite"s client area according to this layout.
protected abstract Point computeSize(Composite composite,int wHint,int hHint, boolean flushCache)