leftComponent.addControlListener(new ControlAdapter() {
	public void controlResized(ControlEvent event) {
		if (leftComponent.getBounds().width < 200) {
			int totalWidth = sashForm.getClientArea().width;
			sashForm.setWeights(new int[] {200, totalWidth - 200});
		}
	}
});

public class SashExample {
	
	public static void main(String[] args) {
		
		final int MIN_SIZE = 150;
		
		Display display = new Display();
		Shell shell = new Shell(display);
		
		shell.setLayout(new FillLayout());
		
		final SashForm sashForm = new SashForm(shell, SWT.NONE);
		
		final Composite left = new Composite(sashForm, SWT.BORDER);
		final Composite right = new Composite(sashForm, SWT.BORDER);
		
		left.setLayout(new RowLayout());
		right.setLayout(new RowLayout());
		
		new Label(left, SWT.NONE).setText("Left side");
		new Label(right, SWT.NONE).setText("Right side");
		
		left.addControlListener(new ControlAdapter() {
			public void controlResized(ControlEvent event) {
				if (left.getBounds().width < MIN_SIZE) {
					int totalWidth = sashForm.getClientArea().width;
					sashForm.setWeights(new int[] {MIN_SIZE, totalWidth - MIN_SIZE});
				}
			}
		});
		
		sashForm.setWeights(new int [] {1, 3});
		
		shell.setSize(1000, 600);
		shell.open();
		
		while (!shell.isDisposed()) {
			if (!display.readAndDispatch()) {
				display.sleep();
			}
		}
		
		display.dispose();
		
	}

}