Building graphical user interfaces - Objects First with Java: A Practical Introduction Using BlueJ - page 404

If we try this out, we can see that we are getting closer, but we still do not have what we
want. Our buttons now are much larger than we intended. The reason is that a container in a
BorderLayout (our toolbar JPanel in this case) always covers its whole area (the WEST area
in our frame). And a GridLayout always resizes its components to fill the whole container.

A FlowLayout does not do this; it is quite happy to leave some empty space around the
components. Our solution is therefore to use both: the GridLayout to arrange the buttons in
a column and a FlowLayout around it to allow some space. We end up with a GridLayout
panel inside a FlowLayout panel inside a BorderLayout . Code 11.15 shows this solution.
Constructions like this are very common. You will often nest various containers inside other
containers to create exactly the look you want.

Code 11.15
Using a nested
GridLayout container
inside a FlowLayout
container

// Create the toolbar with the buttons.
JPanel toolbar = new JPanel();
toolbar.setLayout( new GridLayout(0, 1));

smallerButton = new JButton( "Smaller" );
toolbar.add(smallerButton);

largerButton = new JButton( "Larger" );
toolbar.add(largerButton);

// Add toolbar into panel with flow layout for spacing.
JPanel flow = new JPanel();
flow.add(toolbar);

contentPane.add(flow, BorderLayout.WEST);

Our buttons now look quite close to what we were aiming for. Before adding the finishing
polish, we can first work on making the buttons work.

We need to add two methods, named, for instance, makeLarger and makeSmaller , to do the
actual work, and we need to add action listeners to the buttons that invoke these methods.

Exercise 11.47 In your project, add two method stubs named makeLarger and make
Smaller . Initially, put just a single println statement into these method bodies to see when
they have been called. The methods can be private.

Exercise 11.48 Add action listeners to the two buttons that invoke the two new methods.
Adding action listeners to buttons is identical to adding action listeners to menu items. You
can essentially copy the code pattern from there. Test it. Make sure your makeSmaller and
makeLarger methods get called by activating the buttons.