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Using Swing Components
How to Use Separators
The
JSeparator class provides a horizontal or vertical dividing line
or empty space.
It's most commonly used in menus and tool bars.
In fact, you can use separators
without even knowing that a JSeparator class exists,
since menus
and tool bars
provide convenience methods
that create and add separators
customized for their containers.
Separators are somewhat similar to
borders, except that they are genuine components
and, as such,
are drawn inside a container,
rather than around the edges of a particular component.
Here is a picture of a menu that has three separators,
used to divide the menu into four groups of items:
The code to add the menu items and separators to the menu
is extremely simple,
boiling down to something like this:
menu.add(menuItem1);
menu.add(menuItem2);
menu.add(menuItem3);
menu.addSeparator();
menu.add(rbMenuItem1);
menu.add(rbMenuItem2);
menu.addSeparator();
menu.add(cbMenuItem1);
menu.add(cbMenuItem2);
menu.addSeparator();
menu.add(submenu);
Adding separators to a tool bar is similar.
You can find the full code explained in the
how-to sections for
menus
and tool bars.
If you want more control over separators in menus and tool bars,
you can directly use the
JSeparator subclasses that implement them:
JPopupMenu.Separator and
JToolBar.Separator.
In particular, JToolBar.Separator
has API for specifying the separator's size.
You can use the JSeparator class directly
to provide a dividing line in any container.
The following picture shows a GUI
that has a separator to the right of the button
labeled Fire.
Separators have almost no API and are extremely easy to use
as long as you keep one thing in mind:
In most implementations,
a vertical separator has a preferred height of 0,
and a horizontal separator has a preferred width of 0.
This means a separator
is not visible
unless you either set its preferred size
or put it in under the control of a layout manager
such as BorderLayout or BoxLayout
that stretches it to fill its available display area.
The vertical separator does have a bit of width
(and the horizontal a bit of height),
so you should see some space where the separator is.
However, the actual dividing line isn't drawn
unless the width and height are both non-zero.
The following code snippet
shows how ListDemo puts together
the panel that contains the vertical separator.
You can find the full source code for ListDemo in
ListDemo.java.
JPanel buttonPane = new JPanel();
buttonPane.setLayout(new BoxLayout(buttonPane,
BoxLayout.LINE_AXIS));
buttonPane.add(fireButton);
buttonPane.add(Box.createHorizontalStrut(5));
buttonPane.add(new JSeparator(SwingConstants.VERTICAL));
buttonPane.add(Box.createHorizontalStrut(5));
buttonPane.add(employeeName);
buttonPane.add(hireButton);
buttonPane.setBorder(BorderFactory.createEmptyBorder(5,5,5,5));
As the code shows,
the buttons, separator, and text field
all share a single container —
a JPanel instance
that uses a left-to-right
box layout.
Thanks to the layout manager
(and to the fact that separators have unlimited maximum sizes),
the separator is automatically made as tall
as its available display area.
In the preceding code,
the horizontal struts are
invisible components used to provide space
around the separator.
A 5-pixel empty border
provides a cushion around the panel,
and also serves to prevent the separator from extending
all the way to the component above it
and the window's edge below it.
Here's a picture of another GUI
that uses a separator,
this time to put a dividing line
between a group of controls
and a display area.
You can find the code in the
example index.
Here is the code that sets up the separator's container:
JPanel panel = new JPanel(new BorderLayout());
...
panel.setBorder(BorderFactory.createEmptyBorder(
GAP/2, //top
0, //left
GAP/2, //bottom
0)); //right
panel.add(new JSeparator(JSeparator.VERTICAL),
BorderLayout.LINE_START);
panel.add(addressDisplay,
BorderLayout.CENTER);
As in the last example, the panel uses an empty border
so that the separator doesn't
extend all the way to the edges of its container.
Placing the separator in the leftmost area of the
BorderLayout-controlled container
makes the separator as tall as the
address-display component
that's in the center of the container.
See
How to Use BorderLayout for details on how border layouts work.
The API for using separators is minimal,
since they have no contents
and don't respond to user input.
Several of this lesson's examples use separators,
usually in menus.
Here is a list of some of the more interesting examples.
