Java Reference
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Self-Test Exercises (continued)
16. How would you modify the class definition in Display 17.7 so that the three
labels are displayed as follows?
First label
Second label
Third label
(There may be space between each pair of lines.)
17. Suppose you are defining a windowing GUI class in the usual way, as a derived
class of the class JFrame , and suppose you want to specify a layout manager for
the JFrame so as to produce the following sort of layout (that is, a one-row lay-
out, typically having three columns):
What should the argument to setLayout be?
18. Suppose the situation is as described in Self-Test Exercise 17, except that you
want the following sort of layout (that is, a one-column layout, typically having
three rows):
What should the argument to setLayout be?
Panels
A GUI is often organized in a hierarchical fashion, with window-like containers,
known as panels, inside of other window-like containers. A panel is an object of the
class JPanel , which is a very simple container class that does little more than group
objects. It is one of the simplest container classes, but an extremely useful one. A
JPanel object is analogous to the braces used to combine a number of simpler Java
statements into a single larger Java statement. It groups smaller objects, such as but-
tons and labels, into a larger component (the JPanel ). You can then put the JPanel
object in a JFrame . Thus, one of the main functions of JPanel objects is to subdivide a
JFrame (or other container) into different areas.
For example, when you use a BorderLayout manager, you can place components
in each of the five locations BorderLayout.NORTH , BorderLayout.SOUTH , Border-
Layout.EAST , BorderLayout.WEST , and BorderLayout.CENTER . But what if you want to
put two components at the bottom of the screen in the BorderLayout.SOUTH position?
panel
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