Java How to Program Fourth Edition - DCC

Chapter 13 Graphical User Interface Components: Part 2 779
of each component in the grid is instantiated with line 26. Lines 23 through 30 instantiate
each of the GUI components that will be added to the content pane.
JTextArea textArea1 is the first component added to the GridBagLayout
(line 44). The values for weightx and weighty values are not specified in gbConstraints,
so each has the value zero by default. Thus, the JTextArea will not resize
itself even if space is available. However, the JTextArea spans multiple rows, so the vertical
size is subject to the weighty values of JButtons button2 and button3. When
either button2 or button3 is resized vertically based on its weighty value, the
JTextArea is also resized.
Line 43 sets variable fill in constraints to GridBagConstraints.BOTH,
causing the JTextArea to always fill its entire allocated area in the grid. An anchor
value is not specified in constraints, so the default CENTER is used. We do not use
variable anchor in this program, so all components will use the default. Line 36 calls our
utility method addComponent method (defined at lines 81–95). The JTextArea
object, the row, the column, the number of columns to span and the number of rows to span
are passed as arguments.
Method addComponent’s parameters are a Component reference component
and integers row, column, width and height. Lines 85–86 set the GridBagConstraints
variables gridx and gridy. The gridx variable is assigned the column in
which the Component will be placed, and the gridy value is assigned the row in which
the Component will be placed. Lines 89–90 set the GridBagConstraints variables
gridwidth and gridheight. The gridwidth variable specifies the number of columns
the Component will span in the grid and the gridheight variable specifies the
number of rows the Component will span in the grid. Line 93 sets the GridBagConstraints
for a component in the GridBagLayout. Method setConstraints of
class GridBagLayout takes a Component argument and a GridBagConstraints
argument. Method add (line 94) is used to add the component to the content pane.
JButton object button1 is the next component added (lines 48–49). The values of
weightx and weighty are still zero. The fill variable is set to HORIZONTAL—the
component will always fill its area in the horizontal direction. The vertical direction is not
filled. The weighty value is zero, so the button will become taller only if another component
in the same row has a nonzero weighty value. JButton b1 is located at row 0,
column 1. One row and two columns are occupied.
JComboBox comboBox is the next component added (line 54). The weightx and
weighty values are zero, and the fill variable is set to HORIZONTAL. The JComboBox
button will grow only in the horizontal direction. Note that the weightx,
weighty and fill variables remain set in gbConstraints until they are changed. The
JComboBox button is placed at row 2, column 1. One row and two columns are occupied.
JButton object button2 is the next component added (lines 57–60). JButton
button2 is given a weightx value of 1000 and a weighty value of 1. The area occupied
by the button is capable of growing in the vertical and horizontal directions. The fill
variable is set to BOTH, which specifies that the button will always fill the entire area. When
the window is resized, b2 will grow. The button is placed at row 1, column 1. One row and
one column are occupied.
JButton button3 is added next (lines 64–66). Both the weightx value and
weighty value are set to zero, and the value of fill is BOTH. JButton button3 will