Swing I - Absolute Java - page 994

Text Areas and Text Fields
A text field is an object of the class JTextField and is displayed as a field that allows
the user to enter a single line of text. In Display 17.17, the following creates a text field
named name in which the user will be asked to enter his or her name:

private JTextField name;

text field

JTextField

...
name = new JTextField(NUMBER_OF_CHAR);
In Display 17.17 , the variable name is a private instance variable. The creation of the
JTextField in the last of the previous lines takes place inside the class constructor. The
number NUMBER_OF_CHAR that is given as an argument to the JTextField constructor
specifies that the text field will have room for at least NUMBER_OF_CHAR characters to
be visible. The defined constant NUMBER_OF_CHAR is 30 , so the text field is guaranteed
to have room for at least 30 characters. You can type any number of characters into a
text field but only a limited number will be visible; in this case, you know that at least
30 characters will be visible.
A Swing GUI can read the text in a text field and so receive text input; if this is
desired, it can produce output by causing text to appear in the text field. The method
getText returns the text written in the text field. For example, the following will set
a variable named inputString to whatever string is in the text field name at the time
that the getText method is invoked:

String inputString = name.getText();
The method getText is an input method, and the method setText is an output
method. The method setText can be used to display a new text string in a text field.
For example, the following will cause the text field name to change the text it displays
to the string "This is some output" :

name.setText("This is some output");

Display 17.17 A Text Field (part 1 of 3)

1 import javax.swing.JFrame;
2 import javax.swing.JTextField;
3 import javax.swing.JPanel;
4 import javax.swing.JLabel;
5 import javax.swing.JButton;
6 import java.awt.GridLayout;
7 import java.awt.BorderLayout;
8 import java.awt.FlowLayout;
9 import java.awt.Color;
10 import java.awt.event.ActionListener;
11 import java.awt.event.ActionEvent;

12 public class TextFieldDemo extends JFrame
13 implements ActionListener

14 {
15 public static final int WIDTH = 400;

16 public static final int HEIGHT = 200;