Input with a TextField

Context

You're writing a program with a graphical user interface. You need to get a short piece of textual input from the user. Use a JTextField graphical user interface widget.

Example

When the user types text in the JTextField (the top box) followed by pressing the Enter key, the text is appended to the JTextArea (the bottom box). The JTextArea is set so the user may not edit it. In a Java-enabled browser the actual applet should appear on the left; an image of the applet on the right. This example is illustrated by the following applet.

Applet	Image

package Patterns.TextField;
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.JTextArea;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class TextFieldEx extends JFrame
{  // Instantiate a textfield for input and a textarea for output.
   private JTextField input = new JTextField(15);
   private JTextArea output = new JTextArea(5, 15);

   public TextFieldEx()
   {  // Register a listener with the textfield
      TextFieldListener tfListener = new TextFieldListener();
      input.addActionListener(tfListener);

      // Don't let the user change the output.
      output.setEditable(false);

      // Add all the widgets to the applet
      this.getContentPane().add(input);
      this.getContentPane().add(output);
      input.requestFocus();        // start with focus on this field
   }

   // The listener for the textfield.
   private class TextFieldListener implements ActionListener
   {  public void actionPerformed(ActionEvent evt)
      {  String inputString = input.getText();
         output.append(inputString + "\n");
         input.setText("");
      }
   }
}

Pattern

package <name-of-package>;
import javax.swing.JFrame;
import javax.swing.TextField;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class <name-of-class> extends JFrame
{  private JTextField <textfieldname> = new JTextField(<optional-size>);

   public void init()
   {  <textfieldname>.addActionListener(new <listenerclassname>());
      this.getContentPane().add(<textfieldname>);
   }
   
   private class <listenerclassname> implements ActionListener
   {  public void actionPerformed(ActionEvent evt)
      {  <code-to-execute-when-text-is-entered-in-text-field>
      }
   }
}

Notes

• If more than one line of text is required, use a JTextArea. If the user is selecting one item from a fixed list, then a drop-down menu (Choice) or a JList is usually a better interface.
• There are a number of attributes that can be set on the text field, including the colour of the text (setForeground(Color c)), the background colour (setBackground(Color c)), and the font (setFont(Font f)).
• Textfields may be constructed with a given number of columns, although it doesn't seem to actually bear a relationship to the current font. It can also be constructed with a beginning string.
• The program may set a string to be shown in the text field. Similar to the text area shown in the example, a text field may be set so that it is not editable by the user.
• A text field may request the focus with the method requestFocus(). This directs the next input to be in that particular text field.

Related Patterns

• Input with a JButton
• Output with a JTextArea is used in the example.
• Summary of Listeners, for a summary of many standard gui widgets and the listeners they use.