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JButton Class

A button is a component used to initiate some action. There are three different ways a user can interact with a button:

  1. Click on the button with the left mouse button.
  2. Type the keyboard shortcut.
  3. Tap the spacebar when the button has the focus.

By default, a button's size will be determined by the length of the button text which which will be centered within the button:

Default Button

If a button is stretched horizontally, the button text remains centered:

Stretched Button

Import

The JButton class is imported by the statement:

import javax.swing.*;

Constructor

JButton(String text)

The text parameter is a string variable or a literal string (enclosed by quotation marks) and specifies the text that is to appear on the button. This text should indicate the action that will be performed when the button is clicked.

Selected Methods

addActionListener(this)

Register this application to listen for action events generated by the JButton that invokes this method.

setMnemonic(int mnemonic)

Set the keyboard shortcut. Entering the keyboard shortcut has the same effect as clicking the button. On most systems, the shortcut involves pressing the Alt key in combination with a letter of the alphabet (e.g., Alt-S means to hold down the Alt key while pressing the letter S). The parameter to this method should be a virtual key constant defined in the KeyEvent class (such as VK_A, VK_B, VK_C, and so on). The KeyEvent class is part of the java.awt.event package.

The letter should be part of the button label text (often the first letter). This letter will be underlined when the button label is displayed. The keyboard shortcut is not case sensitive so if the the keyboard shortcut is Alt-S, the user can press either a lowercase "s" or an uppercase "S" while holding the Alt key down.

setToolTipText(String text)

Set the tool tip text. The tool tip text is displayed when the user moves the mouse cursor over the button but does not click it. The tool tip text should briefly describe the purpose of the button.

Responding to User Interaction

To respond when a user clicks on a button (or taps the spacebar when the button has the focus),

  1. The applet must implement the ActionListener interface.
  2. The button must invoke its addActionListener(this) method.
  3. The desired action must be coded in the actionPerformed method of the ActionListener interface.

Once the applet has been set up to respond to a button click, only one more step is needed to allow it to respond to a keyboard shortcut. The button must invoke its setMnemonic method.

The Tab key can be used to move the focus from one component to the next. Shift-Tab can be used to move the focus from one component to the previous component. By default, the tab sequence is determined by the order in which the components are added to the interface.

Example

The Java code below illustrates how to set up a button to exit the application. The keyboard short-cut for this button is Alt-X.

JButton cmdExit;
cmdExit = new JButton("Exit");
cmdExit.setMnemonic(KeyEvent.VK_X);
cmdExit.setToolTipText("Click to exit the program");
cmdExit.addActionListener(this);