Implementing GUIs in Java

● The Java Foundation Classes (JFC) are a set of packages encompassing the following APIs:– Abstract Window Toolkit (AWT): native GUI

components– Swing: lightweight GUI components– 2D: rendering two-dimensional shapes, text, and

images– Accessibility: allowing compatibility with, for

example, screen readers and screen magnifiers

Abstract Window Toolkit (AWT)

● Provides basic UI components:– Buttons, lists, menus, textfields, etc– Event handling mechanism– Clipboard and data transfer– Image manipulation– Font manipulation– Graphics

● Platform independence is achieved through peers, or native GUI components

AWT Packages

java.awt Basic component functionalityjava.awt.accessibility Assistive technologiesjava.awt.color Colors and color spacesjava.awt.datatransfer Clipboard and data transfer supportjava.awt.dnd Drag and dropjava.awt.event Event classes and listenersjava.awt.font 2D API font packagejava.awt.geom 2D API geometry packagejava.awt.im Input methodsjava.awt.image Fundamental image manipulation classesjava.awt.peer Peer interfaces for component peersjava.awt.print 2D API support for printingjava.awt.swing Swing components

Peers and Platform Independence

● The first AWT (Java 1.0) was rolled out in an incredible 6 weeks using peers

● Thus an AWT menu on the Solaris platform, for example, actually creates a Motif menu object as its peer

● UI components that have peers are called heavyweight because– they are rendered in their own (opaque) windows and

thus are expensive to use, – they must be rectangular and cannot have transparent

backgrounds, and– they are not amenable to being subclassed

Using Peers

Java Program

Java AWT

NativeWindowSystemPeers

A Java program creates and displays an AWT component,which creates and displays a native component, or peer.

Lightweight Components● AWT 1.1 introduced the notion of lightweight

components which:– are contained within a heavyweight component's

window– do not have peers– are rendered in their container's window rather than

one of their own– do not incur performance penalties and can have

transparent backgrounds● Almost all Swing components are lightweight

ones that extend either java.awt.Component or java.awt.Container

Some AWT Components

Object

Component

Container ButtonList

Scrollbar

Label

Canvas

JComponent

AWT vs. Swing● Swing does not replace the AWT; it is built on

top of it● All 1.0 AWT components are heavyweight;

corresponding Swing components are lightweight● Swing component names begin with ``J'':

– Component (AWT) vs. JComponent (Swing)– Button (AWT) vs. JButton (Swing)

● Always use Swing components; however, since Swing is built on top of AWT, you will need to know some AWT methods

Some Swing Components

JComponent

AbstractButton

JButtonJMenuItem

JToggleButton

JCheckBox

JLabel

JList

JScrollBar

JFileChooser

JComponents

● Note that JComponents are containers● JComponents do not extend their AWT

counterparts:– For example, the JButton class is not a subclass

(direct or indirect) of Button ● However, some Swing components are not JComponents– For example, some Swing containers are direct

subclasses of their AWT counterparts

Some AWT Containers

Container

JComponent Panel

ScrollPane

Window

Dialog Frame

Applet

Swing Components That Are Not JComponents (in red)

Container

JComponent Panel

ScrollPaneWindow

Dialog Frame Applet

JDialog

JWindow

JFrame JApplet

Some More Swing Components That Are JComponents

JComponent

JLayeredPane JPanel

JScrollPane

JInternalFrame

JDesktopPane

JTable

JTree

Some AWT Component Methods

● void setBackground(Color c)● void setForeground(Color c)● void setEnabled(boolean b)● void setVisible(boolean b)● void setFont(Font f)● void setSize(Dimension d)● void setLocation(int x, int y)

All but setSize and setLocation are overridden by the JComponent class.

Example: A Simple Framed Window

import java.awt.*;import javax.swing.*;

public class SwingTest {

public static void main(String[] args) { JFrame frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); frame.setVisible(true); }

}

Notes on the Example

● setSize and setLocation require java.awt.*; the rest require javax.swing.*

● The JFrame constructor argument is used as a title● The Dimension constructor takes an integer width

and height, respectively● The setLocation method takes a pair of integer

coordinates (x,y) where (0,0) is the upper left corner of the display

● The visibility of a JFrame is set to false by default

Example Output Display

● This window was managed by the K Desktop Environment (KDE)

● Clicking the Close button (X) will cause the display to be hidden, but the program will continue since no listeners are set up yet

● Can use ctl-C to kill the Java Virtual Machine

Adding Color

– Color.black– Color.blue– Color.cyan– Color.darkGray– Color.gray– Color.green– Color.lightGray

– Color.magenta– Color.orange– Color.pink– Color.red– Color.white– Color.yellow

The java.awt.Color class has the following staticfields (data members):

Changing Background Color

import java.awt.*;import javax.swing.*;

public class SwingTest {

public static void main(String[] args) { JFrame frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); Container contentPane = frame.getContentPane(); contentPane.setBackground(Color.red); frame.setVisible(true); }

}

Content Panes● Q: Why not just: frame.setBackground(Color.red); ?● A: In order to be lightweight, Swing's top-level

window objects must be built on top of a lightweight AWT Container object introduced in version 1.1

● This container is called a content pane● Swing top-level window classes:

– JWindow– JFrame– JApplet– JDialog– JInternalFrame

Adding a Label and Button

import java.awt.*;import javax.swing.*;

public class SwingTest {

public static void main(String[] args) { JFrame frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); Container contentPane = frame.getContentPane(); JLabel label = new JLabel("HERE IS A LABEL"); contentPane.add(label, BorderLayout.NORTH); JButton button = new JButton("BUTTON"); contentPane.add(button, BorderLayout.SOUTH); frame.setVisible(true); }

}

New Display

Resized

Notes on the Code

● Since the frame is a top-level Swing window, components must be added to its content pane

● When components are added to a container, how they are placed is dependent upon the container's layout manager

● The default layout manager for a JFrame is a BorderLayout manager (described later)

● When adding to a container whose layout manager is BorderLayout, the second parameter should be a location defined in the BorderLayout class

Adding a List of Optionsimport java.awt.*;import javax.swing.*;

public class SwingTest {

public static void main(String[] args) { JFrame frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); Container contentPane = frame.getContentPane(); JLabel label = new JLabel("HERE IS A LABEL"); contentPane.add(label, BorderLayout.NORTH); JButton button = new JButton("BUTTON"); contentPane.add(button, BorderLayout.SOUTH); String[] options = {"Option 1", "Option 2", "Option 3"}; JList list = new JList(options); contentPane.add(list, BorderLayout.CENTER); frame.setVisible(true); }}

New Display

Note that "Option 3" has been selected.

Adding a Check Box and Slider

public class SwingTest {

public static void main(String[] args) { JFrame frame = new JFrame("Test Frame"); frame.setSize(new Dimension(400,200)); frame.setLocation(100,100); Container contentPane = frame.getContentPane(); JLabel label = new JLabel("HERE IS A LABEL"); contentPane.add(label, BorderLayout.NORTH); JButton button = new JButton("BUTTON"); contentPane.add(button, BorderLayout.SOUTH);

String[] options = {"Option 1", "Option 2", "Option 3"}; JList list = new JList(options); contentPane.add(list, BorderLayout.CENTER);

JCheckBox cbox = new JCheckBox("Check"); contentPane.add(cbox, BorderLayout.WEST);

JSlider slider = new JSlider(); contentPane.add(slider, BorderLayout.EAST);

frame.setVisible(true); }}

New Display

Layout Management

● A layout manager determines the location and size of components placed into a container

● Different layout managers use different algorithms for determining size and location:– BorderLayout: places at compass locations and center– FlowLayout: places components in rows, left to right– GridLayout: places in rectangular grid– BoxLayout: places in a single row or column

Changing the Layout

public class SwingTest {

public static void main(String[] args) { JFrame frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); Container contentPane = frame.getContentPane();

contentPane.setLayout(new FlowLayout()); JLabel label = new JLabel("HERE IS A LABEL"); JButton button = new JButton("BUTTON"); String[] options = {"Option 1", "Option 2", "Option 3"}; JList list = new JList(options); JCheckBox cbox = new JCheckBox("Check"); JSlider slider = new JSlider(); contentPane.add(label); contentPane.add(button); contentPane.add(list); contentPane.add(cbox); contentPane.add(slider);

frame.setVisible(true); }

}

New Display

Resized

Default Layout Managers

● The default layout manager for content panes is BorderLayout. Recall that the following Swing components have content panes:– JWindow– JFrame– JDialog– JApplet– JInternalFrame

● The other Swing container is the JPanel, whose default layout manager is FlowLayout.

JPanels

● A JPanel object can be used for grouping components into a container, which can then be added to another container

● The JPanel constructor with no arguments creates a panel with a FlowLayout manager

● Another JPanel constructor takes any layout manager as an argument

● A JPanel can also be used a a blank area for drawing custom graphics

JPanel Example

JFrame frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); Container contentPane = frame.getContentPane();

JLabel label = new JLabel("HERE ARE SOME BUTTONS", SwingConstants.CENTER);

JButton button1 = new JButton("BUTTON1"); JButton button2 = new JButton("BUTTON2"); JButton button3 = new JButton("BUTTON3"); JPanel panel = new JPanel(); panel.add(button1); panel.add(button2); panel.add(button3);

contentPane.add(label, BorderLayout.NORTH); contentPane.add(panel, BorderLayout.CENTER);

frame.setVisible(true);

JPanel Example Output

Note use of SwingConstants.CENTER argumentin JLabel constructor.

Changing JPanel Layout

JFrame frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); Container contentPane = frame.getContentPane();

JLabel label = new JLabel("HERE ARE SOME BUTTONS", SwingConstants.CENTER);

JButton button1 = new JButton("BUTTON1"); JButton button2 = new JButton("BUTTON2"); JButton button3 = new JButton("BUTTON3"); JPanel panel = new JPanel(); panel.setLayout (new BoxLayout(panel, BoxLayout.Y_AXIS)); panel.add(button1); panel.add(button2); panel.add(button3);

contentPane.add(label, BorderLayout.NORTH); contentPane.add(panel, BorderLayout.CENTER);

frame.setVisible(true);

New Output

● The button panel is to the west because no other component was placed there

● The BoxLayout constructor requires both the component being laid out and either:– BoxLayout.X_AXIS– BoxLayout.Y_AXIS

Tweaking Layouts

● Some layout constructors allow hgap and vgap, integers specifying the number of pixels separating components horizontally and vertically

● FlowLayout allows the specification of whether the line of components should be left-justified, right-justified, or centered

new FlowLayout(int align)new FlowLayout(int align, int hgap, int vgap)

new BorderLayout(int hgap, int vgap)

new GridLayout(int rows, int cols)new GridLayout(int rows, int cols, int hgap, int vgap)

Tweaking Example JFrame frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); Container contentPane = frame.getContentPane(); LayoutManager lm = contentPane.getLayout(); ((BorderLayout)lm).setHgap(25);

JLabel label = new JLabel("HERE ARE SOME BUTTONS", SwingConstants.CENTER);

JButton button1 = new JButton("BUTTON1"); JButton button2 = new JButton("BUTTON2"); JButton button3 = new JButton("BUTTON3"); JPanel panel = new JPanel(); panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS)); panel.add(button1); panel.add(button2); panel.add(button3);

contentPane.add(label, BorderLayout.NORTH); contentPane.add(panel, BorderLayout.CENTER);

frame.setVisible(true);

Tweaking Example Output

● The LayoutManager returned by getLayout() is an interface type that the BorderLayout class implements

● The setHgap method we want is in the BorderLayout class

● So we must cast the LayoutManager to BorderLayout in order to use setHgap

Sizing Hints● Layout managers often need to resize their

components to make things fit– For example, the widths and heights of components in

a BoxLayout are adjusted according to both preferred and maximum heights and widths

● If you don't like the size of the components a layout manager comes up with, you may have to give sizing hints using the following methods from the JComponent class:– void setMinimumSize(Dimension d)– void setPreferredSize(Dimension d)– void setMaximumSize(Dimension d)