Layout Managers

Layout managers are provided to arrange GUI components in a container for presentation purposes. Programmers can use the layout managers for basic layout capabilities instead of determining the exact position and size of every GUI component. This functionality enables the programmer to concentrate on the basic look-and-feel and lets the layout managers process most of the layout details. All layout managers implement the interface LayoutManager (in package java.awt). Class Container's setLayout method takes an object that implements the LayoutManager interface as an argument. There are basically three ways for you to arrange components in a GUI:

  1. Absolute positioning: This provides the greatest level of control over a GUI's appearance. By setting a Container's layout to null, you can specify the absolute position of each GUI component with respect to the upper-left corner of the Container. If you do this, you also must specify each GUI component's size. Programming a GUI with absolute positioning can be tedious unless you have an integrated development environment (IDE) that can generate the code for you.
  2. Layout managers: Using layout managers to position elements can be simpler and faster than creating a GUI with absolute positioning, but you lose some control over the size and the precise positioning of GUI components.
  3. Visual programming in an IDE: IDEs provide tools that make it easy to create GUIs. Each IDE typically provides a GUI design tool that allows you to drag and drop GUI components from a tool box onto a design area. You can then position, size and align GUI components as you like. The IDE generates the Java code that creates the GUI. In addition, you can typically add event-handling code for a particular component by double-clicking the component. Some design tools also allow you to use the layout managers described in this chapter and in Chapter 22.

Look-and-Feel Observation 11.16

Most Java programming environments provide GUI design tools that help a programmer graphically design a GUI; the design tools then write the Java code to create the GUI. Such tools often provide greater control over the size, position and alignment of GUI components than do the built-in layout managers.

Look-and-Feel Observation 11.17

It is possible to set a Container's layout to null, which indicates that no layout manager should be used. In a Container without a layout manager, the programmer must position and size the components in the given container and take care that, on resize events, all components are repositioned as necessary. A component's resize events can be processed by a ComponentListener.

Figure 11.38 summarizes the layout managers presented in this chapter. Other layout managers are discussed in Chapter 22.

Figure 11.38. Layout managers.

(This item is displayed on page 568 in the print version)

Layout manager

Description

FlowLayout

Default for javax.swing.JPanel. Places components sequentially (left to right) in the order they were added. It is also possible to specify the order of the components by using the Container method add, which takes a Component and an integer index position as arguments.

BorderLayout

Default for JFrames (and other windows). Arranges the components into five areas: NORTH, SOUTH, EAST, WEST and CENTER.

GridLayout

Arranges the components into rows and columns.

 

11.17.1. FlowLayout

FlowLayout is the simplest layout manager. GUI components are placed on a container from left to right in the order in which they are added to the container. When the edge of the container is reached, components continue to display on the next line. Class FlowLayout allows GUI components to be left aligned, centered (the default) and right aligned.

The application of Fig. 11.39 and Fig. 11.40 creates three JButton objects and adds them to the application, using a FlowLayout layout manager. The components are center aligned by default. When the user clicks Left, the alignment for the layout manager is changed to a left-aligned FlowLayout. When the user clicks Right, the alignment for the layout manager is changed to a right-aligned FlowLayout. When the user clicks Center, the alignment for the layout manager is changed to a center-aligned FlowLayout. Each button has its own event handler that is declared with an inner class that implements ActionListener. The sample output windows show each of the FlowLayout alignments. Also, the last sample output window shows the centered alignment after the window has been resized to a smaller width. Notice that the button Right flows onto a new line.

Figure 11.39. FlowLayout allows components to flow over multiple lines.

(This item is displayed on pages 568 - 570 in the print version)

 1 // Fig. 11.39: FlowLayoutFrame.java
 2 // Demonstrating FlowLayout alignments.
 3 import java.awt.FlowLayout;
 4 import java.awt.Container;
 5 import java.awt.event.ActionListener;
 6 import java.awt.event.ActionEvent;
 7 import javax.swing.JFrame;
 8 import javax.swing.JButton;
 9
10 public class FlowLayoutFrame extends JFrame
11 {
12 private JButton leftJButton; // button to set alignment left
13 private JButton centerJButton; // button to set alignment center
14 private JButton rightJButton; // button to set alignment right
15 private FlowLayout layout; // layout object
16 private Container container; // container to set layout
17
18 // set up GUI and register button listeners
19 public FlowLayoutFrame()
20 {
21 super( "FlowLayout Demo" );
22
23 layout = new FlowLayout(); // create FlowLayout
24 container = getContentPane(); // get container to layout
25 setLayout( layout ); // set frame layout
26
27 // set up leftJButton and register listener
28 leftJButton = new JButton( "Left" ); // create Left button
29 add( leftJButton ); // add Left button to frame
30 leftJButton.addActionListener(
31
32 new ActionListener() // anonymous inner class
33 {
34 // process leftJButton event
35 public void actionPerformed( ActionEvent event )
36 {
37 layout.setAlignment( FlowLayout.LEFT );
38
39 // realign attached components
40 layout.layoutContainer( container );
41 } // end method actionPerformed
42 } // end anonymous inner class
43 ); // end call to addActionListener
44
45 // set up centerJButton and register listener
46 centerJButton = new JButton( "Center" ); // create Center button
47 add( centerJButton ); // add Center button to frame
48 centerJButton.addActionListener(
49
50 new ActionListener() // anonymous inner class
51 {
52 // process centerJButton event
53 public void actionPerformed( ActionEvent event )
54 {
55 layout.setAlignment( FlowLayout.CENTER );
56
57 // realign attached components
58 layout.layoutContainer( container );
59 } // end method actionPerformed
60 } // end anonymous inner class
61 ); // end call to addActionListener
62
63 // set up rightJButton and register listener
64 rightJButton = new JButton( "Right" ); // create Right button
65 add( rightJButton ); // add Right button to frame
66 rightJButton.addActionListener(
67
68 new ActionListener() // anonymous inner class
69 {
70 // process rightJButton event
71 public void actionPerformed( ActionEvent event )
72 {
73 layout.setAlignment( FlowLayout.RIGHT );
74
75 // realign attached components
76 layout.layoutContainer( container );
77 } // end method actionPerformed
78 } // end anonymous inner class
79 ); // end call to addActionListener
80 } // end FlowLayoutFrame constructor
81 } // end class FlowLayoutFrame

Figure 11.40. Test class for FlowLayoutFrame.

(This item is displayed on page 570 in the print version)

 1 // Fig. 11.40: FlowLayoutDemo.java
 2 // Testing FlowLayoutFrame.
 3 import javax.swing.JFrame;
 4
 5 public class FlowLayoutDemo
 6 {
 7 public static void main( String args[] )
 8 {
 9 FlowLayoutFrame flowLayoutFrame = new FlowLayoutFrame();
10 flowLayoutFrame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
11 flowLayoutFrame.setSize( 300, 75 ); // set frame size
12 flowLayoutFrame.setVisible( true ); // display frame
13 } // end main
14 } // end class FlowLayoutDemo
 

As seen previously, a container's layout is set with method setLayout of class Container. Line 25 sets the layout manager to the FlowLayout declared at line 23. Normally, the layout is set before any GUI components are added to a container.

Look-and-Feel Observation 11.18

Each container can have only one layout manager. Separate containers in the same application can use different layout managers.

Note in this example that each button's event handler is specified with a separate anonymous inner class object (lines 3043, 4861 and 6671, respectively). Each button's actionPerformed event handler executes two statements. For example, line 37 in method actionPerformed for button left uses FlowLayout method setAlignment to change the alignment for the FlowLayout to a left-aligned (FlowLayout.LEFT) FlowLayout. Line 40 uses LayoutManager interface method layoutContainer (which is inherited by all layout managers) to specify that the JFrame should be rearranged based on the adjusted layout. According to which button was clicked, the actionPerformed method for each button sets the FlowLayout's alignment to FlowLayout.LEFT (line 37), FlowLayout.CENTER (line 55) or FlowLayout.RIGHT (line 73).

11.17.2. BorderLayout

The BorderLayout layout manager (the default layout manager for a JFrame) arranges components into five regions: NORTH, SOUTH, EAST, WEST and CENTER. NORTH corresponds to the top of the container. Class BorderLayout extends Object and implements interface LayoutManager2 (a subinterface of LayoutManager that adds several methods for enhanced layout processing).

A BorderLayout limits a Container to containing at most five componentsone in each region. The component placed in each region can be a container to which other components are attached. The components placed in the NORTH and SOUTH regions extend horizontally to the sides of the container and are as tall as the components placed in those regions. The EAST and WEST regions expand vertically between the NORTH and SOUTH regions and are as wide as the components placed in those regions. The component placed in the CENTER region expands to fill all remaining space in the layout (which is the reason the JTextArea in Fig. 11.36 occupies the entire window). If all five regions are occupied, the entire container's space is covered by GUI components. If the NORTH or SOUTH region is not occupied, the GUI components in the EAST, CENTER and WEST regions expand vertically to fill the remaining space. If the EAST or WEST region is not occupied, the GUI component in the CENTER region expands horizontally to fill the remaining space. If the CENTER region is not occupied, the area is left emptythe other GUI components do not expand to fill the remaining space. The application of Fig. 11.41 and Fig. 11.42 demonstrates the BorderLayout layout manager by using five JButtons.

Figure 11.41. BorderLayout containing five buttons.

(This item is displayed on page 572 in the print version)

 1 // Fig. 11.41: BorderLayoutFrame.java
 2 // Demonstrating BorderLayout.
 3 import java.awt.BorderLayout;
 4 import java.awt.event.ActionListener;
 5 import java.awt.event.ActionEvent;
 6 import javax.swing.JFrame;
 7 import javax.swing.JButton;
 8
 9 public class BorderLayoutFrame extends JFrame implements ActionListener
10 {
11 private JButton buttons[]; // array of buttons to hide portions
12 private final String names[] = { "Hide North", "Hide South",
13 "Hide East", "Hide West", "Hide Center" };
14 private BorderLayout layout; // borderlayout object
15
16 // set up GUI and event handling
17 public BorderLayoutFrame()
18 {
19 super( "BorderLayout Demo" );
20
21 layout = new BorderLayout( 5, 5 ); // 5 pixel gaps
22 setLayout( layout ); // set frame layout 
23 buttons = new JButton[ names.length ]; // set size of array
24
25 // create JButtons and register listeners for them
26 for ( int count = 0 ; count < names.length; count++ )
27 {
28 buttons[ count ] = new JButton( names[ count ] );
29 buttons[ count ].addActionListener( this );
30 } // end for
31
32 add( buttons[ 0 ], BorderLayout.NORTH ); // add button to north 
33 add( buttons[ 1 ], BorderLayout.SOUTH ); // add button to south 
34 add( buttons[ 2 ], BorderLayout.EAST ); // add button to east 
35 add( buttons[ 3 ], BorderLayout.WEST ); // add button to west 
36 add( buttons[ 4 ], BorderLayout.CENTER ); // add button to center
37 } // end BorderLayoutFrame constructor
38
39 // handle button events
40 public void actionPerformed( ActionEvent event )
41 {
42 // check event source and layout content pane correspondingly
43 for ( JButton button : buttons )
44 {
45 if ( event.getSource() == button )
46 button.setVisible( false ); // hide button clicked
47 else
48 button.setVisible( true ); // show other buttons
49 } // end for
50
51 layout.layoutContainer( getContentPane() ); // layout content pane
52 } // end method actionPerformed
53 } // end class BorderLayoutFrame

Figure 11.42. Test class for BorderLayoutFrame.

(This item is displayed on page 573 in the print version)

 1 // Fig. 11.42: BorderLayoutDemo.java
 2 // Testing BorderLayoutFrame.
 3 import javax.swing.JFrame;
 4
 5 public class BorderLayoutDemo
 6 {
 7 public static void main( String args[] )
 8 {
 9 BorderLayoutFrame borderLayoutFrame = new BorderLayoutFrame();
10 borderLayoutFrame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
11 borderLayoutFrame.setSize( 300, 200 ); // set frame size
12 borderLayoutFrame.setVisible( true ); // display frame
13 } // end main
14 } // end class BorderLayoutDemo
 

Line 21 of Fig. 11.41 creates a BorderLayout. The constructor arguments specify the number of pixels between components that are arranged horizontally (horizontal gap space) and between components that are arranged vertically (vertical gap space), respectively. The default is one pixel of gap space horizontally and vertically. Line 22 uses method setLayout to set the content pane's layout to layout.

We add Components to a BorderLayout with another version of Container method add that takes two argumentsthe Component to add and the region in which the Component should appear. For example, line 32 specifies that buttons[ 0 ] should appear in the NORTH region. The components can be added in any order, but only one component should be added to each region.

Look-and-Feel Observation 11.19

If no region is specified when adding a Component to a BorderLayout, the layout manager assumes that the Component should be added to region BorderLayout.CENTER.

Common Programming Error 11.6

When more than one component is added to a region in a BorderLayout, only the last component added to that region will be displayed. There is no error that indicates this problem.

Note that class BorderLayoutFrame implements ActionListener directly in this example, so the BorderLayoutFrame will handle the events of the JButtons. For this reason, line 29 passes the this reference to the addActionListener method of each JButton. When the user clicks a particular JButton in the layout, method actionPerformed (lines 4052) executes. The enhanced for statement at lines 4349 uses an if...else to hide the particular JButton that generated the event. Method setVisible (inherited into JButton from class Component) is called with a false argument (line 46) to hide the JButton. If the current JButton in the array is not the one that generated the event, method setVisible is called with a true argument (line 48) to ensure that the JButton is displayed on the screen. Line 51 uses LayoutManager method layoutContainer to recalculate the layout of the content pane. Notice in the screen captures of Fig. 11.41 that certain regions in the BorderLayout change shape as JButtons are hidden and displayed in other regions. Try resizing the application window to see how the various regions resize based on the window's width and height. For more complex layouts, group components in JPanels, each with a separate layout manager. Place the JPanels on the JFrame using either the default BorderLayout or some other layout.

11.17.3. GridLayout

The GridLayout layout manager divides the container into a grid so that components can be placed in rows and columns. Class GridLayout inherits directly from class Object and implements interface LayoutManager. Every Component in a GridLayout has the same width and height. Components are added to a GridLayout starting at the top-left cell of the grid and proceeding left to right until the row is full. Then the process continues left to right on the next row of the grid, and so on. The application of Fig. 11.43 and Fig. 11.44 demonstrates the GridLayout layout manager by using six JButtons.

Figure 11.43. GridLayout containing six buttons.

(This item is displayed on pages 574 - 575 in the print version)

 1 // Fig. 11.43: GridLayoutFrame.java
 2 // Demonstrating GridLayout.
 3 import java.awt.GridLayout;
 4 import java.awt.Container;
 5 import java.awt.event.ActionListener;
 6 import java.awt.event.ActionEvent;
 7 import javax.swing.JFrame;
 8 import javax.swing.JButton;
 9
10 public class GridLayoutFrame extends JFrame implements ActionListener
11 {
12 private JButton buttons[]; // array of buttons
13 private final String names[] =
14 { "one", "two", "three", "four", "five", "six" };
15 private boolean toggle = true; // toggle between two layouts
16 private Container container; // frame container
17 private GridLayout gridLayout1; // first gridlayout 
18 private GridLayout gridLayout2; // second gridlayout
19
20 // no-argument constructor
21 public GridLayoutFrame()
22 {
23 super( "GridLayout Demo" );
24 gridLayout1 = new GridLayout( 2, 3, 5, 5 ); // 2 by 3; gaps of 5
25 gridLayout2 = new GridLayout( 3, 2 ); // 3 by 2; no gaps 
26 container = getContentPane(); // get content pane
27 setLayout( gridLayout1 ); // set JFrame layout
28 buttons = new JButton[ names.length ]; // create array of JButtons
29
30 for ( int count = 0; count < names.length; count++ )
31 {
32 buttons[ count ] = new JButton( names[ count ] );
33 buttons[ count ].addActionListener( this ); // register listener
34 add( buttons[ count ] ); // add button to JFrame
35 } // end for
36 } // end GridLayoutFrame constructor
37
38 // handle button events by toggling between layouts
39 public void actionPerformed( ActionEvent event )
40 {
41 if ( toggle )
42 container.setLayout( gridLayout2 ); // set layout to second
43 else
44 container.setLayout( gridLayout1 ); // set layout to first 
45
46 toggle = !toggle; // set toggle to opposite value
47 container.validate(); // re-layout container
48 } // end method actionPerformed
49 } // end class GridLayoutFrame

Figure 11.44. Test class for GridLayoutFrame.

(This item is displayed on page 575 in the print version)

 1 // Fig. 11.44: GridLayoutDemo.java
 2 // Testing GridLayoutFrame.
 3 import javax.swing.JFrame;
 4
 5 public class GridLayoutDemo
 6 {
 7 public static void main( String args[] )
 8 {
 9 GridLayoutFrame gridLayoutFrame = new GridLayoutFrame();
10 gridLayoutFrame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
11 gridLayoutFrame.setSize( 300, 200 ); // set frame size
12 gridLayoutFrame.setVisible( true ); // display frame
13 } // end main
14 } // end class GridLayoutDemo
 

Lines 2425 create two GridLayout objects (variables grid1 and grid2 are declared at lines 1718). The GridLayout constructor used at line 24 specifies a GridLayout with 2 rows, 3 columns, 5 pixels of horizontal-gap space between Components in the grid and 5 pixels of vertical-gap space between Components in the grid. The GridLayout constructor used at line 25 specifies a GridLayout with 3 rows and 2 columns that uses the default gap space (1 pixel).

The JButton objects in this example initially are arranged using grid1 (set for the content pane at line 27 with method setLayout). The first component is added to the first column of the first row. The next component is added to the second column of the first row, and so on. When a JButton is pressed, method actionPerformed (lines 3948) is called. Every call to actionPerformed toggles the layout between grid2 and grid1, using boolean variable toggle to determine the next layout to set.

Line 47 illustrates another way to reformat a container for which the layout has changed. Container method validate recomputes the container's layout based on the current layout manager for the Container and the current set of displayed GUI components.

Introduction to Computers, the Internet and the World Wide Web

Introduction to Java Applications

Introduction to Classes and Objects

Control Statements: Part I

Control Statements: Part 2

Methods: A Deeper Look

Arrays

Classes and Objects: A Deeper Look

Object-Oriented Programming: Inheritance

Object-Oriented Programming: Polymorphism

GUI Components: Part 1

Graphics and Java 2D™

Exception Handling

Files and Streams

Recursion

Searching and Sorting

Data Structures

Generics

Collections

Introduction to Java Applets

Multimedia: Applets and Applications

GUI Components: Part 2

Multithreading

Networking

Accessing Databases with JDBC

Servlets

JavaServer Pages (JSP)

Formatted Output

Strings, Characters and Regular Expressions

Appendix A. Operator Precedence Chart

Appendix B. ASCII Character Set

Appendix C. Keywords and Reserved Words

Appendix D. Primitive Types

Appendix E. (On CD) Number Systems

Appendix F. (On CD) Unicode®

Appendix G. Using the Java API Documentation

Appendix H. (On CD) Creating Documentation with javadoc

Appendix I. (On CD) Bit Manipulation

Appendix J. (On CD) ATM Case Study Code

Appendix K. (On CD) Labeled break and continue Statements

Appendix L. (On CD) UML 2: Additional Diagram Types

Appendix M. (On CD) Design Patterns

Appendix N. Using the Debugger

Inside Back Cover

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Java(c) How to Program
Java How to Program (6th Edition) (How to Program (Deitel))
ISBN: 0131483986
EAN: 2147483647
Year: 2003
Pages: 615
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