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Streib ePub. Daniel June 7, Paperback Download. The window on the right shows the same window after it has been resized by dragging the lowerright corner, resulting in components flowing down onto other lines. Chap-3 Adv Prog. If you narrow it further, no change will happen on positions. Because the top container, the window, really contains only two components: the field panel and the button panel.
You can control alignment and spacing of the components. Complicated layouts can be made by combining many panels, some with horizontal layout and some with vertical layouts. However, the Box class has a number of necessary methods for working with BoxLayouts. Creating Boxes 0 You can create the two kinds of boxes with: import javax.
BoxLayout cont BoxLayout spacing Fillers - rigid areas, glue, struts, and custom Fillers 0 Invisible components can be added to a layout to produce empty space between components. The most useful are rigid areas or struts. Glue may be insert expandable empty space in a layout. Generally use rigid areas instead of struts. Used if you want spacing to increase when a window is resized. Filler is a more general spacing component with specified minimum, preferred, and maximum sizes.
BoxLayout cont BoxLayout spacing cont Rigid Area 0 A rigid area is very is a fixed size filler with vertical and horizontal dimensions. It can be used in either horizontal or vertical layouts. Create a rigid area by specifying its width and height in pixels in a Dimension object, which has two parameters, an x distance and a y distance.
Typically the dimension you are not interested in is set to zero. BoxLayout cont BoxLayout spacing cont Glue p. It's more like a spring or. Put a glue component where extra space should appear disappear from when a window is resized. Use vertical glue in a vertical BoxLayout and horizontal glue in a horizontal layout.
For example, this will allow extra vertical space between two buttons. BoxLayout cont BoxLayout spacing cont Struts p. Create a strut by specifying its size in pixels, and adding it to the panel at the point you want the space between other components.
To avoid problems from nested panels, you are generally better off using a rigid area. BoxLayout cont BoxLayout spacing cont Box. Filler 0 You can create your own filler by using the Box. Filler constructor and specifying the minimum, preferred, and maximum size. Each size must be in a Dimension object, which has width and height.
Filler min, pref, max ; For example, to create a new horizontal filler that can get no smaller that 4 pixels, that prefers to be 16 pixels wide, and that will expand to no more than 32 pixels, you could do this: Box. Filler new Dimension 4,0 , new Dimension 16, 0 , new Dimension 32, 0 ;. BoxLayout alignment 0 Component alignment 0 Alignment is a property of the component, not the layout. Set the X alignment for components in a vertical box and the Y alignment for components in a horizontal box.
Component class. It is almost impossible to align them, because they are in two different panels. Constructors of other layouts do not need containers.
This makes the statement looks very strange: p. Rows and columns are equally divided. To Create a GridLayout 0 There are three constructors: p. Columns expand.
There are 5 pixels of horizontal space between components and 10 pixels of space between vertical components. To add Components to a GridLayout 0 Use the. You do not can not use the row and column to tell where to add the components -- add them in starting at the top left and going across the row first. Empty Cells 0 There is no way to leave a cell empty in a grid layout.
Although it often works ok to leave the final cells empty, there are reports of problems, so you should fill out the last cells in the final row too. The easiest way to do this is to put an empty label in any cell you want to skip.
GridLayout cont import java. RIGHT ; p. But all components are having wrong sizes. And it :. Rows and columns are not equally divided. If you're familiar with HTML tables, you'll feel comfortable with the idea. Unfortunately, GridBagLayout is awkward and error-prone to use. It is very difficult to work with because eleven constraint values are used for each component! These constraint values are bundled in a java.
GridBagConstraints object. Fortunately, most of them have reasonable defaults. The layout doesn't have features which specifically help to use Sun's or anyone else's Human Interface Guidelines, for example in handling gaps.
Spacing is specified in pixels, which provides no flexibility for screen resolution and font changes. Quote from Otaku, Cedric's blog: "GridBagLayout is an absolute disaster and the perfect example of something that is completely flawed and violates with a stunning regularity the principle of least surprise.
If some groups of components eg, radio buttons, groups of buttons, checkboxes are unrelated to alignment with other components, put them in their own panel and use an appropriate layout often GridLayout. Describe this area by giving the row, column, width in columns , and height in rows. GridBagLayout cont Three categories of constraint parameters 0 GridBagLayout performs three functions using values from the GridBagConstraints parameter in the add method.
The names are the following. Grid position, width and height describe the display area.. Position within the display area. Identifying rows and columns which receive extra space on expansion.
They are computed by the layout manager as a consequence of what you add. The add method clones copies the constraint object, so typically the same GridBagConstraints object is reused to pass constraints for the next component.
It is common to use only one GridBagConstraints object to add all components, changing fields as necessary between adds, but this practice also leads to many errors and code which is difficult to change. Necessary - row and column coordinates, width and height in terms of rows and columns. Describe the location and size in rows and columns of the component using gridx, gridy, gridwidth, and gridheight. Common - expandability and alignment. If a column or row can use extra space when a window is expanded, set weightx and weighty to 1.
Use fill depending on what the type of component is. For example, a JTextField can usually grow horizontally, so you would use gbc. Use anchor to tell which edges of the display area a component should be attached to. Less common - surrounding spacing. You can also control the space around a component with ipadx and ipady. To control the amount of unused space around a component, use insets.
Use GridBagConstraints. RELATIVE the default value to specify that the component be placed just to the right of for gridx or just below for gridy the component that was added to the container just before this component was added. We recommend specifying the gridx and gridy values for each component rather than just using GridBagConstraints. These constraints specify the number of cells the component uses, not the number of pixels it uses. The default value is 1. We recommend specifying the gridwidth and gridheight values for each component rather than just using GridBagConstraints.
Note: GridBagLayout does not allow components to span multiple rows unless the component is in the leftmost column or you have specified positive gridx and gridy values for the component. GridBagLayout cont GridBagConstraints cont 0 ipadx, ipady Specifies the internal padding: how much to add to the size of.
The default value is zero. The value is specified as an Insets object. By default, each component has no external padding. Here is a picture of how these values are interpreted in a container that has the default, leftto-right component orientation.
Previous releases require values named after points of the compass. We recommend that you use the new constants, instead, since they enable easier localization. Weights are used to determine how to distribute space among columns weightx and among rows weighty ; this is important for specifying resizing behavior.
Unless you specify at least one non-zero value for weightx or weighty, all the components clump together in the center of their container.
This is because when the weight is 0. Generally weights are specified with 0. Larger numbers indicate that the component's row or column should get more space. For each column, the weight is related to the highest weightx specified for a component within that column, with each multicolumn component's weight being split somehow between the columns the component is in. Similarly, each row's weight is related to the highest weighty specified for a component within that row.
Extra space tends to go toward the rightmost column and bottom row. GridBagLayout cont gridx gridy The int column gridx and row gridy of the component. The row and columns start at zero. The value GridBagConstraints. Number of columns or rows the component occupies. Default 1. These double variables default value 0 are used in calculating where space is allocated in rows and columns when a window is resized.
A column width is originally calculated as the maximum width of the preferred sizes of the components in that column. If a component is narrower than the column width, the value of the fill attribute is used in deciding how to use that extra space see below. Extra horizontal space is allocated to a column in proportion to the maximum weightx value for that column. If weightx is zero, no extra space is allocated.
Because these weights are relative, you can assign any arbitrary positive double value to produce the desired layout, although it is common to work in the range The analogous procedure is performed for the row heights using weighty. If a component isn't expanded to fill the space, the anchor attribute see below is used to specify where it goes within that cell. Default value GridBagConstraints. If the component doesn't occupy its entire display area, anchor specifies where it should be placed.
The location is usually given as a compass direction. A relative system which works for both right-to-left and left-toright languages is also available. GridBagLayout cont insets A java. Insets object adds padding space to the component. Insets should be rarely used because they often produce bad results, with the exception of JPanels and JLabels.
The constructor parameters in pixels specify the top, left, bottom, right. For example, gbc. Default value 0. Negative values can be used to tighten the spacing. These values are rarely useful, although they can be used for fine-tuning spacing.
Java Idiom 0 It's common to write a utility method which sets the fields of a GridBagConstraints object. The GridBagConstraints object is awkward to use. It allows you to make careless mistakes in the layout without warnings or error messages.
GridBagLayout cont import javax. EAST ; p. WEST ; p. EAST ;p. And all components are properly sized. Layout cont General advice 0 Initial Iterations. To quickly get the first iterations of a program running, use FlowLayout. The result is often ugly, but if there are only a few components, it's quick. FlowLayout is rarely the correct layout to use in a finished program.
BorderLayout is often a good layout for simple programs. More complicated layouts can often be nesting BorderLayouts within other BorderLayouts. This and FlowLayout are the most important layouts for beginners. Getting a GridBagLayout to work correctly can be time-consuming, but it produces excellent results. Null Layout 0 You can set the layout manager to null cont.
Set the positions of components. Problems with null layout that regular layouts don't have 0 Difficult to change, therefore hard expensive to maintain Moving, adding, removing, etc require a lot of recalculation.
Relatively little work is required with regular layouts. I once wrote a Java program that I proudly showed to an important colleague, who unfortunately was using a different system.
The layout looked really, really bad - gaps, overlaps. What happened? Null layout! It was one of my early programs and the last null layout I ever wrote. It's not big, but if you have things carefully aligned in null layouts, they may not be in the next version of Java.
I once decided I wanted larger fonts on the high resolution screen I had. Of course, this completely breaks any null layouts altho regular layouts will adjust properly. Impossible with null layouts, automatic with regular layouts. Events -- Introduction 0 Events come from User Controls When you define a user interface, you will usually have some way to.
For example, buttons, menus, sliders, mouse clicks, User interface event objects are passed from an event source, such as a button or mouse click, to an event listener, a user method which will process them.
Needs an Event Listener If you want a control to do something when the user alters the control, you must have a listener. Types of Events There are several kinds of events. Although these events usually come from the user interface, they can have other sources eg, a Timer. Button listener example 0 After a button is created, you will add a listener to it. Eg, b. An ActionEvent object is passed as a parameter to it actionPerformed.
Listeners The listener method must be called actionPerformed 0 There is unfortunately no way to use any given method as a listener -- the listener must be called actionPerformed.
Because there can only be one actionPerformed method in a class, a new class is needed for every separate listener, or you have to share an actionPerformed method and use the ugly technique of figuring out who caused the call.
To address this awkward situation C has delegates, which allow any method to be a listener, provided it has the correct return type and parameters. This doesn't provide any additional functionality, but it is more convenient.
Listeners cont Common Listener Strategies - Here are common ways to write listeners: 0 Named Inner Class Listeners are one of the most common ways to write small programs. They are convenient to write and be shared with several components. This is a little quicker to write, but lacks some of the flexibility of inner class listeners. For example, if you are defining a subclass of JPanel to use for drawing with a mouse, an instance of this new class will typically also be the mouse event listener.
Similarly with a JPanel used for animation -- the panel itself may serve as the ActionListener for the Timer. Do not use "this" as a listener for buttons, menus, etc. In some ways they are more powerful than other action listener strategies in that they can easily be used to enable or disable multiple controls. An action object can be shared with many controls; eg, the same action is sometimes accomplished with a menu item and a button or toolbar tool.
They encapsulate the name, description, enabled status, listener, and icon information. This is a good choice for large interfaces, and quite compatible with the MVC pattern see below. There are a number of ways to split the controller actions from the View part of the interface. This peculiar scheme certainly isn't general, and I've never seen it done in a serious program.
Do not do this. Inner-class Listeners cont 0 Defining an inner class listener to handle events is a very popular. Use an inner class rather than an outer class to access instance variables of the enclosing class. In the example below, the myGreetingField can be referenced by the listener class. Because simple program listeners typically get or set values of other widgets in the interface, it is very convenient to use an inner class.
Unlike anonymous inner class listeners, it's easy to reuse the same listener for more than one control, eg, the click of a button might perform the same action as the equivalent menu item, and might be the same as hitting the enter key in a text field. It's easier to group all the listeners together with inner classes than with anonymous inner class listeners.
Layout the panel. Anonymous Listeners A common idiom 0 There is no need to define a named class simply to add a listener object to a button. Java has a somewhat obscure syntax for creating an anonymous innner class listener that implements an interface. Top-level Listeners Using this as a listener 0 A common way to write simple applets is to use the applet itself as a. For example 0 This doesn't seem to be a problem for small programs, but what happens if there is more than one button?
There can be only one actionPerformed method in a class. Problem: One listener for many components 0 Inside the listener method, it's possible to check the parameter to find out which component caused the event. Top-level Listeners cont 0 Using one listener makes the response slower, and forces all events to.
This uses the event model which was introduced in Java 1. Although you will see this style used in some Java books, don't use it.
It doesn't scale up from one control to many. Buttons, menu items, toolbar items, etc use action listeners; imagine what this method would look like for Microsoft Word! AbstractAction, provide a useful mechanism to implement action listeners that can be shared and coordinated.
Actions can be used with most buttons, including toobox buttons and menu items, text fields, etc. They can be shared with all controls which do the same thing. They can specify text, icons, tooltip text, accelerator, and mnemonic keys.
You must subclass AbstractAction the hint is the word "abstract" in the class name. The minimum you need to do is override actionPerformed See examples below. Action, AbstractAction cont Constructors for abstract classes? The Java syntax for anonymous classes allows an abstract class's constructor to be called if it is followed by the body for the anonymous class. See Example - Simple anonymous class below.
Action, AbstractAction cont Specifies name for button, etc. Must defineactionPerformed Must define actionPerformed Sets the status of this Action. Sets the value of property key to value. Gets the value of property key. Accelerator key. Mnemonic key. Name for buttons and menu items. Used as tooltip text. Used for toolbars. Action, AbstractAction cont Usage 0 Actions can be used directly in the add method of some containers eg, menus.
Uses Action's text, icon. SOUTH ; f. If you release the "On" button, you will see 5 more messages. If you continue to press the "Off" button and hold it, you will see 2 more messages.
If you release the "Off" button, you will see 7 more messages. Here is the list of all the messages:. This tells us that a button has more than 2 states: selected and deselected. Events 10 and 11 show that when "Off" is selected, "On" is deselected.
If you click the button, the button label text will change from "On" to "Off", and the text of the label component will also change. Mouse Events Overview 0 The mouse listeners allow you to receive events to process: Button clicks, presses, or releases by the left, middle, or right buttons.
Moves and drags. Which Modifier keys shift, control, alt were down when the event occurred. Notification when the mouse enters or exits the component. Scroll wheel movements. Mouse Events cont Overview 0 Normally handled for you. The mouse is handled automatically by most components, so you never have to know about it.
For example, if someone clicks on a button JButton , the JButton translates that click into an ActionEvent, which is a higher level event that can be caused by a number of things. You don't need to know and shouldn't care whether the ActionEvent was from a mouse click on the button, or from a keyboard shortcut, or hitting enter while that button had focus, or If you are drawing your own graphics eg, on a JComponent or JPanel and need to know where the user clicks, then you need to know about mouse events.
You can easily add a mouse listener to a JComponent or JPanel. The most useful information in a MouseEvent is the x and y coordinates of the mouse cursor. Interface for mouse presses, releases, clicks, enters, and exits. Interface for mouse moves and drags. Class useful for writing anonymous listener for mouse button presses, entering, Class useful for writing anonymous listener for mouse movement.
Object passed to mouseWheelMoved. Subclass of MouseEvent. Interface to handle wheel movements. Adapter classes - You only have to override the methods you need. Here are the actions that a MouseListener catches. This is perhaps the most commonly used. Often used to change cursor. Often used to restore cursor. Even if you care only about mouse clicks, if your class directly implements MouseListener, then you must implement all five MouseListener methods.
Methods for those events you do not care about can have empty bodies. Here is an example:.
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