Frequently Asked Questions during Java applet development

This can be done by using the following code:

applet.getAppletContext().

showDocument(new URL(“javascript:” + js));

Where applet is an object of JApplet class, js is a JavaScript function name, for example: show().

With that, the applet should be launched from JavaScript

To do this, the applet should be launched from JavaScript, e.g.

In this case, the applet is launched from JavaScript with id= ‘mainApplet’. In the applet, there is appletMethod() method, which we want to call. To do this, you need to call the method related to JavaScript object with id name. You can also pass parameters to the called methods, as demonstrated in this example.

Also, it is necessary to know that the applet method called from JavaScript should be privileged. To do this, you need to wrap the method code in the following “wrapper”:

AccessController.doPrivileged(new PrivilegedAction() { public Object run() { //method code return new Object(); } });

In the above example in “var parameters = {};” construction, you need to add the necessary parameters, for example:

var parameters = {firstParameter:true, secondParameter:false};

In the applet, the parameters are read as follows:

String parameter=applet.getParameter(“firstParameter”);

Where applet is an object of JApplet class (or Applet class).

This method also works when you run the applet using <object> and <applet> tag.

This can be done in 2 ways. In both cases, the component must be visible.

Without the robot use:

BufferedImage image = new BufferedImage(component.getWidth(), component.getHeight(), BufferedImage.TYPE_INT_ARGB); Graphics g = image.getGraphics(); component.paint(g);

With the robot use:

In this case, the screenshot of the area in which this component is situated will be made. That is, if the component overlaps another application window, then the screenshot will contain an area of this another window.

Point point = new Point(0, 0); SwingUtilities.convertPointToScreen(point, component); Rectangle region = component.getBounds(); region.x = point.x; region.y = point.y; BufferedImage image= new Robot().createScreenCapture(region);

To do this, run the applets in different JVM. To run applets in different JVM, you need to run them with separate_jvm=true parameter.

Below, there is an example of such JTextField: public class TextField extends JTextField { public static final int TEXT_FIELD_PADDING = 4; public TextField() { addAutoResize(); init(); } public void init() { setStyleByParentStyle(); this.setPreferredSize(new Dimension(Constant. TEXT_FIELD_MINIMAL_WIDTH, this.getPreferredSize().height)); } private void addAutoResize() { this.getDocument().addDocumentListener (new DocumentListener() { @Override public void insertUpdate(DocumentEvent e) { checkResize(); } @Override public void removeUpdate(DocumentEvent e) { checkResize(); } @Override public void changedUpdate(DocumentEvent e) { checkResize(); } }); } private void checkResize() { FontMetrics fontMetrics = this.getFontMetrics(this. getFont()); if (fontMetrics.stringWidth(this.getText()) + TEXT_FIELD_PADDING * 2 < Constant.TEXT_FIELD_MINIMAL_WIDTH) { this.setPreferredSize(new Dimension(Constant.TEXT_FIELD_MINIMAL_WIDTH, this.getHeight())); this.getParent().revalidate(); } else { //NOTE 2 - border width*2 this.setPreferredSize(new Dimension(fontMetrics.stringWidth(this.getText()) + 2 + Constant.CARRET_WIDTH + TEXT_FIELD_PADDING * 2, this.getHeight())); this.getParent().revalidate(); } } }

The quite convenient mechanism for the implementation of Drag&Drop has appeared in Java 6, but it does have its disadvantages. For example, you should explicitly specify a Drop Target, what is not very useful when you need to lay down the object near the Drop Target. Also, in the standard implementation, there is no guarantee of execution order of listeners’ methods. I’ll tell you the concept of implementing a more extensible Drag&Drop.

Initially, the mouse listeners (Mouse Listener and MouseMotionListener) should be assigned to all Drag Sources. It’s need to implement 3 methods: a method of mouse click on the object, a method of moving the mouse while holding the mouse button on the object (mouseDragged in MouseMotionListener) and the mouse up method.

The listener’s assignment looks as follows:

component.addMouseListener(new MouseAdapter() { @Override public void mousePressed(MouseEvent e) { //block click right mouse button if (MouseEvent.BUTTON1 == e.getButton()) { startDrag(e); } } @Override public void mouseReleased(MouseEvent e) { //block click right mouse button if (MouseEvent.BUTTON1 == e.getButton()) { endDrag(e); } } }); component.addMouseMotionListener(new MouseMotionAdapter() { @Override public void mouseDragged(MouseEvent e) { drag(e); } });

Accordingly, when you click on the object Drag&Drop starts, when you move the mouse the object should be moved, when you release the mouse the object should change its position and be moved to a new container. If the object will be moved in the frame of one container, then it is possible to implement only mouseDragged () method, in which the coordinates of the dragged object will be changed:

@Override public void mouseDragged(MouseEvent e) { Point mouseLocation = e.getLocationOnScreen(); Component draggedComponent = (Component) e.getSource(); SwingUtilities.convertPointFromScreen(mouseLocation, draggedComponent.getParent()); draggedComponent.setLocation(mouseLocation); }

But dragged object coordinates can be set relative to the container in which it is located. Accordingly, when the mouse is moved to another container it is necessary to add a component to a new container and to calculate the new coordinates, etc. This method is not very beautiful and extensible, so I suggest using GlassPane to display the dragged object.

The algorithm looks as follows:

With this approach, we have no any dependences on the container on which the cursor should be placed to make drop and correspondingly the object can be “Dropped” anywhere.

public class GhostGlassPane extends JPanel { private final AlphaComposite composite; private BufferedImage ghostImage = null; private Point location = new Point(0, 0); public GhostGlassPane() { setOpaque(false); composite = AlphaComposite.getInstance(AlphaComposite. SRC_OVER, 0.7f); } public void paintComponent(Graphics g) { if (ghostImage == null) return; Graphics2D g2 = (Graphics2D) g; g2.setComposite(composite); g2.drawImage(ghostImage, (int) (location.getX()), (int) (location.getY()), null); } }

In this response, only concept of implementation is given.

Read our “Java applet signing” article.

Read our “Support of MP3 and WAV format files playback in Java” article.

The following swing components support HTML formatting: buttons, menu items, labels, tool tips and tabbed panes. To use the formatting, it is necessary at the beginning of the text to put <html> the tag (closing it at the end is optional.) For example:

<html><b><u>T</u>wo</b>

The following text will be displayed on the component: Two

Most probably, the cycle is used in the flow of drawing Swing. In order to perform any action on the UI and not to block the flow of drawing, you can use Swing Timer.
An example usage is:

Timer timer= new Timer(1000, new ActionListener() { @Override public void actionPerformed(ActionEvent e) { //actions } }); timer.setRepeats(true); timer.start();

This code will perform some actions once per second without blocking the flow of drawing.
You can also set the property of repeats = false, to perform an action only 1 time.
To stop the timer, perform timer.stop()

I can offer the following effect:

A white background appears smoothly on the one frame (you can insert a picture or a screenshot of the frame), then the white background disappears gradually, and the next frame appears under it.

The algorithm looks like:

You can run an applet on HTML page in 3 different ways: using <applet> tag, using <object> tag or using JavaScript.

<Applet> tag is considered obsolete, but still most of the applets are run in this way:

<applet code="HelloWorldApplet.class" width="320" height="120"> </applet>

The example of the applet run  <object> tag:

<object type="application/x-java-applet" name="previewersGraph" width="360" height="320"> <param name="codebase" value="/applets" /> <param name="code" value="my.full.class.Name" /> <param name="archive" value="applets.jar" /> </object>

An example of JavaScript run of applet can be found above.

I can offer the following algorithm:

Implementation of the algorithm:

/** * Class for adding components to container with random * coordinates */ public class ComponentRandomizer { /** * Add all components to container with random * coordinates */ public static void addComponentsToContainer(Container container, List<JComponent> components) { List<Rectangle> cells = getContainerCells(container, components); for (JComponent component : components) { Rectangle randomCell = cells.get(new Random(). nextInt(cells.size())); cells.remove(randomCell); addComponentToContainer(container, component, randomCell); } } /** * Add one component to container by it cell */ private static void addComponentToContainer(Container container, JComponent component, Rectangle cell) { container.add(component); int xCoordinate = getRandomInt(cell.getX(), cell.getWidth() - component.getWidth()); int yCoordinate = getRandomInt(cell.getY(), cell.getHeight() - component.getHeight()); component.setLocation(xCoordinate, yCoordinate); } /** * Get random integer */ private static int getRandomInt(double minimal, double interval) { if (interval > 0) { return (int) (minimal + new Random().nextInt((int) interval)); } else { return (int) minimal; } } /** * Create a cells that serves to adding component into it */ private static List<Rectangle> getContainerCells(Container container, List<JComponent> components) { Pair<Integer, Integer> optimalCellsNumber = getOptimalNumberOfContainerCells(container, components); Dimension cellSize = getCellSize(container, optimalCellsNumber); List<Rectangle> cells = new LinkedList<Rectangle>(); for (int stringIndex = 0; stringIndex < optimalCellsNumber.getKey(); stringIndex++) { for (int columnIndex = 0; columnIndex < optimalCellsNumber.getValue(); columnIndex++) { cells.add(createCell(cellSize, stringIndex, columnIndex)); } } return cells; } /** * Create a cell that serves to adding component into it */ private static Rectangle createCell(Dimension cellSize, int stringIndex, int columnIndex) { int xCellCoordinate = (int) (columnIndex * cellSize.getWidth()); int yCellCoordinate = (int) (stringIndex * cellSize.getHeight()); return new Rectangle(xCellCoordinate, yCellCoordinate, (int) cellSize.getWidth(), (int) cellSize.getHeight()); } /** * Create cell size */ private static Dimension getCellSize(Container container, Pair<Integer, Integer> optimalCellsNumber) { double cellWidth = container.getWidth() / optimalCellsNumber.getValue(); double cellHeight = container.getHeight() / optimalCellsNumber.getKey(); return new Dimension((int) Math.floor(cellWidth), (int) Math.floor(cellHeight)); } /** * Get the optimal amount of cells that can fit all of the * components * * @return count strings and columns */ private static Pair<Integer, Integer> getOptimalNumberOfContainerCells(Container container, List<JComponent> components) { //<Strings count, Columns count> Pair<Integer, Integer> numberOfContainerCells = getNumberOfContainerCells(container, components); //remove extra columns while (Math.ceil((double) components.size() / numberOfContainerCells.getKey()) < numberOfContainerCells.getValue()) { numberOfContainerCells = new Pair<Integer, Integer>(numberOfContainerCells.getKey(), numberOfContainerCells.getValue() - 1); } //remove extra strings while (Math.ceil((double) components.size() / numberOfContainerCells.getValue()) < numberOfContainerCells.getKey()) { numberOfContainerCells = new Pair<Integer, Integer>(numberOfContainerCells.getKey() - 1, numberOfContainerCells.getValue()); } return numberOfContainerCells; } /** * Split container on the cells by largest component and get * the number of cells of the container * * @return count strings and columns */ private static Pair<Integer, Integer> getNumberOfContainerCells(Container container, List<JComponent> components) { JComponent largestComponent = getLargestComponent(components); int stringsCount = (int) Math.floor(container.getHeight() / largestComponent.getHeight()); int columnsCount = (int) Math.floor(container.getWidth() / largestComponent.getWidth()); if (stringsCount * columnsCount < components.size()) { throw new IllegalArgumentException(Constant. MESSAGE_THE_COMPONENTS_CANNOT_BE_ACCOMMODATED_IN_THE_CONTAINER); } return new Pair<Integer, Integer>(stringsCount, columnsCount); } /** * Get component with largest square */ private static JComponent getLargestComponent(List<JComponent> components) { JComponent largestComponent = components.get(0); for (JComponent checkedComponent : components) { if (getComponentSquare( checkedComponent.getPreferredSize()) > getComponentSquare(largestComponent.getPreferredSize())) { largestComponent = checkedComponent; } } return largestComponent; } /** * Create component square */ private static double getComponentSquare(Dimension checkedComponentSize) { return checkedComponentSize.getHeight() * checkedComponentSize.getWidth(); } /** * Class for saving pair values */ public static class Pair<K, V> { private K key; private V value; public K getKey() { return key; } public V getValue() { return value; } public Pair(K k, V v) { key = k; value = v; } } }

To do this you need to have 2 projects: the applet project that will be built as a JAR file and the web application project that need to be deployed (as a result you will get WAR file).

Correspondingly, you should create a multimodule project which as modules will contain 2 of the above projects:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>org.jazzteam</groupId> <artifactId>applet-project</artifactId> <packaging>pom</packaging> <modules> <module>applet</module> <module>applet-web</module> </modules> <version>1.0-SNAPSHOT</version> </project>

After the applet module building, you should copy the resulting JAR file in the web application.

<build> <plugins> <plugin> <artifactId>maven-antrun-plugin</artifactId> <version>1.4</version> <executions> <execution> <id>copy</id> <phase>site</phase> <configuration> <tasks> <copy file="target\applet-eugliajar" tofile="..\euglia-applet\src\main\webapp\applet-euglia.jar"/> </tasks> </configuration> <goals> <goal>run</goal> </goals> </execution> </executions> </plugin> </plugins> </build>

In this case the general project should be built with “clean package site” command, because in the “site” phase the files copying begin.

In this approach, the web application should contain HTML page that runs the applet.