Containers

Contents:
Container
Panel
Insets
Window
Frames
Dialogs
FileDialog

This chapter covers a special type of Component called Container. A Container is a subclass of Component that can contain other components, including other containers. Container allows you to create groupings of objects on the screen. This chapter covers the methods in the Container class and its subclasses: Panel, Window, Frame, Dialog, and FileDialog. It also covers the Insets class, which provides an internal border area for the Container classes.

Every container has a layout associated with it that controls how the container organizes the components in it. The layouts are described in Layouts.

Java 1.1 introduces a special Container called ScrollPane. Because of the similarities between scrolling and ScrollPane, the new ScrollPane container is covered with the Scrollbar class in Scrolling.

Container

Container is an abstract class that serves as a general purpose holder of other Component objects. The Container class holds the methods for grouping the components together, laying out the components inside it, and dealing with events occurring within it. Because Container is an abstract class, you never see a pure Container object; you only see subclasses that add specific behaviors to a generic container.

Container Methods

The abstract Container class contains a single constructor to be called by its children. Prior to Java 1.1, the constructor was package private.

• protected Container()
• The constructor for Container creates a new component without a native peer. Since you no longer have a native peer, you must rely on your container to provide a display area. This allows you to create containers that require fewer system resources. For example, if you are creating panels purely for layout management, you might consider creating a LightweightPanel class to let you assign a layout manager to a component group. Using LightweightPanel will speed things up since events do not have to propagate through the panel and you do not have to get a peer from the native environment. The following code creates the LightweightPanel class:

  import java.awt.*;
  public class LightweightPanel extends Container {
   LightweightPanel () {} LightweightPanel (LayoutManager lm) {
   setLayout(lm);
  }
  } 
  

A Container holds a set of objects within itself. This set of methods describes how to examine and add components to the set.

• public int getComponentCount ()
  public int countComponents ()
• The getComponentCount() method returns the number of components within the container at this level. getComponentCount() does not count components in any child Container (i.e., containers within the current container).

  countComponents() is the Java 1.0 name for this method.

• public Component getComponent (int position)
• The getComponent() method returns the component at the specific position within it. If position is invalid, this method throws the run-time exception ArrayIndexOutOfBoundsException.
• public Component[] getComponents ()
• getComponents() returns an array of all the components held within the container. Since these are references to the actual objects on the screen, any changes made to the components returned will be reflected on the display.
• public Component add (Component component, int position)
• The add() method adds component to the container at position. If position is -1, add() inserts component as the last object within the container. What the container does with position depends upon the LayoutManager of the container. If position is invalid, the add() method throws the run-time exception IllegalArgumentException. If you try to add component's container to itself (anywhere in the containment tree), this method throws an IllegalArgumentException. In Java 1.1, if you try to add a Window to a container, add() throws the run-time exception IllegalArgumentException. If you try to add component to a container that already contains it, the container is removed and re-added, probably at a different position.

  Assuming that nothing goes wrong, the parent of component is set to the container, and the container is invalidated. add() returns the component just added.

  Calling this method generates a ContainerEvent with the id COMPONENT_ADDED.

• public Component add (Component component)
• The add() method adds component to the container as the last object within the container. This is done by calling the earlier version of add() with a position of -1. If you try to add component's container to itself (anywhere in the containment tree), this method throws the run-time exception IllegalArgumentException. In Java 1.1, if you try to add a Window to a container, add() throws the run-time exception IllegalArgumentException.

  Calling this method generates a ContainerEvent with the id COMPONENT_ADDED.

• public void add (Component component, Object constraints)
  public Component add (String name, Component component)
• This next version of add() is necessary for layouts that require additional information in order to place components. The additional information is provided by the constraints parameter. This version of the add() method calls the addLayoutComponent() method of the LayoutManager. What the container does with constraints depends upon the actual LayoutManager. It can be used for naming containers within a CardLayout, specifying a screen area for BorderLayout, or providing a set of GridBagConstraints for a GridBagLayout. In the event that this add() is called and the current LayoutManager does not take advantage of constraints, component is added at the end with a position of -1. If you try to add component's container to itself (anywhere in the containment tree), this method throws the run-time exception IllegalArgumentException. In Java 1.1, if you try to add a Window to a container, add() throws the run-time exception IllegalArgumentException.

  The add(String, Component) method was changed to add(component, object) in Java 1.1 to accommodate the LayoutManager2 interface (discussed in Layouts) and to provide greater flexibility. In all cases, you can just flip the parameters to bring the code up to 1.1 specs. The string used as an identifier in Java 1.0 is just treated as a particular kind of constraint.

  Calling this method generates a ContainerEvent with the id COMPONENT_ADDED.

• public void add (Component component, Object constraints, int index)
• This final version of add() is necessary for layouts that require an index and need additional information to place components. The additional information is provided by the constraints parameter. This version of add() also calls the addLayoutComponent() method of the LayoutManager. component is added with a position of index. If you try to add component's container to itself (anywhere in the containment tree), this method throws the run-time exception IllegalArgumentException. In Java 1.1, if you try to add a Window to a Container, add() throws the run-time exception IllegalArgumentException.

  Some layout managers ignore any index. For example, if you call add(aButton, BorderLayout.NORTH, 3) to add a Button to a BorderLayout panel, the Button appears in the north region of the layout, no matter what the index.

  Calling this method generates a ContainerEvent with the id COMPONENT_ADDED.

• protected void addImpl(Component comp, Object constraints, int index)
• The protected addImpl() method is the helper method that all the others call. It deals with synchronization and enforces all the restrictions on adding components to containers.

  The addImpl() method tracks the container's components in an internal list. The index with which each component is added determines its position in the list. The lower the component's index, the higher it appears in the stacking order. In turn, the stacking order determines how components are displayed when sufficient space isn't available to display all of them. Components that are added without indices are placed at the end of the list (i.e., at the end of the stacking order) and therefore displayed behind other components. If all components are added without indices, the first component added to the container is first in the stacking order and therefore displayed in front.

  You could override addImpl() to track when components are added to a container. However, the proper way to find out when components are added is to register a ContainerListener and watch for the COMPONENT_ADDED and the COMPONENT_REMOVED events.

• public void remove (int index)
• The remove() method deletes the component at position index from the container. If index is invalid, the remove() method throws the run-time exception IllegalArgumentException. This method calls the removeLayoutComponent() method of the container's LayoutManager.

  removeAll() generates a ContainerEvent with the id COMPONENT_REMOVED.

• public void remove (Component component)
• The remove() method deletes component from the container, if the container directly contains component. remove() does not look through nested containers trying to find component. This method calls the removeLayoutComponent() method of the container's LayoutManager.

  When you call this method, it generates a ContainerEvent with the id COMPONENT_REMOVED.

• public void removeAll ()
• The removeAll() method removes all components from the container. This is done by looping through all the components, setting each component's parent to null, setting the container's reference to the component to null, and invalidating the container.

  When you call this method, it generates a ContainerEvent with the id COMPONENT_REMOVED for each component removed.

• public boolean isAncestorOf(Component component)
• The isAncestorOf() method checks to see if component is a parent (or grandparent or great grandparent) of this container. It could be used as a helper method for addImpl() but is not. If component is an ancestor of the container, isAncestorOf() returns true; otherwise, it returns false.

Every container has a LayoutManager. The LayoutManager is responsible for positioning the components inside the container. The Container methods listed here are used in sizing the objects within the container and specifying a layout.

• public LayoutManager getLayout ()
• The getLayout() method returns the container's current LayoutManager.
• public void setLayout (LayoutManager manager)
• The setLayout() method changes the container's LayoutManager to manager and invalidates the container. This causes the components contained inside to be repositioned based upon manager's rules. If manager is null, there is no layout manager, and you are responsible for controlling the size and position of all the components within the container yourself.
• public Dimension getPreferredSize ()
  public Dimension preferredSize ()
• The getPreferredSize() method returns the Dimension (width and height) for the preferred size of the components within the container. The container determines its preferred size by calling the preferredLayoutSize() method of the current LayoutManager, which says how much space the layout manager needs to arrange the components. If you override this method, you are overriding the default preferred size.

  preferredSize() is the Java 1.0 name for this method.

• public Dimension getMinimumSize ()
  public Dimension minimumSize ()
• The getMinimumSize() method returns the minimum Dimension (width and height) for the size of the components within the container. This container determines its minimum size by calling the minimumLayoutSize() method of the current LayoutManager, which computes the minimum amount of space the layout manager needs to arrange the components. It is possible for getMinimumSize() and getPreferredSize() to return the same dimensions. There is no guarantee that you will get this amount of space for the layout.

  minimumSize() is the Java 1.0 name for this method.

• public Dimension getMaximumSize ()
• The getMaximumSize() method returns the maximum Dimension (width and height) for the size of the components within the container. This container determines its maximum size by calling the maximumLayoutSize() method of the current LayoutManager2, which computes the maximum amount of space the layout manager needs to arrange the components. If the layout manager is not an instance of LayoutManager2, this method calls the getMaximumSize() method of the Component, which returns Integer.MAX_VALUE for both dimensions. None of the java.awt layout managers use the concept of maximum size yet.
• public float getAlignmentX ()
• The getAlignmentX() method returns the alignment of the components within the container along the x axis. This container determines its alignment by calling the current LayoutManager2's getLayoutAlignmentX() method, which computes it based upon its children. The return value is between 0.0 and 1.0. Values nearer 0 indicate that the component should be placed closer to the left edge of the area available. Values nearer 1 indicate that the component should be placed closer to the right. The value 0.5 means the component should be centered. If the layout manager is not an instance of LayoutManager2, this method calls Component's getAlignmentX() method, which returns the constant Component.CENTER_ALIGNMENT. None of the java.awt layout managers use the concept of alignment yet.
• public float getAlignmentY ()
• The getAlignmentY() method returns the alignment of the components within the container along the y axis. This container determines its alignment by calling the current LayoutManager2's getLayoutAlignmentY() method, which computes it based upon its children. The return value is between 0.0 and 1.0. Values nearer 0 indicate that the component should be placed closer to the top of the area available. Values nearer 1 indicate that the component should be placed closer to the bottom. The value 0.5 means the component should be centered. If the layout manager is not an instance of LayoutManager2, this method calls Component's getAlignmentY() method, which returns the constant Component.CENTER_ALIGNMENT. None of the java.awt layout managers use the concept of alignment yet.
• public void doLayout ()
  public void layout ()
• The doLayout() method of Container instructs the LayoutManager to lay out the container. This is done by calling the layoutContainer() method of the current LayoutManager.

  layout()is the Java 1.0 name for this method.

• public void validate ()
• The validate() method sets the container's valid state to true and recursively validates all of its children. If a child is a Container, its children are in turn validated. Some components are not completely initialized until they are validated. For example, you cannot ask a Button for its display dimensions or position until it is validated.
• protected void validateTree ()
• The validateTree() method is a helper for validate() that does all the work.
• public void invalidate ()
• The invalidate() method invalidates the container and recursively invalidates the children. If the layout manager is an instance of LayoutManager2, its invalidateLayout() method is called to invalidate any cached values.

The event model for Java is described in Events. These methods help in the handling of the various system events at the container level.

• public void deliverEvent (Event e)
• The deliverEvent() method is called by the system when the Java 1.0 Event e happens. deliverEvent() tries to locate a component contained in the container that should receive it. If one is found, the x and y coordinates of e are translated for the new target, and Event e is delivered to this by calling its deliverEvent(). If getComponentAt() fails to find an appropriate target, the event is just posted to the container with postEvent().
• public Component getComponentAt (int x, int y)
  public Component locate (int x, int y)
• The container's getComponentAt() method calls each component's contains() method to see if the x and y coordinates are within it. If they are, that component is returned. If the coordinates are not in any child component of this container, the container is returned. It is possible for getComponentAt() to return null if the x and y coordinates are not within the container. The method getComponentAt() can return another Container or a lightweight component.

  locate()is the Java 1.0 name for this method.

• public Component getComponentAt (Point p)
• This getComponentAt() method is identical to the previous method, with the exception that the location is passed as a single point, rather than as separate x and y coordinates.

With the 1.1 event model, you register listeners, which are told when events occur. Container events occur when a component is added or removed.

• public synchronized void addContainerListener(ContainerListener listener)
• The addContainerListener() method registers listener as an object interested in receiving notifications when an ContainerEvent passes through the EventQueue with this Container as its target. The listener.componentAdded() or listener.componentRemoved() method is called when these events occur. Multiple listeners can be registered. The following code demonstrates how to use a ContainerListener to register action listeners for all buttons added to an applet. It is similar to the ButtonTest11 example in Button Events. The trick that makes this code work is the call to enableEvents() in init(). This method makes sure that container events are delivered in the absence of listeners. In this applet, we know there won't be any container listeners, so we must enable container events explicitly before adding any components.

// Java 1.1 only import java.awt.*; import java.applet.*; import java.awt.event.*;
public class NewButtonTest11 extends Applet implements ActionListener {
 Button b;
public void init () {
 enableEvents (AWTEvent.CONTAINER_EVENT_MASK); add (b = new Button ("One")); add (b = new Button ("Two")); add (b = new Button ("Three")); add (b = new Button ("Four"));
}
protected void processContainerEvent (ContainerEvent e) {
 if (e.getID() == ContainerEvent.COMPONENT_ADDED) {
 if (e.getChild() instanceof Button) {
 Button b = (Button)e.getChild(); b.addActionListener (this);
}
}
}
public void actionPerformed (ActionEvent e) {
 System.out.println ("Selected: " + e.getActionCommand());
}
} 

• public void removeContainerListener(ContainerListener listener)
• The removeContainerListener() method removes listener as an interested listener. If listener is not registered, nothing happens.
• protected void processEvent(AWTEvent e)
• The processEvent() method receives all AWTEvents with this Container as its target. processEvent() then passes them along to any listeners for processing. When you subclass Container, overriding processEvent() allows you to process all events yourself, before sending them to any listeners. There is no equivalent under the 1.0 event model.

  If you override processEvent(), remember to call super.processEvent(e) last to ensure that regular event processing can occur. If you want to process your own events, it's a good idea to call enableEvents() (inherited from Component) to ensure that events are delivered even in the absence of registered listeners.

• protected void processContainerEvent(ContainerEvent e)
• The processContainerEvent() method receives all ContainerEvents with this Container as its target. processContainerEvent() then passes them along to any listeners for processing. When you subclass Container, overriding the processContainerEvent() method allows you to process all container events yourself, before sending them to any listeners. There is no equivalent under the 1.0 event model.

  If you override the processContainerEvent() method, remember to call super.processContainerEvent(e) last to ensure that regular event processing can occur. If you want to process your own events, it's a good idea to call enableEvents() (inherited from Component) to ensure that events are delivered even in the absence of registered listeners.

The following methods are early vestiges of an approach to painting and printing. They are not responsible for anything that couldn't be done with a call to paintAll() or printAll(). However, they are available if you wish to call them.

• public void paintComponents (Graphics g)
• The paintComponents() method of Container paints the different components it contains. It calls each component's paintAll() method with a clipped graphics context g, which is eventually passed to paint().
• public void printComponents (Graphics g)
• The printComponents() method of Container prints the different components it contains. It calls each component's printAll() method with a clipped graphics context g, which is passed to print(), and eventually works its way to paint().

Since it is the container's responsibility to deal with painting lightweight peers, the paint() and print() methods are overridden in Java 1.1.

• public void paint(Graphics g)
• The paint() method of Container paints the different lightweight components it contains.

• public void print(Graphics g)
• The print() method of Container prints the different lightweight components it contains.

NOTE:

If you override paint() or print() in your containers (especially applets), call super.paint(g) or super.print(g), respectively, to make sure that lightweight components are rendered. This is a good practice even if you don't currently use any lightweight components; you don't want your code to break mysteriously if you add a lightweight component later.Peers

The container is responsible for creating and destroying all the peers of the components within it.

• public void addNotify ()
• The addNotify() method of Container creates the peer of all the components within it. After addNotify() is called, the Container is invalid. It is useful for top-level containers to call this method explicitly before calling the method setVisible(true) to guarantee that the container is laid out before it is displayed.
• public void removeNotify ()
• The removeNotify() method destroys the peer of all the top-level objects contained within it. This in effect destroys the peers of all the components within the container.

• protected String paramString ()
• When you call the toString() method of a container, the default toString() method of Component is called. This in turn calls paramString() which builds up the string to display. At the Container level, paramString() appends the layout manager name, like layout=java.awt.BorderLayout, to the output.
• public Insets getInsets ()
  public Insets insets ()
• The getInsets() method gets the container's current insets. An inset is the amount of space reserved for the container to use between its edge and the area actually available to hold components. For example, in a Frame, the inset for the top would be the space required for the title bar and menu bar. Insets exist for top, bottom, right, and left. When you override this method, you are providing an area within the container that is reserved for free space. If the container has insets, they would be the default. If not, the default values are all zeroes.

  The following code shows how to override insets() to provide values other than the default. The top and bottom have 20 pixels of inset. The left and right have 50. Insets describes the Insets class in more detail.

  public Insets insets () {
   // getInsets() for Java 1.1 return new Insets (20, 50, 20, 50);
  }
  

  To find out the current value, just call the method and look at the results. For instance, for a Frame the results could be the following in the format used by toString():

  java.awt.Insets[top=42,left=4,right=4,bottom=4] 
  

  The 42 is the space required for the title and menu bar, while the 4 around the edges are for the window decorations. These results are platform specific and allow you to position items based upon the user's run-time environment.

  When drawing directly onto the graphics context of a container with a large inset such as Frame, remember to work around the insets. If you do something like g.drawString("Hello World", 5, 5) onto a Frame, the user won't see the text. It will be under the title bar and menu bar.

  insets() is the Java 1.0 name for this method.

• public void list (PrintWriter output, int indentation)
  public void list (PrintStream output, int indentation)
• The list() method is very helpful if you need to find out what is inside a container. It recursively calls itself for each container level of objects inside it, increasing the indentation at each level. The results are written to the PrintStream or PrintWriter output.