public abstract class Component
extends Object
implements ImageObserver, MenuContainer, Serializable
A component is an object having a graphical representation
that can be displayed on the screen and that can interact with the
user. Examples of components are the buttons, checkboxes, and scrollbars
of a typical graphical user interface.
The Component class is the abstract superclass of
the nonmenu-related Abstract Window Toolkit components. Class
Component can also be extended directly to create a
lightweight component. A lightweight component is a component that is
not associated with a native opaque window.
Serialization
It is important to note that only AWT listeners which conform
to the
Serializable protocol will be saved when
the object is stored. If an AWT object has listeners that
aren't marked serializable, they will be dropped at
writeObject time. Developers will need, as always,
to consider the implications of making an object serializable.
One situation to watch out for is this:
import java.awt.*;
import java.awt.event.*;
import java.io.Serializable;
class MyApp implements ActionListener, Serializable
{
BigObjectThatShouldNotBeSerializedWithAButton bigOne;
Button aButton = new Button();
MyApp()
{
// Oops, now aButton has a listener with a reference
// to bigOne!
aButton.addActionListener(this);
}
public void actionPerformed(ActionEvent e)
{
System.out.println("Hello There");
}
}
In this example, serializing
aButton by itself
will cause
MyApp and everything it refers to
to be serialized as well. The problem is that the listener
is serializable by coincidence, not by design. To separate
the decisions about
MyApp and the
ActionListener being serializable one can use a
nested class, as in the following example:
import java.awt.*;
import java.awt.event.*;
import java.io.Serializable;
class MyApp java.io.Serializable
{
BigObjectThatShouldNotBeSerializedWithAButton bigOne;
Button aButton = new Button();
static class MyActionListener implements ActionListener
{
public void actionPerformed(ActionEvent e)
{
System.out.println("Hello There");
}
}
MyApp()
{
aButton.addActionListener(new MyActionListener());
}
}
Note: For more information on the paint mechanisms utilitized
by AWT and Swing, including information on how to write the most
efficient painting code, see
Painting in AWT and Swing.
For details on the focus subsystem, see
How to Use the Focus Subsystem,
a section in The Java Tutorial, and the
Focus Specification
for more information.