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 window. On the contrary, a heavyweight component is associated with a native window. The isLightweight() method may be used to distinguish between the two kinds of the components.

Lightweight and heavyweight components may be mixed in a single component hierarchy. However, for correct operating of such a mixed hierarchy of components, the whole hierarchy must be valid. When the hierarchy gets invalidated, like after changing the bounds of components, or adding/removing components to/from containers, the whole hierarchy must be validated afterwards by means of the Container.validate() method invoked on the top-most invalid container of the hierarchy.

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 implements 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 utilized 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.

implements ImageObserver, MenuContainer, Serializable