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Why do we make a distinction between 'system-triggered' and. A common mistake is to always invoke the no-arg version, which causes a repaint of the entire component, often resulting in unnecessary paint processing. To enable app-triggered painting, the AWT provides the following methods to allow programs to asynchronously request a paint operation:Р”В¬omponents that render complex output should invoke repaint() with arguments defining only the region that requires updating. It is not recommended that programs invoke paint() directly.

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Why? Because such code may be invoked at times when it is not appropriate to paint - for instance, before the component is visible or has access to a valid Graphics object. In general, programs should avoid placing rendering code at any point where it might be invoked outside the scope of the paint callback.

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draw circle (color already set to foreground)Р”Вevelopers who are new to AWT might want to take a peek at the PaintDemo example, which provides a runnable program example of how to use the paint callback in an AWT program. Int d = Math.min(size.width, size.height) Dynamically calculate size information Here is a simple example of a paint callback which renders a filled circle in the bounds of a component:

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They are free to change the state of the Graphics object as necessary.

Programs must use this Graphics object (or one derived from it) to render output.

The Graphics object's clip rectangle is set to the area of the component that is in need of repainting.

The Graphics object's translation is set such that the coordinate (0,0) represents the upper left corner of the component.

The Graphics object's font is set to the component's font property.

The Graphics object's color is set to the component's foreground property.

When AWT invokes this method, the Graphics object parameter is pre-configured with the appropriate state for drawing on this particular component: This means that a program should place the component's rendering code inside a particular overridden method, and the toolkit will invoke this method when it's time to paint.

Regardless of how a paint request is triggered, the AWT uses a 'callback' mechanism for painting, and this mechanism is the same for both heavyweight and lightweight components. a button detects that a mouse button has been pressed and determines that it needs to paint a 'depressed' button visual).

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In an application-triggered painting operation, the component decides it needs to update its contents because its internal state has changed. (For example, something that previously obscured the component has moved, and a previously obscured portion of the component has become exposed).

The component has damage that needs to be repaired.

The component is first made visible on the screen.

In a system-triggered painting operation, the system requests a component to render its contents, usually for one of the following reasons:

In AWT, there are two kinds of painting operations: system-triggered painting, and application-triggered painting. To understand how AWT's painting API works, helps to know what triggers a paint operation in a windowing environment. But it also introduces some differences in the mechanism, as well as new APIs that make it easier for applications to customize how painting works. For the most part, the Swing painting mechanism resembles and relies on the AWT's. Consequently, there are subtle differences in how painting works for heavyweight and lightweight components.Р”Р„fter JDK 1.1, when the Swing toolkit was released, it introduced its own spin on painting components. With the introduction of lightweight components in JDK 1.1 (a 'lightweight' component is one that reuses the native window of its closest heavyweight ancestor), the AWT needed to implement the paint processing for lightweight components in the shared Java code. This scheme took care of details such as damage detection, clip calculation, and z-ordering. This allowed the AWT to rely heavily on the paint subsystem in each native platform. When the original AWT API was developed for JDK 1.0, only heavyweight components existed ('heavyweight' means that the component has it's own opaque native window). The main topics covered in this article are:

While the article covers the general paint mechanism ( where and when to render), it does not tell how to use Swing's graphics APIs to render a correct output. Its purpose is to help developers write correct and efficient GUI painting code. This article explains the AWT and Swing paint mechanisms in detail.