Thursday, 6 April 2017

Awt Package

The Java programming language class library provides a user interface toolkit called the Abstract Windowing Toolkit, or the AWT. The AWT is both powerful and flexible. Newcomers, however, often find that its power is veiled. The class and method descriptions found in the distributed documentation provide little guidance for the new programmer. Furthermore, the available examples often leave many important questions unanswered. Of course, newcomers should expect some difficulty. Effective graphical user interfaces are inherently challenging to design and implement, and the sometimes complicated interactions between classes in the AWT only make this task more complex. However, with proper guidance, the creation of a graphical user interface using the AWT is not only possible, but relatively straightforward.

This article covers some of the philosophy behind the AWT and addresses the practical concern of how to create a simple user interface for an applet or application.
What is a user interface
The user interface is that part of a program that interacts with the user of the program. User interfaces take many forms. These forms range in complexity from simple command-line interfaces to the point-and-click graphical user interfaces provided by many modern applications.

At the lowest level, the operating system transmits information from the mouse and keyboard to the program as input, and provides pixels for program output. The AWT was designed so that programmers don't have worry about the details of tracking the mouse or reading the keyboard, nor attend to the details of writing to the screen. The AWT provides a well-designed object-oriented interface to these low-level services and resources.

Because the Java programming language is platform-independent, the AWT must also be platform-independent. The AWT was designed to provide a common set of tools for graphical user interface design that work on a variety of platforms. The user interface elements provided by the AWT are implemented using each platform's native GUI toolkit, thereby preserving the look and feel of each platform. This is one of the AWT's strongest points. The disadvantage of such an approach is the fact that a graphical user interface designed on one platform may look different when displayed on another platform.

Contains all of the classes for creating user interfaces and for painting graphics and images. A user interface object such as a button or a scrollbar is called, in AWT terminology, a component. The Component class is the root of all AWT components. See Component for a detailed description of properties that all AWT components share.
Some components fire events when a user interacts with the components. The AWTEvent class and its subclasses are used to represent the events that AWT components can fire. See AWTEvent for a description of the AWT event model.
Components and containers
A graphical user interface is built of graphical elements called components. Typical components include such items as buttons, scrollbars, and text fields. Components allow the user to interact with the program and provide the user with visual feedback about the state of the program. In the AWT, all user interface components are instances of class Component or one of its subtypes.

Components do not stand alone, but rather are found within containers. Containers contain and control the layout of components. Containers are themselves components, and can thus be placed inside other containers. In the AWT, all containers are instances of class Container or one of its subtypes.

Spatially, components must fit completely within the container that contains them. This nesting of components (including containers) into containers creates a tree of elements, starting with the container at the root of the tree and expanding out to the leaves, which are components such as buttons.

A container is a component that can contain components and other containers. A con tainer can also have a layout manager that controls the visual placement of components in the container. The AWT package contains several layout manager classes and an interface for building your own layout manager. See Container and LayoutManager for more information.

Each Component object is limited in its maximum size and its location because the values are stored as an integer. Also, a platform may further restrict maximum size and location coordinates. The exact maximum values are dependent on the platform. There is no way to change these maximum values, either in Java code or in native code. These limitations also impose restrictions on component layout. If the bounds of a Component object exceed a platform limit, there is no way to properly arrange them within a Container object. The object's bounds are defined by any object's coordinate in combination with its size on a respective axis.
Types of components
Class Component defines the interface to which all components must adhere.
The AWT provides nine basic non-container component classes from which a user interface may be constructed. (Of course, new component classes may be derived from any of these or from class Component itself.) These nine classes are class Button, Canvas, Checkbox, Choice, Label, List, Scrollbar, TextArea, and TextField.

Border Layout Example

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