A example JHotDraw application

JHotDraw is a framework for the creation of drawing editors. It provides support for a range of programs from simple paint package style editors to more complex programs that have rules about how their elements can be used and altered (e.g. a UML diagramming tool). The framework provides support for the creation of geometric and user defined shapes, editing those shapes, creating behavioural constraints in the editor and animation.

The applications created by JHotDraw are not 'bitmaped' (or rasterised) paint programs. They maintain a collection of shapes that are separate from each other and can be manipulated as separate entities (unless explicitly grouped together). The Drawing class is responsible for keeping track of which shapes have been added to the drawing. To increase its flexibility JHotDraw follows the principals of the MVC design pattern by separating its model (data) from its view (appearance). The Drawing can be thought of as the model as it contains the current state of the application, a separate class called DrawingView represents the drawing's appearance in the application's GUI.

Applications in JHotDraw are defined by the DrawApplication class. This class is responsible for the creation of the graphical user interface (GUI) and for controlling access to the main elements of a JHotDraw application. In JHotDraw all applications are comprised of a drawing; an associated drawing view, a set of drawing shapes and at least one tool.
 

• To create your application see [Customising DrawApplication]

The shapes that make up a drawing are represented by the concept of Figures in JHotDraw. Figures are defined as rectangular areas of a drawing but are free to define the visual area inside that rectangle (i.e. the bit the user sees) as they wish. JHotDraw comes presupplied with many comonly used figures but it is possible for developers to create custom figures for use in their own applications.
 

• To reuse existing figures in your application see [Identifying Existing Figures]
• To modify existing figures for your application see [Modifying Existing Figures]
• To compose existing figures together see [Creating Composite Figures]

Most applications in JHotDraw will require mechanisms for creating and editing the figures that make up a drawing. Figure creation is usually performed with a tool (a button that, when selected, alters the behaviour of the mouse).Editing figures is largely dependant on the type of figure and the set of attributes it allows to be altered. By default most figures allow their size and position to be changed, some allow their colours (outline and fill) to be changed and a few will support more esoteric options such as text editing etc. Editing behaviour is usually limited to the current selection on a drawing
 

• To create construction tools for figures see [Using CreationTool]

There are three main ways to define editing behaviour in JHotDraw, using handles (small icons that appear on the selected figure in the drawing), ActionTools (a special type of tool) or menus. In many cases these different mechanisms can be used interchangeably but occasionally the unique benefits and limitations of each approach have to be considered to create suitable edit functionality for an application.
 

• To create handles to edit figure attributes see [Creating Handles]
• To create tools to edit figure attributes see [Using ActionTools]
• To create menus to edit figure attributes see [Creating Custom Menus]

Constraints are used to impose limits on an application's behavior and are useful when creating applications that require specific relationships between their elements (example constraints include, controlling where figures can be positioned, which of their attributes can be altered, connecting two figures together, etc).

In JHotDraw there are a number of ways in which constraints may be modelled. Constraints can be local to a class of Figure (for example to force the figure to be a certain size or stay in a given position). Or constraints can be applied to subclasses of the Drawing (for constraints that affect groups of figures). Finally it is possible to create connections between figures in the drawing. These are represented on the Drawing by a special type of figure called a ConnectionFigure.
 

• To create constraints on figures and drawings see [Modifying Existing Figures] (NB the standard implementation of drawing is a subclass of Figure)
• To create connections between figures see [Creating ConnectionFigures]

All the different elements mentioned above (i.e. Figures, Handles etc) need to be positioned on the drawing. This can be done by speciying absolute coordinates for that element's position or by using Locators, special classes that can be used to describe positions relative to other elements in the drawing (for example a Locator could be used to create a handle in the centre of a figure).
 

• To use Locators in your application see [Using Locators]

JHotDraw  also supports simple animation capabilites at both the drawing and figure levels of the application.
 

• To create animation in your application see [Creating Animation]

• For an index of all availible patterns click here [JHD Pattern Index]