C SC 225 Lecture 8: Design Patterns: Composite

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The Composite pattern

• Use this when primitive object needs to be grouped into composite which is itself treated like the primitive
• In other words, the composite has the same behavior as its parts
• The primitive object is called a leaf. Both the leaf and the composite implement the same interface
• This is a Structural Pattern
• Some common examples are:
1. Hierarchical GUI menus. Menu item can either be primitive or a submenu. Submenu is composite that is treated like a primitive menu item
2. Some Java GUI components are primitive (e.g. JButton), other components are also containers (e.g. JPanel). Containers are like the submenus above.
• A given composite behavior (from interface) is implemented by having it invoke the same behavior on its primitives and combining the results.
• Example is submenu that displays by having all its menu items display
• Example is container component that calculates its preferred size by getting the preferred sizes of all its subcomponents
• Cost of composite object is sum of costs of its components. Sound like Decorator?
• Composite superficially resembles Decorator.
• Compare their class diagrams. See Horstmann pages 194 and 197.
• They differ in their intent. Decorator enhances its component, Composite merely groups its components
• Note the decorator wraps around a single "decoratee", while the composite wraps around a collection of composites/primitives.
• Note the composite is really playing a dual role here, as both composite and primitive
• It may have behaviors that the leaf would not, like adding components
• A leaf may have behaviors that a composite would not, like getters
• If both implement a common interface, then both may have both sets of behaviors
• This is not an ideal situation; how to deal with it?
• Is there any way around it? Yes, but complicated.