Applied Design Patterns with Java

Behavioral :: Command (233) {C ch 17}

Examples - UML : fullCommand and undoCmd

The first example Java program is called 'fullCommand'.

The UML diagram is above, and the list of Java files is below:

The second example Java program is called 'undoCmd'.

The UML diagram is above, and the list of Java files is below:

Example - Java :: Patterns\Behavioral\Command

Pattern Concept: to handle a request by forwarding it to a SPECIFIC object. Details on the specific actions needed and a specific interface are enclosed in the forwarded object. The key here is that the knowledge of the specific actions and interface do not need to be embedded in the requesting object. This allows easy changing of the actions without changing the program itself.

Issues of the Command pattern include:

• The pattern helps keep the program and the user interface (UI) objects completely separate from the actions they initiate, with the result that program objects have no knowledge of - and dependencies on - each other;
• This separation allows the actions to be postponed and 'scheduled' later;
• The Command pattern readily supports 'undo' actions, by reversing the sequence of Command objects sent, and reversing the sequence of each forwarded Command object's 'do' steps;
• Program requestors are CONTAINERS holding, spawning, and forwarding Command objects, so that the actions can be changed independently of the program's UI;
• Use of the Command pattern may result in a proliferation of many small objects; Java has a language specific remedy: Anonymous Inner Classes, which are unnamed types.

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