This series walks you through Unified Modeling Language (UML) design and notation for Java-based application development. This article introduces one of the building blocks of the Unified Modeling Language: sequence diagramming. In the UML, sequence diagrams are used throughout the design process to demonstrate the internal interactions between actors and objects as a system executes over time. Sequence diagrams are used to model the use case scenarios, protocols in a framework, subsystems, classes and method logic.