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What Is a State Diagram?

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  • Written By: Andrew Kirmayer
  • Edited By: Shereen Skola
  • Last Modified Date: 21 November 2016
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    Conjecture Corporation
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A state diagram is an abstract representation of a process. It is often used in computer science to model the flow of programming languages, and can help analysts map business processes as well. Elements of a system are generally called objects, which can undergo changes in state; these transitions in the Unified Modeling Language (UML) are tracked throughout the chart, also called a UML state machine diagram. Typically used to describe the behavior of a system in general, a state diagram helps to track objects using different symbols. It does not usually describe the interaction of objects.

The UML diagrams typically illustrate how an object behaves under different use cases, and a few symbols are often used to identify different elements. Generally similar to a flowchart, the state diagram usually has a large dot at the top to represent the initial state of an object. Changes in state can be shown in rounded boxes, with the name of the state, variables, and actions separated in a stacked format. Horizontal lines are typically used to separate each.

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Straight lines outside of the boxes in a state diagram can connect to different elements of the diagram. The lines usually define transitions, and often feature arrows at one end to mark the path of one state to another. At the bottom of the diagram is typically a large black dot surrounded by a circle. An entire chart can describe a complex series of events and at what states they can occur in, which are often more than one.

The flow of a state diagram is typically determined by the transitions that occur. Some object states can convert to a redundant one; sometimes multiple transitions can lead to the same state which can be confusing. In this case, more than one box can be enclosed in another, which is usually called a super-state. This format can make a state diagram easier to read if the events and transitions in a system are complex.

A state diagram can represent computational models of machine operations or the function of many machines in a production system. It can also help college or seminar instructors map out their curriculum based on the students who have enrolled. Semantics, or rules, are often applied to state diagrams. There are alternative rules, and even variant patterns that can be used depending on the system, such as a production process for an electronics device like a stopwatch or controller.

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