A UML activity diagram shows what happens during a process or an operation. Activity diagrams are similar to flow charts so if you've worked with flow charts in the past you already have the basic idea. You might use an activity diagram to explore a business process, a software process, what happens in an individual use case or across several use cases. We have here an example of a very simple activity diagram so let's look at the various parts that go into it. Your starting point is represented as a filled circle and that's called the initial node, now an activity is made up of a series of actions. An activity is a process and each step in the process is an action and in UML those individual actions are represented in this sort of rounded rectangle shape. Connecting the various actions and the other elements are these arrows. Notice the arrows show the direction of the flow, which step in the process comes next. And those arrows are called control flows, you might hear them called just flows or you might hear them called edges as well. Now in any process you're going to come to various decision points where you have to decide which path to take. These are represented by a diamond shape, that's called a decision node and a decision node simply represents a branching point. The path depends on a decision that's made right here. Notice that each flow that comes out of that decision node has a guard condition attached to it and a guard condition basically is true or false. If it's true you can continue along this path, if it's not true you can't continue along this path and you have to take another path. Guard conditions as you see are always shown in square brackets. So we have a branching point here with the decision node and we have two different alternative paths that you might take at this point. They come back together in another diamond and that's called a merge. A number of different flows can all lead to the same action and the best way to show that with UML is to show them merging and then from that merge point moving on to the next action. Finally we have the end point which is called the final node and that shows where your process ends. So that's the concept, let's look at an example, fill in this chart with some specific actions so we can show a process. So let's say our process is going out on Saturday night and action one we'll say is pick up date, whatever you're doing on Saturday night that's the first step. Now we come to a decision point, and we'll say the decision point is whether you've got money or whether you've got no money. If you've got money you can take various actions, go to a restaurant and then maybe from there go to the movies. If you've got no money you've got to take a different course of action so we'll go for a walk. Nice cheap date. So you see the different alternative paths, if this condition having money is true you can do these activities, if this condition, no money is true you do this activity. We'll say it was a good night and in either case you come to the merge point and the final action of your Saturday night is get a good night kiss. So whichever path you take you merge and come back here and then that's the end of your date. So based on the symbols we've just learned you should be able to read this activity diagram with no trouble. What we have here is a process for human resources department, manage employee and we start up here up at the top with our initial node and here's what we're doing, manage employee. Now when you want to manage an employee you immediately come to a decision point, are we talking about a new employee or are we talking about an existing employee? If we're talking about a new employee we need to create new employee record. If we're talking about an existing employee immediately we come to another decision point, is that employee still employed or no longer employed? If the employee is still employed we modify the employee record, if the employee is no longer employed then we terminate the employee. Now whichever one of these actions you take we come here to a merge point. At a merge point any one of the actions leading to that merge point can move you along to the next step. So if we create a new employee record we come to the merge point, we move on to the next step, update records and then we're done. Now the diagrams that we've been looking at so far have been vertically oriented. If you like you can also use a horizontal orientation to convey the same information. Here's an example this is the same activity diagram we just looked at except its laid out horizontally. So starting with your initial node you have manage employee as your first step, your decision point going off in the same two directions, another decision point again going off in the same two directions, the merge point coming together with update records and the end point there. So its entirely a matter of your preference whether you start at the top and go down for vertically orientation or whether you start at the left and move right for horizontal orientation.
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