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Figure 6-6. Specifications on sequence diagrams.

Objects communicate among them by sending messages. Figure 6-7 shows an example of objects sending messages to each other. A message is shown by an arrow. A message has a sender, which in the case of Figure 6-7 is object farmer, and a receiver, which is object GUI. When object farmer sends a message to object GUI, it means that farmer needs to use some of the behavior defined in object GUI.

When an object receives a message, it need some time to execute the message and send the results to the sender. The time during which an object is performing an operation is referred to as the focus of control [BRJ99]. In Figure 6-7, object GUI receives a message and it has the focus of control.

farmer: Farmer

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