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UML knows two communication forms between objects: One form lets an object
invoke an operation at another object. This means that the first object determines
the behavior the second object should execute. In the second form, one object
sends a signal to the other object.
The most important difference between these two forms is that, though both
forms transmit information, the second form lets the receiver object decide how
it will respond. When an operation is invoked, then the sender decides on the
behavior to be executed.
The signal is a type and is written in a notation similar to the one for classes. In
addition, the keyword ?«signal?» is denoted above the signal ??™ s name ( Figure 3.21 ). A
signal has only attributes; it does not normally have operations.
9 Within the ball-and-socket notation , the ball by itself is commonly referred to as lollipop notation .
3.3 The Class Diagram
FIGURE 3-20
Example for an interface realization.
class Interface realization CarKey
?«signal?» Key(removed:boolean)
?«signal?» Key(removed:boolean)
?«interface?»
CarKey
On-board computer control
On-board computer
control
class Interface realization CarKey (alternative)
KeyDeposit
KeyDeposit
CarKey
CarKey
Defi nition
A signal describes the structure of a communication object.
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