UML: signal classifier vs class classifier

Question

If I understand signal right this is an asynchronous message between two or more objects. For example in UML we have a signal classifier:

-----------------
|  << signal >> |
|  SomeEvent    |
-----------------
|id:Int         |
|text:String    |
-----------------
|getId()        |
|getText()      |
-----------------

Then we can write this signal in Java as following:

class SomeEvent{
  private final int id;
  private final String text;
  //+constructor + getters
}

However, in Java we have a CLASS, but in UML we have a SIGNAL classifier, but not a CLASS classifier(Update: I mean in this example). How to explain it?

Answer 1

Your discussion about UML signals is correct.

From the spec.:

10.3.3.1 Signals

A Signal is a specification of a kind of communication between objects in which a reaction is asynchronously triggered in the receiver without a reply. The receiving object handles Signals as specified by clause 13.3. The data carried by the communication are represented as attributes of the Signal. A Signal is defined independently of the Classifiers handling it.

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However, in Java we have a CLASS, but in UML we have a SIGNAL classifier, but not a CLASS classifier. How to explain it?

1. UML does have a CLASS classifier, from the spec. (emphasis mine):

11.4 Classes

... The purpose of a Class is to specify a classification of objects and to specify the Features that characterize the structure and behavior of those objects.

11.4.4 Notation

A Class is shown using the Classifier symbol. As Class is the most widely used Classifier, no keyword is needed to indicate that the metaclass is Class.

2. UML is programming-language-independent.

   In Java, UML class and signal classifiers are implemented as classes.

   Another example: UML has a interface classifier, but C++ does not have interfaces. An interface in C++ would be an abstract (pure virtual) class.

Answer 2

In fact UML is richer than Java. In UML, Signal, Class, Component, and Collaboration classifier concepts would correspond to Java Class.