From diagram to code

Turning a class diagram into Java code: visibilities, attributes, an association becoming an attribute or a collection depending on the multiplicity, navigability, generalisation, realisation and composition.

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4 topics
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Translating a visibility: what is it?
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+ becomes public, - becomes private, # becomes protected, and ~ has no keyword: you write nothing.
Java's default level, with no keyword, is exactly package visibility.
In the diagram, Equipe is joined to Joueur by an association navigable from Equipe to Joueur, with 0..* on the Joueur side. Which declaration goes in Equipe?
- private List<Joueur> joueurs = new ArrayList<>();
- private Joueur joueur;
- private List<Equipe> joueurs = new ArrayList<>();
- private static Joueur joueurs;
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private List<Joueur> joueurs = new ArrayList<>();
Zero or more players: the upper bound exceeds 1, so a collection of Joueur, created empty so it is usable. A single reference can only hold one, and static would share the attribute across all teams.
An underlined member translates to …; an italic member, to ….
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An underlined member translates to static; an italic member, to abstract.
An abstract member forces its class to be abstract too: the class name itself goes italic in the box.
Navigability: what is it?
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The direction in which one object can reach the other, shown by an open arrow at the end of the line; only the starting object keeps a reference.
Order → Line gives an attribute in Order and none in Line. With no arrow, the direction is left open, or both sides get an attribute.
An association is navigable from Commande to Ligne only. Why does only Commande get an attribute?
- The arrow says which class keeps a reference to the other
- Java forbids two classes from referencing each other
- An attribute placed in Ligne would have to be public
- An attribute in Ligne would cause an endless loop at construction
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The arrow says which class keeps a reference to the other
A reference is only worth keeping if the code needs it. Adding an attribute in Ligne would create a useless dependency and two references to keep consistent; two classes can of course reference each other when you decide so.

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