SE450: Basics: Multiplicity [43/63] Previous pageContentsNext page

Multiplicities put constraints on number of references held by an object.

Recall java.util.List and java.io.BufferedReader. 

   +---------------+
   | <<interface>> |       * +--------+
   |     List      |<>------>| Object |
   +---------------+         +--------+

   +----------------+   0..1 +--------+
   | BufferedReader |<>----->| Reader |
   +----------------+        +--------+

Note: if mutiplicity is 1, then reference must be non-null when constructor terminates.

This may require that we throw and IllegalArgumentException if an argument to the constructor is null.

file:Main.java [source] [doc-public] [doc-private]
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package basics.checkargs;
public class Main {
  private Main() {}
  static public void main (final String[] args) {
    //stdlib.Trace.graphvizShowSteps (true); stdlib.Trace.run ();
    try {
      System.out.println(new Person("bob"));
      System.out.println(new Person(null));
    } catch (IllegalArgumentException e) {
      System.out.println("Error creating Person: " + e); 
    }
  }
}
final class Person {
  final private String name;
  public Person(String name) {
    if (name == null)
      throw new IllegalArgumentException("null name");
    this.name = name;
  }
  public String toString() { return "Person(" + name + ")"; };
}

   +--------+   name  1 +--------+
   | Person |<>-------->| String |
   +--------+           +--------+

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