Iterator

Intent

Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

Class diagram of the original GoF pattern

Improvement in MixJuice

Solved Problem(s)

Information hiding problem

Internal state of Aggregate should be accessible from Iterator but hidden from clients. (This problem is mentioned at p.262,line.18-23 in the GoF book.) Note that this problem is also solved by the package/nested-class mechanisms of Java.

Used programming technique(s)

Namespace separation of ConcreteAggregate and ConcreteIterator.

Consequences

• (+) Internal state of Aggregate are hidden from clients.

Structure and pseudo code

module aggregate {
  define class Aggregate {
    define abstract Iterator createIterator();
  }

  define class Iterator {
    define abstract void first();
    define abstract void next();
    define abstract boolean isDone();
    define abstract Object currentItem();
  }

  define class ConcreteAggregate extends Aggregate {
  }
  define class ConcreteIterator extends Iterator {
  }
}

module aggregate.implementation extends aggregate {
  class ConcreteAggregate {
    Iterator createIterator() {...}
  }
  class ConcreteIterator {
    // accessible to ConcreteAggregate directly.
    void first() {...}
    void next() {...}
    boolean isDone() {...}
    Object currentItem() {...}
  }
}

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