Temporal Patterns, source code from http://martinfowler.com/eaaDev/timeNarrative.html

snapshot.java

class Customer...
  private TemporalCollection addresses = new SingleTemporalCollection();
  public Address getAddress(MfDate date) {
    return (Address) addresses.get(date);
  }
  public Address getAddress() {
    return getAddress(MfDate.today());
  }
  public void putAddress(MfDate date, Address value) {
    addresses.put(date, value);
  }
class CustomerSnapshot...
  private Customer base;
  private MfDate validDate;
  public CustomerSnapshot (Customer base, MfDate validDate) {
    this.base = base;
    this.validDate = validDate;
  }
  public Address getAddress() {
    return base.getAddress(validDate);
  }