In addition to support for the new Java date/time API, iCal4j v4 will also aim to improve support for Functional Programming patterns and features included since Java 8.

Immutable Collections

In earlier releases we introduced custom collections (i.e. lists) for parameters, properties and components. The primary purpose was to support custom string representations and other convenience methods.

To improve support for Java 8 features like Lambda functions and streaming APIs, the following changes to these collections will be introduced in iCal4j v4:

  • Lists will no longer inherit from ArrayList - instead a new method getAll() provides access the underlying collection
  • The underlying collections are now “Unmodifiable” - any mutation operations will now return a new instance of the list instead of modifying inline
  • Convenience methods on property, component and calendar classes replace direct collection mutation

So as an example, thia approach to modify property parameters with earlier iCal4j versions will now fail:

property.getParameters().add(newParam); // throws UnsupportedOperationException

Instead, this can be replaced with:

property.add(newParam); // more concise and Thread-safe

Also, accessor methods have been deprecated in favour of new methods added to collection classes:

property.getParameter(Parameter.VALUE); // old method: now Deprecated
property.getParameters().getFirst(Parameter.VALUE); // new method: improves consistency of APIs

Similar methods are also available on calendar and component classes.

Deep vs Shallow Copies

Often when modifying hierarchical data structures a deep copy is required to create new instances that don’t impact the original instances.

A common way to create a deep copy of a component, property or parameter is to use the corresponding copy() method. In previous versions the implementations of this method haven’t always been a true deep copy, and could result in shared references across multiple instances.

With iCal4j v4 the deep copy implementation will ensure copies cannot impact the source instance, improving the support for FP patterns.

For example, with iCal4j v3:

VEvent eventCopy = event.copy(); // creates a new instance with a new property list and new alarm list

The concern here is that whilst the underlying collections have been replicated, the property and alarm instances contained in them are not. This can be problematic if the properties in the copy are modified, as changes will also reflect in the source instance.

With iCal4j v4:

VEvent eventCopy = event.copy(); // creates a new instance with a new property list and new alarm list

Whilst the method signature is the same, underlying properties and alarms will also be copied to construct the new instance. So any changes to properties and alarms in the copy won’t be reflected in the source instance.

Sometimes however a shallow copy is sufficient and provides improved performance, especially with large data structures.

As a general rule, a shallow copy of iCal4j content is sufficient when mutation occurs only via direct mutator methods.

VEvent copy = new VEvent(event.getProperties());
copy.replace(new Summary("updated summary")); // safe

copy.getProperties().getRequired(DTSTART).replace(new TzId("Australia/Melbourne")); // not safe!!

DtStart dtstart = new DtStart(copy.getProperties().getRequired(DTSTART).getParameters());
dtstart.replace(new TzId("Australia/Melbourne")); // safe

The above example can be written slightly more succinctly using the copy() method:

DtStart dtstart = copy.getProperties().getRequired(DTSTART).copy();
dtstart.replace(new TzId("Australia/Melbourne")); // safe

Practical Examples

In practice these changes will have the greatest impact on how object instances are retrieved, copied and modified. The following examples demonstrate some of the most significant changes.

Change VEVENT summary

calendar.getComponents().getRequired(VEVENT).replace(new Summary("updated summary"));

The above example retrieves the first VEVENT component from the calendar and creates a new property list instance including the new summary and other existing properties.

Change DTSTART timezone

calendar.getComponents().getRequired(VEVENT).getProperties().getRequired(DTSTART).replace(new TzId("Australia/Melbourne"));

This example will create a new parameter list for the DTSTART property that includes the new timezone identifier.

These changes are available from iCal4j Alpha4. We welcome any feedback and bug reports via the Github issues page.