Construct with Collaborators, Call with Work

This article was adapted from a Google Tech on the Toilet (TotT) episode. You can download a printer-friendly version of this TotT episode and post it in your office.

By Shahar Roth

Classes require various objects and parameters to function. The “Construct with Collaborators, Call with Work” guideline can help you construct effective inputs:

• Use the constructor for collaborators—the dependencies that establish the object’s identity. Collaborators stay with the object for its lifetime to enable it to fulfill its ongoing duties.
• Pass work—the parameters that change with each interaction—to methods. Unique to each call, these inputs provide the specific data needed for an operation such as a file path or database query.

Consider a ReportGenerator that needs a database, a formatter, and a date range to generate a report. The database and formatter, as collaborators, are injected via the constructor, while dateRange, which varies per report generation, is passed as a method parameter to the generate method:

A single ReportGenerator object can generate multiple reports with different date ranges:

Following the “Construct with Collaborators, Call with Work” guideline promotes:

• Reusability: Enables instances to be used for multiple, distinct operations.
• Testability: Separates dependency setup from business logic.
• Cleaner code: Hides implementation dependencies from the object’s users.
• Predictable behavior: Locks in dependencies at creation time.

Note that the definition of “collaborator” versus “work” depends on the object’s identity. For example, a RequestMessage could be a collaborator for a RequestHandler if the handler operates on a single request, or work if the handler processes different requests with each method call.