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Change Value to Reference

Problem

So you have many identical instances of a single class that you need to replace with a single object.

Solution

Convert the identical objects to a single reference object.

Before
Change Value to Reference - Before
After
Change Value to Reference - After

Why Refactor

In many systems, objects can be classified as either values or references.

  • References: when one real-world object corresponds to only one object in the program. References are usually user/order/product/etc. objects.

  • Values: one real-world object corresponds to multiple objects in the program. These objects could be dates, phone numbers, addresses, colors, and the like.

The selection of reference vs. value isn't always clear-cut. Sometimes there's a simple value with a small amount of unchanging data. Then it becomes necessary to add changeable data and pass these changes every time the object is accessed. In this case it becomes necessary to convert it to a reference.

Benefits

  • An object contains all the most current information about a particular entity. If the object is changed in one part of the program, these changes are accessible from the other parts of the program that make use of the object.

Drawbacks

  • References are much harder to implement.

How to Refactor

  1. Use Replace Constructor with Factory Method on the class from which the references are to be generated.

  2. Determine which object will be responsible for providing access to references. Instead of creating a new object, when you need one you now need to get it from a storage object or static dictionary field.

  3. Determine whether references will be created in advance or dynamically as necessary. If objects are created in advance, make sure to load them before use.

  4. Change the factory method so that it returns a reference. If objects are created in advance, decide how to handle errors when a non-existent object is requested. You may also need to use Rename Method to inform that the method returns only existing objects.