Monday, 3 October 2016

Constant Object Anti Pattern

Most constants are used to remove magic numbers or variables that lack context. A classic example would be code littered with the number 7. What does this refer to exactly? If this was replaced with DaysInWeek or similar, much clarity is provided. You can determine that code performing offsets would be adding days, rather than a mysterious number seven.

Sadly a common pattern which uses constants is the use of a single constant file or object.

The beauty of constants is clarity, and the obvious fact such variables are fixed. When a constant container is used, constants are simply lumped together. These can grow in size and often become a dumping ground for all values within the application.

A disadvantage of this pattern is the actual value is hidden. While a friendly variable name is great, there will come a time where you will want to know the actual value. This forces you to navigate, if only to peek at the value within the constant object. A solution is to simple perform a refactor to move the variable closer to use. If this is within a single method, let the constant live within the method. If a class, let the constant live at a field level. Finally if the constant is used across multiple classes, find a shared home and rely on a well thought out namespace.

A similar issue regarding constants is the use of configuration files or similar to set the values. While the const keyword is dropped in this case, the object performs the same role. A public key, followed by a value that is used. The anti pattern in this case is treating all values as requiring configuration. Unless you need to change such values at runtime or based on deployment models, inline constants are much preferred. Literal values, mainly strings can often be left inline with limited downsides also. For example, a fixed, relative file path is much better left inline. If you are worried about lack of context, then the use of named arguments can help.

Lessons

  • Keep constants local to methods, or classes.
  • Avoid constant objects or files as these will become bloated and lack context.
  • Only introduce configuration for aspects that need or will change. Defer second guessing.
  • Use named arguments to add context for inline variables.

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