Refactoring: Convert static into instance

You want to ensure reusing functionality does not have any other side effects in other places that might be using it.

Make the static instance explicit by introduce a static instance encapsulating any static state and delegate items to it. Use the instance.

Imagine you had this sort of code (below) and there’s dozens of use throughout a codebase. Static imports make it easy for people to simply increment a count, but in order to use the Counter, you have no assurance nothing else will try to use it.

public class Counter {
  static int totalCount;

  public static void count() {
    totalCount++;
  }
}

The following transformations will help you keep existing clients happy:

• Introduce new method (instance level)
• Introduce global instance
• Delegate static method to new method (instance level)
• Turn static state into instance state

The final result is below (note that this code is not thread-safe and not designed for concurrent programs, but is there to demonstrate the result of the refactoring steps):

public class Counter {
   private static final Counter INSTANCE = new Counter();

   private int totalCount;

   public static void count() {
     INSTANCE.recordCount();
   }

   public void recordCount() {
     totalCount++;
   }
}

Now we had a bit more of a complex example, but this refactoring helped us move to the right direction of using instances where possible, instead of sharing global state.