Static Factory Method Pattern

Although Java's new C(...) syntax for creating new objects is straightforward, it causes problems when used indiscriminately in large programs.

In Java, that syntax requires the creating method to know the name of the class for that new object. That may sound reasonable for simple data types such as UTC, but what if there are two or more classes that implement the UTC interface? What if choosing which class to use is complicated?

One of the classes that implements the UTC interface might be preferred during the early phases of a program's execution, but a different representation might be preferred during later phases. If users of the UTC abstract data type create new values of that type using the new operator, then every use of that operator would need to be augmented by conditional code that determines the current phase of the program before deciding whether to create the new value via new UTC1(...) or new UTC2(...).

To give another example, suppose it becomes necessary to improve the efficiency of a program, and someone concludes its efficiency could be improved by creating at most one UTC object for any given combination of hour and minute. Then every occurrence of new UTC1(h,m) would have to be replaced by a fairly complicated computation that determines whether a UTC with hour and minute components equal to h and m is already available. If so, it would use that previously created UTC. If not, it would create a new UTC and enter that new UTC into a database of available UTC values.

The computations implied by those two examples are too complicated to be written out in full everywhere a new UTC value is needed. Writing those computations out in many different places would violate the programming principle that says Don't repeat yourself.

Static factory methods solve the problem. Instead of writing out the logic needed to decide whether it is necessary to create a brand new object, and to decide which representation to use for that object if a new object is needed, we can define a single static factory method that encapsulates the necessary logic. Everyone who needs to create new values of that data type can then call the static factory method instead of writing code that contains explicit uses of the new syntax.