Design Patterns and Best Practices in Java

Pure functions

A pure function is a function that has no side effects and its output is the same for the same input (predictable and cacheable). A side effect is an action that modifies the outside context of the function. Examples of this include to the following:

• Writing to a file/console/network/screen
• Modifying an outside variable/state
• Calling a non-pure function
• Starting a process

Side effects are sometimes unavoidable or even desirable—I/O or low-level operations are examples of code with side effects (von Neumann machines work because of side effects). As a rule of thumb, try to isolate the functions with side effects from the rest of the code. Haskell and other functional programming languages use monads for the task. We will have ...