Pragmatic Unit Testing in Java with JUnit, 3rd Edition

Verifying Side Effects

An impure function creates side effects. The prototypical side effect: you call a void method that changes the value of one or more fields in the containing object. The Location class does just that in its move method (highlighted):

utj3-units/02/src/main/java/units/Location.java

import java.util.Objects;

public class Location {
   enum Heading {North, East, South, West}
   private int x, y;
   private Heading heading;

   public Location(int x, int y, Heading heading) {
      this.x = x;
      this.y = y;
      this.heading = heading;
   }

   public void move(int distance) {
      switch (heading) {
         case North -> y = y + distance;
         case East -> x = x + distance;
         case South -> y = y - distance; ...