Functional Programming in Java, 2nd Edition

Keep Separate Conditions in Separate Filters

A common question I hear from developers is "should I write multiple filters with one condition each or one filter with multiple conditions?" The short and quick answer is to use multiple filter calls with one condition each.

Consider the following code:

drivers.stream()
  .filter(driver -> driver.getAge() > 21 && driver.isDriversLicenseValid())
  //Please don't code like the previous line
  .map(driver -> driver.getPrimaryCar())
  .map(car -> car.getRegistration())
  .forEach(registration -> System.out.println(registration));

The lambda expression passed to the filter method does two separate checks: if the driver is more than 21 years old and if the driver has a valid driver’s license. Two conditions ...