Functional Programming in Java, 2nd Edition

Refactoring the Traditional for Loop

Existing codebases are littered with for loops. We use them to iterate over collections of data but also over a range of indexes. To illustrate how we can refactor the traditional imperative for loop into functional style code, we’ll look at using a for loop to compute the factorial of numbers in a range. But, of course, we’ll start with a test to provide the safety net we just discussed.

refactoring/fpij/FactorialTest.java

public class FactorialTest {
  Factorial factorial;

  @BeforeEach
  public void init() {
    factorial = new Factorial();
  }

  @Test
  public void computeFactorial() {
    assertAll(
      () -> assertEquals(BigInteger.ONE, factorial.compute(1)),
 () -> assertEquals(BigInteger.TWO, ...