Functional Programming in Java, 2nd Edition

Refactoring File Processing

Reading data from a file is a common operation in programming and I’m sure you’ve done that countless number of times. But the code to perform that operation was rather verbose and messy in the past. Let’s take a look at a function that reads the contents of a file and counts the number of occurrences of a word in it. Then we’ll refactor to use the functional style.

Let’s start with a small set of tests to verify that a countInFile function works as expected:

refactoring/fpij/WordCountTest.java

public class WordCountTest {
  WordCount wordCount;

  @BeforeEach
  public void init() {
    wordCount = new WordCount();
  }

  @Test
  public void count() {
    assertAll(
      () -> assertEquals(2,
 wordCount.countInFile( ...