Testing a method that takes a lambda as a parameter can be accomplished by passing different lambdas to this method. For example, let's assume that we have the following functional interface:

@FunctionalInterface
public interface Replacer<String> {
  String replace(String s);
}

Let's also assume that we have a method that takes lambdas of the String -> String type, as follows:

public static List<String> replace(
    List<String> list, Replacer<String> r) {

  List<String> result = new ArrayList<>();
  for (String s: list) {
    result.add(r.replace(s));
  }

  return result;
}

Now, let's write a JUnit test for this method using two lambdas:

@Test
public void testReplacer() throws Exception {

  List<String> names = Arrays.asList(
    "Ann a 15", "Mir el 28", "D oru 33");

  List<String> resultWs = replace(
    names, (String s) -> s.replaceAll("\s", ""));
  List<String> resultNr = replace(
    names, (String s) -> s.replaceAll("\d", ""));

  assertEquals(Arrays.asList(
    "Anna15", "Mirel28", "Doru33"), resultWs);
  assertEquals(Arrays.asList(
    "Ann a ", "Mir el ", "D oru "), resultNr);
}