Lambdas and the Stream API

Lambdas, the Stream API and Optional in tests

A lambda is a short way to write an implementation of a functional interface (an interface with a single abstract method). The most common ones:

• Predicate<T> — takes a value, returns boolean (filter);
• Function<T,R> — transforms a value (map);
• Consumer<T> — takes a value, returns nothing (forEach);
• Supplier<T> — takes nothing, returns a value (orElseGet);
• Runnable, Callable<V> — an action without arguments.

A method reference shortens a lambda: WebElement::getText instead of e -> e.getText(). A lambda sees only effectively final local variables, so a counter cannot be changed inside it (use AtomicInteger or count()).

Selenium waits are lambdas too: wait.until(d -> d.findElement(By.id("ready")).isDisplayed()), because ExpectedCondition<T> extends Function<WebDriver, T>.

List<String> titles = rows.stream()
    .filter(WebElement::isDisplayed)
    .map(WebElement::getText)
    .filter(t -> !t.isBlank())
    .toList();             // Java 16+, an unmodifiable list

A Stream is a pipeline: a source → intermediate operations (filter, map, sorted, distinct), which are lazy and do nothing by themselves, → a terminal operation (toList, collect, count, anyMatch, findFirst, forEach) that triggers the computation. A stream can be used only once. Typical tasks: check that a list is sorted, that all elements satisfy a condition (allMatch), group (Collectors.groupingBy). Avoid parallelStream() in tests with a shared driver or state.

Optional is a "value may be absent" container for a method result (findFirst). Extract with orElseThrow() and a clear message, orElse, orElseGet, ifPresent. get() without a check throws NoSuchElementException. Do not use Optional as a field or parameter.