What is a functional interface in Java 8? Explain with examples.

@FunctionalInterface  // optional but recommended — compiler enforces 1 method
public interface Greeting {
    void greet(String name);  // single abstract method
    // void anotherMethod();  // ❌ would make it non-functional
}

// Lambda implements the interface
Greeting g = name -> System.out.println("Hello, " + name + "!");
g.greet("Alice");  // "Hello, Alice!"

Predicate<String> isLong  = s -> s.length() > 5;
Predicate<Integer> isEven = n -> n % 2 == 0;

System.out.println(isLong.test("Hello"));   // false
System.out.println(isLong.test("Welcome")); // true

// Combine predicates
Predicate<Integer> isPositiveEven = isEven.and(n -> n > 0);
Predicate<String> isLongOrEmpty = isLong.or(String::isEmpty);
Predicate<Integer> isOdd = isEven.negate();

Function<String, Integer> strLen = String::length;
Function<Integer, String> intToStr = n -> "Number: " + n;

System.out.println(strLen.apply("Hello"));  // 5
System.out.println(intToStr.apply(42));     // "Number: 42"

// Chain functions
Function<String, String> upper = String::toUpperCase;
Function<String, String> exclaim = s -> s + "!";
Function<String, String> shout = upper.andThen(exclaim);
System.out.println(shout.apply("hello")); // "HELLO!"

Consumer<String> print   = System.out::println;
Consumer<String> logInfo = msg -> System.out.println("[INFO] " + msg);

print.accept("Test passed");
logInfo.accept("Login successful");

// andThen — chain consumers
Consumer<String> printAndLog = print.andThen(logInfo);
printAndLog.accept("Hello"); // prints then logs

Supplier<String> greeting   = () -> "Good morning!";
Supplier<Double> randomNum  = Math::random;
Supplier<List<String>> newList = ArrayList::new;

System.out.println(greeting.get());   // "Good morning!"
System.out.println(randomNum.get());  // 0.7823...

BiFunction<String, Integer, String> repeat = (s, n) -> s.repeat(n);
System.out.println(repeat.apply("Ha", 3)); // "HaHaHa"

@FunctionalInterface
public interface TriFunction<A, B, C, R> {
    R apply(A a, B b, C c);
}

TriFunction<Integer, Integer, Integer, Integer> sum = (a, b, c) -> a + b + c;
System.out.println(sum.apply(1, 2, 3)); // 6

// Define custom functional interface for wait conditions
@FunctionalInterface
interface PageCondition {
    boolean isMet(WebDriver driver);
}

public void waitFor(PageCondition condition) {
    new FluentWait<>(driver)
        .withTimeout(Duration.ofSeconds(10))
        .pollingEvery(Duration.ofMillis(500))
        .until(condition::isMet);
}

// Usage
waitFor(driver -> driver.findElement(By.id("result")).isDisplayed());
waitFor(driver -> driver.getTitle().equals("Dashboard"));