Lambdas and functional interfaces

// Before Java 8: an anonymous class
Comparator<String> byLength = new Comparator<String>() {
    @Override
    public int compare(String a, String b) {
        return Integer.compare(a.length(), b.length());
    }
};

// Java 8+: a lambda
Comparator<String> byLength2 = (a, b) -> Integer.compare(a.length(), b.length());

Runnable hello = () -> System.out.println("Namaste");           // no parameters
Function<Integer, Integer> square = x -> x * x;                  // one parameter: brackets optional
BinaryOperator<Integer> add = (a, b) -> a + b;                   // two parameters
Function<Double, Double> withGst = amount -> {                   // a block body needs return
    double tax = amount * 0.18;
    return amount + tax;
};

Predicate<String> isValidPin = pin -> pin.matches("[1-9][0-9]{5}");
System.out.println(isValidPin.test("560001"));     // true
Predicate<String> notBlank = s -> !s.isBlank();
Predicate<String> both = isValidPin.and(notBlank);  // predicates combine with and, or, negate
Function<Integer, Integer> plusGst = p -> p + p * 18 / 100;
Function<Integer, String> label = p -> "Rs " + p;
System.out.println(plusGst.andThen(label).apply(100));   // Rs 118

double rate = 18;
Function<Double, Double> tax = amt -> amt * rate / 100;   // OK: rate is never changed
// rate = 5;                                              // would make the lambda above a compile error

int count = 0;
// list.forEach(x -> count++);   // compile error: count must be effectively final
// Use a stream count(), or an AtomicInteger, instead.