什么是 lambda 和函数式接口？

// ❌ verbose anonymous class (pre-Java 8)
Calculator add = new Calculator() {
    public int calculate(int a, int b) { return a + b; }
};

// ✅ lambda — same thing, far less boilerplate
Calculator add = (a, b) -> a + b;
Calculator multiply = (a, b) -> a * b;
add.calculate(2, 3);   // 5

Function<String, Integer> length = s -> s.length();          // takes T, returns R
Predicate<Integer> isPositive = n -> n > 0;                   // takes T, returns boolean
Consumer<String> printer = s -> System.out.println(s);        // takes T, returns void
Supplier<String> greeting = () -> "Hello";                     // takes nothing, returns T
BiFunction<Integer, Integer, Integer> sum = (a, b) -> a + b;  // two args

// when a lambda just calls an existing method, use a method reference
list.forEach(System.out::println);          // instead of x -> System.out.println(x)
list.stream().map(String::toUpperCase);      // instead of s -> s.toUpperCase()
list.stream().map(Integer::parseInt);        // static method ref

// passing behavior as arguments — the core enabler of the Stream API
people.stream()
    .filter(p -> p.getAge() > 18)            // Predicate lambda
    .map(Person::getName)                     // method reference
    .forEach(System.out::println);            // Consumer

list.sort((a, b) -> a.compareTo(b));          // Comparator lambda
button.addActionListener(e -> handleClick()); // event handler