Java function函数式接口的使用方法与实例

@FunctionalInterface
public interface Function<T, R> {
    R apply(T t); // 如同细胞核中的DNA链
}

Function<Integer, Integer> square = x -> x * x;
Function<Integer, String> toString = Object::toString;
// 函数组合：数学的复合函数f(g(x))
Function<Integer, String> pipeline = square.andThen(toString); 
// 输出："25"
System.out.println(pipeline.apply(5));

Predicate<String> isLong = s -> s.length() > 5;
Predicate<String> containsA = s -> s.contains("a");
List<String> words = Arrays.asList("lambda", "stream", "function");
words.stream()
.filter(isLong.and(containsA.negate())) // 长度>5且不含a
.forEach(System.out::println); // 输出：function

BiFunction<Integer, Integer, Double> hypotenuse = 
(a, b) -> Math.sqrt(a*a + b*b);
// 输出：5.0
System.out.println(hypotenuse.apply(3, 4));

interface ValidationStrategy {
    boolean execute(String s);
}
class LengthStrategy implements ValidationStrategy {
    public boolean execute(String s) {
        return s.length() > 8;
    }
}

Predicate<String> lengthStrategy = s -> s.length() > 8;
Predicate<String> digitStrategy = s -> s.matches(".*\\d.*");

button.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        handleClick();
    }
});

List<Integer> shadowList = new ArrayList<>();
Consumer<Integer> riskyConsumer = num -> {
    shadowList.add(num); // 修改外部状态
    System.out.println(num * 2);
};
IntStream.range(1,5).forEach(riskyConsumer::accept);

Function<String, Integer> parser = Integer::parseInt;
Object funcObj = parser;
// 编译通过但运行时抛出ClassCastException
Function<Date, String> dangerous = (Function<Date, String>) funcObj;

record Point(int x, int y) {}
Function<Point, String> pointDesc = p -> 
    String.format("(%d,%d)", p.x(), p.y());

Object obj = "function";
String result = switch(obj) {
    case String s when Predicate.not(String::isEmpty).test(s) -> 
    "Non-empty string";
    default -> "Other";
};