java8之lambda表达式用法总结

() -> 42  //参数为空，返回一个int
(List<String> list) -> list.isEmpty() //参数为list，返回一个boolean
(int x, int y) -> x*y //参数为两个int，返回一个int
(String s) -> System.out.println(s); //参数为一个String,不返回结果
(String s) -> {System.out.println(s);} //参数为一个String，打印字符串

public void execute(Runnable r){
  r.run(); 
}
execute(() -> {}); //使用lambda，Runnable是参数为空，没有返回值的函数式接口，即() -> void 
//fetch返回一个函数式接口，() -> String
public Callable<String> fetch() { 
  return () -> "Tricky example ;-)";
}

FunctionalInterface
public interface Predicate<T> {
  //接口方法，入参为泛型T，返回bool。即：(T t) -> bool
  boolean test(T t);
  //默认方法，and操作
  default Predicate<T> and(Predicate<? super T> other) {
    Objects.requireNonNull(other);
    return (t) -> test(t) && other.test(t);
  }
  //默认方法，取反操作
  default Predicate<T> negate() {
    return (t) -> !test(t);
  }
  //默认方法，or 操作
  default Predicate<T> or(Predicate<? super T> other) {
    Objects.requireNonNull(other);
    return (t) -> test(t) || other.test(t);
  }
  //默认方法，判断是否相等
  static <T> Predicate<T> isEqual(Object targetRef) {
    return (null == targetRef)
        ? Objects::isNull
        : object -> targetRef.equals(object);
  }

Predicate<String> nonEmptyStringPredicate = (String s) -> !s.isEmpty();

@FunctionalInterface
public interface Consumer<T> {
  //接口方法，入参为泛型T，返回void。即：(T t) -> void
  void accept(T t);
  //默认方法，可以执行级联操作
  default Consumer<T> andThen(Consumer<? super T> after) {
    Objects.requireNonNull(after);
    return (T t) -> { accept(t); after.accept(t); };
  }

@FunctionalInterface
public interface Function<T, R> {
  //接口方法，入参为泛型T，返回泛型R。即：(T t) -> R
  R apply(T t);
  //默认方法，实现级联操作。before方法输入V，输出T，本function输入T，输出R。
  default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
    Objects.requireNonNull(before);
    return (V v) -> apply(before.apply(v));
  }
  //默认方法，级联操作
  default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
    Objects.requireNonNull(after);
    return (T t) -> after.apply(apply(t));
  }
  //默认方法，输入啥，输出啥
  static <T> Function<T, T> identity() {
    return t -> t;
  }

// Predicate返回了一个boolean 
Predicate<String> p = s -> list.add(s); 
// Consumer返回了一个void 
Consumer<String> b = s -> list.add(s);

(Apple a) -> a.getWeight() 等价于Apple::getWeight 
() -> Thread.currentThread().dumpStack() 等价于 Thread.currentThread()::dumpStack
(str, i) -> str.substring(i) 等价于 String::substring
(String s) -> System.out.println(s) 等价于 System.out::println
(list, element) -> list.contains(element) 等价于 List::contains

Supplier<User> c1 = User::new; // c1 = () -> new User();
User user = c1.get();

Function<Long, User> c2 = User::new;

User user = c2.apply(110L);

@Data
@AllArgsConstructor
public class User {
  private String name;
  private Long userId;
  private Integer age;
}
@FunctionalInterface
public interface TriFunction<T,U,V,R> {
  R create(T t, U u, V v);
}
public static void main(String[] args) {
  TriFunction<String, Long, Integer, User> triFunction = User::new;
  User user = triFunction.create("tina", 12L, 13);
}