@FunctionalInterface
public interface Function<T, R> {R apply(T t);default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {Objects.requireNonNull(before);return (V v) -> apply(before.apply(v));} default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {Objects.requireNonNull(after);return (T t) -> after.apply(apply(t));} static <T> Function<T, T> identity() {return t -> t;}
}

package aaa;import java.util.NoSuchElementException;
import java.util.Optional;public class OptionalDemo {public static void main(String[] args) {//创建Optional实例，也可以通过方法返回值得到。Optional<String> name = Optional.of("Sanaulla");//创建没有值的Optional实例，例如值为'null'Optional<Object> empty = Optional.ofNullable(null);//isPresent方法用来检查Optional实例是否有值。if (name.isPresent()) {//调用get()返回Optional值。
          System.out.println(name.get());}try {//在Optional实例上调用get()抛出NoSuchElementException。
          System.out.println(empty.get());} catch (NoSuchElementException ex) {System.out.println(ex.getMessage());}//ifPresent方法接受lambda表达式参数。//如果Optional值不为空，lambda表达式会处理并在其上执行操作。name.ifPresent((value) -> {System.out.println("The length of the value is: " + value.length());});//如果有值orElse方法会返回Optional实例，否则返回传入的错误信息。System.out.println(empty.orElse("There is no value present!"));System.out.println(name.orElse("There is some value!"));//orElseGet与orElse类似，区别在于传入的默认值。//orElseGet接受lambda表达式生成默认值。System.out.println(empty.orElseGet(() -> "Default Value"));System.out.println(name.orElseGet(() -> "Default Value"));try {//orElseThrow与orElse方法类似，区别在于返回值。//orElseThrow抛出由传入的lambda表达式/方法生成异常。empty.orElseThrow(Exception::new);} catch (Throwable ex) {System.out.println(ex.getMessage());}//map方法通过传入的lambda表达式修改Optonal实例默认值。 //lambda表达式返回值会包装为Optional实例。Optional<String> upperName = name.map((value) -> value.toUpperCase());System.out.println(upperName.orElse("No value found"));//flatMap与map（Funtion）非常相似，区别在于lambda表达式的返回值。//map方法的lambda表达式返回值可以是任何类型，但是返回值会包装成Optional实例。//但是flatMap方法的lambda返回值总是Optional类型。upperName = name.flatMap((value) -> Optional.of(value.toUpperCase()));System.out.println(upperName.orElse("No value found"));//filter方法检查Optiona值是否满足给定条件。//如果满足返回Optional实例值，否则返回空Optional。Optional<String> longName = name.filter((value) -> value.length() > 6);System.out.println(longName.orElse("The name is less than 6 characters"));//另一个示例，Optional值不满足给定条件。Optional<String> anotherName = Optional.of("Sana");Optional<String> shortName = anotherName.filter((value) -> value.length() > 6);System.out.println(shortName.orElse("The name is less than 6 characters"));}}

View Code

        List<String> a = Arrays.asList("we i di ao is great".split(" "));a.forEach((s) -> System.out.println(s));// 表达式a.forEach((String s) -> {System.out.println(s);});// 语句块a.forEach(System.out::println);// 函数

public class LambdaIntro {// functional interface 函数式接口public static interface ItemWithIndexVisitor<E> {public void visit(E item, int index);}public static <E> void eachWithIndex(List<E> list,ItemWithIndexVisitor<E> visitor) {for (int i = 0; i < list.size(); i++) {visitor.visit(list.get(i), i);}}// 一个普通函数，用作函数指针public static <E> void printItem(E value, int index) {String output = String.format("%d -> %s", index, value.toString());System.out.println(output);}public static void main(String[] args) {List<String> list = Arrays.asList("A", "B", "C");// 第一种方式eachWithIndex(list, (value, index) -> {String output = String.format("%d -> %s", index, value);System.out.println(output);});// 第二种方式
        eachWithIndex(list, LambdaIntro::printItem);}
}

        //of：通过枚举方式创建流Stream<Integer> one = Stream.of(1, 2, 3);//流是可以拼接的，从而产生新流Stream<Integer> two = Stream.concat(one, Stream.of(4, 5, 6));two.forEach(System.out::println);//逐个加入，那就用Builder构建器来实现Builder<Integer> builder = Stream.builder();Stream<Integer> three = builder.add(3).add(4).build();three.forEach((s) -> System.out.println(s));//产生器流generatorRandom random = new Random();Stream<Integer> four = Stream.generate(() -> random.nextInt());four.limit(10).forEach(System.out::println);//迭代器iterator，UnaryOperator一元运算符可以通过lambda表达式来创建Stream<Integer>five=Stream.iterate(2, new UnaryOperator<Integer>() {@Overridepublic Integer apply(Integer t) {return t = (t * 5 + 7) % 13;}});five.limit(10).forEach(System.out::println);

        Arrays.asList(1, 2, 3).stream().flatMap(new Function<Integer, Stream<Integer>>() {@Overridepublic Stream<Integer> apply(Integer t) {return Arrays.asList(t, t + 10, t + 100).stream();}}).forEach(System.out::println);

        Stream.of("one", "two", "three", "four").filter(e -> e.length() > 3).peek(e -> System.out.println("Filtered value: " + e)).map(String::toUpperCase).peek(e -> System.out.println("Mapped value: " + e)).collect(Collectors.toList());

Filtered value: three
Mapped value: THREE
Filtered value: four
Mapped value: FOUR

         Stream.of("one", "two", "three", "four").reduce(new BinaryOperator<String>() {@Overridepublic String apply(String t, String u) {System.out.println(t+":"+u);return t + "," + u;}}).ifPresent(System.out::println);

one:two
one,two:three
one,two,three:four
one,two,three,four

        String s = Stream.of("one", "two", "three", "four").reduce("baga",new BinaryOperator<String>() {@Overridepublic String apply(String t, String u) {return t +","+ u;}});System.out.println(s);

baga,one,two,three,four

Map<String, Map<String, List<Person>>> peopleByStateAndCity= personStream.collect(Collectors.groupingBy(Person::getState,Collectors.groupingBy(Person::getCity)));

        String s=Stream.of("one", "two", "three", "four").collect(Collectors.joining(","));System.out.println(s);