lambda表达式

1、 lambda简介

Lambda 表达式是 JDK8 的一个新特性，可以取代大部分的匿名内部类，写出更优雅的 Java 代码，尤其在集合的遍历和其他集合操作中，可以极大地优化代码结构。 JDK 也提供了大量的内置函数式接口供我们使用，使得 Lambda 表达式的运用更加方便、高效。

2、 lambda表达式使用

@FunctionalInterface

@FunctionalInterface
public interface PersonService {

  String getData(String data);
}

public class PersonServiceTest {

public static void main(String[] args) {
    PersonService personService = (x) -> {
        System.out.println(x);
        x = x + ":aaa";
        return x;
    };
    String result = personService.getData("aa");
    System.out.println(result);
}
}

Consumer使用

public class ConsumerDemo {

    public static void main(String[] args) {
        Consumer<String> consumer = (x) -> {
            x = x + ":aa";
            System.out.println(x);
        };
        consumer.accept("aaa");
    }
}

Supplier使用

public class SupplierDemo {

   public static void main(String[] args) {
       Supplier<String> supplier = () -> {
           System.out.println("===========================");
           return "success";
       };
       String result = supplier.get();
       System.out.println(result);
   }
}

Function使用

public class FunctionDemo {

    public static void main(String[] args) {
        Function<String, String> function = (x) -> {
            x = x + ":aa";
            System.out.println(x);
            return x;
        };
        String result = function.apply("aa");
        System.out.println(result);
    }
}

filter使用

public class FilterDemo {

    public static void main(String[] args) {
        List<Integer> list = List.of(1, 2, 3, 4, 5, 6);
        List<Integer> integerList = list.stream().filter(e -> e % 2 == 0).collect(Collectors.toList());
        System.out.println(integerList);
    }
}

map使用

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class Map1Test {

    public static void main(String[] args) {
        List<Integer> integerList = new ArrayList<>(10);
        integerList.add(1);
        integerList.add(2);
        integerList.add(3);
        List<Integer> list = integerList.stream().map(e -> e + 1).collect(Collectors.toList());
        System.out.println(list);
    }
}

allMatch使用

public class AllMatchDemo {

  public static void main(String[] args) {
    List<String> strList = Arrays.asList("a", "b", "c", "d");
    boolean a = Optional.ofNullable(strList).orElseGet(ArrayList::new)
        .stream()
        .allMatch("a"::equals);
    System.out.println("allMatch()测试多元素结果：" + a);
  }
}

anyMatch使用

public class AnyMatchDemo {

  public static void main(String[] args) {
    List<String> strList = Arrays.asList("a", "b", "c", "d");
    boolean a = Optional.ofNullable(strList).orElseGet(ArrayList::new)
        .stream()
        .anyMatch("a"::equals);
    System.out.println("anyMatch()测试多元素结果：" + a);
  }
}

flatMap使用

public class FlatMapTest {

  public static void main(String[] args) {
    List<Integer> list = List.of(1, 2, 3, 4, 5, 6);
    List<Integer> list1 = List.of(7, 8, 9, 10, 11, 12);
    List<List<Integer>> list2 = new ArrayList<>(10);
    list2.add(list);
    list2.add(list1);
    List<Integer> list3 = list2.stream().flatMap(Collection::stream).collect(Collectors.toList());
    System.out.println(list3);
  }
}

List转Map

（1）对象定义

@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {

  private Long id;

  private String name;

}

（2）程序转换

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class UserMapDemo {

    public static void main(String[] args) {
        List<User> userList = new ArrayList<>(10) {{
            add(new User(1L, "aa"));
            add(new User(2L, "bb"));
            add(new User(3L, "cc"));
            add(new User(4L, "dd"));
            add(new User(5L, "ee"));
            add(new User(6L, "ff"));
        }};
        Map<Long, User> map = userList.stream().collect(Collectors.toMap(User::getId, Function.identity(), (x, y) -> y));
        System.out.println(map);
    }
}

List转Map,value是数组

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class ListToMapListDemo {

    public static void main(String[] args) {
        List<User> userList = new ArrayList<>(10) {{
            add(new User(1L, "aa"));
            add(new User(2L, "bb"));
            add(new User(3L, "cc"));
            add(new User(4L, "dd"));
            add(new User(5L, "ee"));
            add(new User(6L, "aa"));
        }};
        Map<String, List<User>> map = userList.stream().collect(Collectors.groupingBy(User::getName, Collectors.toList()));
        System.out.println(map);
    }
}

总结

jdk8还提供了许多的方法，简化操作