Stream系列（三） FlatMap方法使用

package com.example.demo;

import lombok.Data;

import lombok.ToString;

import lombok.extern.log4j.Log4j2;

import org.junit.Test;

import java.util.Collection;

import java.util.List;

import java.util.stream.Collectors;

import java.util.stream.LongStream;

import java.util.stream.Stream;

import static org.junit.Assert.assertEquals;

import static org.junit.Assert.assertTrue;

@Log4j2

public class EmployeeTestCase extends BaseTest{

    @Test

    public void flatMap() {

        List<Employee> employeesAll = listFlat.stream().flatMap(Collection::stream).collect(Collectors.toList());

        assertTrue(employeesAll.size() == 5);

        List<Employee> employeesAll2 = listFlat.stream().flatMap(employees -> {

            Stream<Employee> stream = employees.stream();

            return stream;

        }).collect(Collectors.toList());

        assertEquals(employeesAll, employeesAll2);

        List<Long> listFlatLong = listFlat.stream()

                .flatMap(employees -> employees.stream())

                .peek(System.out::println)

                .flatMapToLong(employee -> LongStream.of(employee.getId()))

                .peek(System.out::println)

                .boxed()

                .collect(Collectors.toList());

        log.info("listFlatLong:{}", listFlatLong.toString());

        List<Long> listFlatLong2 = listFlat.stream()

                .flatMap(employees -> employees.stream())

                .peek(System.out::println)

                .mapToLong(Employee::getId)

                .peek(System.out::println)

                .boxed()

                .collect(Collectors.toList());

        List<Double> listFlatName = listFlat.stream().flatMap(employees -> employees.stream())

                .peek(System.out::println)

                .map(employee -> employee.getSalary())

                .peek(System.out::println)

                .collect(Collectors.toList());

    }

}

package com.example.demo;

import java.util.Arrays;

import java.util.List;

public class BaseTest {

    protected static final List<Employee> list = Arrays.asList(

            new Employee(1, "Alex", 1000),

            new Employee(2, "Michael", 2000),

            new Employee(3, "Jack", 1500),

            new Employee(4, "Owen", 1500),

            new Employee(5, "Denny", 2000));

    protected static final List<List<Employee>> listFlat = Arrays.asList(

            Arrays.asList(new Employee(1, "Alex", 1000),

                    new Employee(2, "Michael", 2000)),

            Arrays.asList(new Employee(3, "Jack", 1500),

                    new Employee(4, "Owen", 1500)),

            Arrays.asList(new Employee(5, "Denny", 2000)));

}