Optional为List<Object>的情况

User.java

package org.caucoder.optional;

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

public class User {
    public User(Integer id) {
        this.id = id;
    }
    private Integer id;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public static void main(String[] args) {

        List<User> list = new ArrayList<>();
        list.add(new User(3));
        list.add(new User(5));
        Optional<List<User>>  optionalA = Optional.ofNullable(list);
        getIdList(optionalA).stream().forEach(System.out::println);
        list = null;
        Optional<List<User>>  optionalB = Optional.ofNullable(list);
        getIdList(optionalB).stream().forEach(System.out::println);
    }

    /**
     * 3
     * 5
     * List<User>为null时
     *
     */

    public static List<Integer> getIdList(Optional<List<User>> optional){
        return optional.orElseGet(()->{
            System.out.println("List<User>为null时");
            return Collections.emptyList();
        }).stream().map(item->{
            return item.getId();
        }).collect(Collectors.toList());
    }
}