Java8操作List＜Object＞小结

在项目开发中，经常遇到对List<Object>的操作，比如取最值、排序、汇总求和、转Map等等。今天在此做个小结，希望和大家一起学习，如有遗漏的地方还请大家指正！

一、准备一个测试类User:

public class User {

    private Integer id;
    private String name;
    private Integer age;

    public User(Integer id, String name, Integer age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name=" + name +  +
                ", age=" + age +
                };
    }
}

二、编码：

①获取id集合

userList.stream().map(User::getId).collect(Collectors.toList());

②获取key为id，value为对象的Map

userList.stream().collect(Collectors.toMap(User::getId, User -> User));

③获取age最大的user

userList.stream().max(Comparator.comparingInt(User::getAge)).get();

④获取age大于10的user

userList.stream().filter(u -> u.getAge() > 10).collect(Collectors.toList());

⑤按年龄正序排序

userList.stream().sorted(Comparator.comparingInt(User::getAge)).collect(Collectors.toList());

⑥按年龄逆序排序

userList.stream().sorted(Comparator.comparingInt(User::getAge).reversed()).collect(Collectors.toList());

⑦按年龄求和

userList.stream().mapToInt(User::getAge).sum();

三、编写测试类：

public class TestUser {

    private static List<User> userList = new ArrayList<>();

    static{
        userList.add(new User(1,"张三",10));
        userList.add(new User(2,"李四",15));
        userList.add(new User(3,"王五",20));
    }

    public static void main(String[] args) {

        System.out.println(userList);
        //获取id集合
        List<Integer> ids = userList.stream().map(User::getId).collect(Collectors.toList());
        System.out.println("获取id集合:"+ids);

        //获取key为id，value为对象的Map
        Map<Integer, User> userMap = userList.stream().collect(Collectors.toMap(User::getId, User -> User));
        System.out.println("获取key为id，value为对象的Map:"+userMap);

        //获取age最大的user
        User user = userList.stream().max(Comparator.comparingInt(User::getAge)).get();
        System.out.println("获取age最大的user:"+user);


        //获取age大于10的user
        List<User> filterUser = userList.stream().filter(u -> u.getAge() > 10).collect(Collectors.toList());
        System.out.println("获取age大于10的user:"+filterUser);

        //按年龄正序排序
        List<User> ascList = userList.stream().sorted(Comparator.comparingInt(User::getAge)).collect(Collectors.toList());
        System.out.println("按年龄排序：正序:"+ascList);

        //按年龄逆序排序
        List<User> descList = userList.stream().sorted(Comparator.comparingInt(User::getAge).reversed()).collect(Collectors.toList());
        System.out.println("按年龄逆序排序:"+descList);

        //按年龄求和
        int sumAge = userList.stream().mapToInt(User::getAge).sum();
        System.out.println("按年龄求和:"+sumAge);
    }
}

四、结果输出：