java streams:按字段分组和添加字段

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

public class Main {
    public static void main(String[] args) {
        // 假设有一个Person类，包含name和age字段
        List<Person> persons = Arrays.asList(
                new Person("Alice", 25),
                new Person("Bob", 30),
                new Person("Alice", 35),
                new Person("Bob", 40)
        );

        // 按name字段分组
        Map<String, List<Person>> groupedByName = persons.stream()
                .collect(Collectors.groupingBy(Person::getName));

        // 输出分组结果
        groupedByName.forEach((name, group) -> {
            System.out.println("Name: " + name);
            System.out.println("Group: " + group);
            System.out.println();
        });
    }
}

class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

Name: Alice
Group: [Person{name='Alice', age=25}, Person{name='Alice', age=35}]

Name: Bob
Group: [Person{name='Bob', age=30}, Person{name='Bob', age=40}]

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

public class Main {
    public static void main(String[] args) {
        // 假设有一个Person类，包含name和age字段
        List<Person> persons = Arrays.asList(
                new Person("Alice", 25),
                new Person("Bob", 30),
                new Person("Charlie", 35)
        );

        // 添加字段
        List<PersonWithGender> personsWithGender = persons.stream()
                .map(person -> new PersonWithGender(person.getName(), person.getAge(), getGender(person.getName())))
                .collect(Collectors.toList());

        // 输出结果
        personsWithGender.forEach(System.out::println);
    }

    private static String getGender(String name) {
        // 根据姓名判断性别的逻辑
        // 这里只是一个示例，实际应用中可能需要更复杂的逻辑
        if (name.equals("Alice")) {
            return "Female";
        } else {
            return "Male";
        }
    }
}

class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

class PersonWithGender {
    private String name;
    private int age;
    private String gender;

    public PersonWithGender(String name, int age, String gender) {
        this.name = name;
        this.age = age;
        this.gender = gender;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public String getGender() {
        return gender;
    }

    @Override
    public String toString() {
        return "PersonWithGender{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", gender='" + gender + '\'' +
                '}';
    }
}

PersonWithGender{name='Alice', age=25, gender='Female'}
PersonWithGender{name='Bob', age=30, gender='Male'}
PersonWithGender{name='Charlie', age=35, gender='Male'}