Java 8 Stream：高效统计List中特定字段出现次数的教程

import java.time.LocalDate;

public class FamilyMember {

    private String memberType;
    private String fullName;
    private LocalDate dateOfBirth;
    private String gender;

    public FamilyMember(String memberType, String fullName, 
                        LocalDate dateOfBirth, String gender) {
        this.memberType = memberType;
        this.fullName = fullName;
        this.dateOfBirth = dateOfBirth;
        this.gender = gender;
    }

    // Getters for all fields
    public String getMemberType() {
        return memberType;
    }

    public String getFullName() {
        return fullName;
    }

    public LocalDate getDateOfBirth() {
        return dateOfBirth;
    }

    public String getGender() {
        return gender;
    }

    // Setters (optional, depending on immutability needs)
    public void setMemberType(String memberType) {
        this.memberType = memberType;
    }

    public void setFullName(String fullName) {
        this.fullName = fullName;
    }

    public void setDateOfBirth(LocalDate dateOfBirth) {
        this.dateOfBirth = dateOfBirth;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    @Override
    public String toString() {
        return "FamilyMember{" +
               "memberType='" + memberType + '\'' +
               ", fullName='" + fullName + '\'' +
               '}';
    }
}

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

public class TestFamilyMemberCount {

    public static void main(String[] args) {
        // 1. 创建FamilyMember对象实例
        FamilyMember member1 = new FamilyMember("Sibling", "Sibling name",
                LocalDate.of(1990, 12, 12), "Male");
        FamilyMember member2 = new FamilyMember("Sibling", "Sibling name2",
                LocalDate.of(1990, 12, 12), "Male");
        FamilyMember member3 = new FamilyMember("Sibling", "Sibling name3",
                LocalDate.of(1990, 12, 12), "Male");
        FamilyMember member4 = new FamilyMember("Child", "Child name",
                LocalDate.of(2010, 12, 12), "Male");
        FamilyMember member5 = new FamilyMember("Child", "Child name2",
                LocalDate.of(2000, 12, 12), "Female");
        FamilyMember member6 = new FamilyMember("Spouse", "Spouse name",
                LocalDate.of(1990, 12, 12), "Male");

        // 2. 将FamilyMember对象放入列表中
        List<FamilyMember> listOfFamilyMember = Arrays.asList(member1, member2,
                member3, member4, member5, member6);

        // 3. 使用Stream API进行分组和计数
        Map<String, Long> countMembers = listOfFamilyMember.stream()
                .collect(Collectors.groupingBy(FamilyMember::getMemberType,
                        Collectors.counting()));

        // 4. 打印结果
        System.out.println(countMembers);
    }
}

{Spouse=1, Sibling=3, Child=2}