Java Stream API：按嵌套字段分组对象的正确姿势

public class Project {
    private int id;
    private String name; // 假设还有其他字段
    // 构造函数、Getter/Setter
    public Project(int id, String name) {
        this.id = id;
        this.name = name;
    }
    public int getId() { return id; }
    public String getName() { return name; }
    // 重写equals和hashCode方法，确保Project对象基于id进行比较
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Project project = (Project) o;
        return id == project.id;
    }
    @Override
    public int hashCode() {
        return Objects.hash(id);
    }
}

import java.util.Objects;

public class Task {
    private Project project;
    private String description; // 假设还有其他字段
    // 构造函数、Getter/Setter
    public Task(Project project, String description) {
        this.project = project;
        this.description = description;
    }
    public Project getProject() { return project; }
    public String getDescription() { return description; }
}

// 假设 tasks 是 List<Task>
// 错误的尝试 1：按Project对象本身分组
// Map<Project, List<Task>> groupedByProjectObject = tasks.stream()
//                                                      .collect(Collectors.groupingBy(Task::getProject));
// 这种方式会根据Project对象的引用（或其equals/hashCode实现）来分组，
// 如果有两个Task对象引用了不同的Project实例，即使这些Project实例的id相同，
// 它们也可能被分到不同的组中（除非Project的equals和hashCode已正确实现）。

// 错误的尝试 2：链式方法引用（语法错误）
// tasks.stream().collect(Collectors.groupingBy(task::getProject::getId)); // 编译错误

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

public class GroupingByNestedField {
    public static void main(String[] args) {
        // 示例数据
        Project project1 = new Project(1, "Project Alpha");
        Project project2 = new Project(2, "Project Beta");
        Project project3 = new Project(1, "Project Gamma"); // ID与project1相同

        List<Task> tasks = Arrays.asList(
            new Task(project1, "Task A for Alpha"),
            new Task(project2, "Task B for Beta"),
            new Task(project1, "Task C for Alpha"),
            new Task(project3, "Task D for Gamma (same ID as Alpha)")
        );

        // 使用Lambda表达式按Project的ID分组
        Map<Integer, List<Task>> groupedTasksById = tasks.stream()
            .collect(Collectors.groupingBy(task -> task.getProject().getId()));

        // 打印结果
        groupedTasksById.forEach((projectId, taskList) -> {
            System.out.println("Project ID: " + projectId);
            taskList.forEach(task -> System.out.println("  - " + task.getDescription()));
        });
    }
}

Project ID: 1
  - Task A for Alpha
  - Task C for Alpha
  - Task D for Gamma (same ID as Alpha)
Project ID: 2
  - Task B for Beta

// 假设我们有一个Task对象实例
Task someTask = new Task(new Project(100, "Specific Project"), "Specific Task");

// 我们可以获取到其内部的Project对象
Project specificProject = someTask.getProject();

// 然后对这个特定的Project对象的方法进行方法引用
Function<Project, Integer> getProjectId = Project::getId; // 引用Project类的getId方法
int projectId = getProjectId.apply(specificProject); // 100

// 也可以直接引用特定Project实例的getId方法
Function<Void, Integer> getSpecificProjectId = specificProject::getId;
int anotherProjectId = getSpecificProjectId.apply(null); // 100