java - 创建不可变类时，集合是否应该只包含不可变对象(immutable对象)？

Question

目前正在备考...在过去的一篇论文中遇到了这个问题。

Consider the following Student and Tutor classes:

public class Student {
    private String name;
    private String course;

    public Student(String name, String course) {
        this.name = name;
        this.course = course;
    }

    public String getName() {
        return name;
    }

    public String getCourse() {
        return course;
    }

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

    public void setCourse(String course) {
        this.course = course;
    }
}

导师:

public final class Tutor {
    private String name;
    private final Set<Student> tutees;

    public Tutor(String name, Student[] students) {
        this.name = name;
        tutees = new HashSet<Student>();
        for (int i = 0; i < students.length; i++) {
            tutees.add(students[i]);
        }
    }

    public Set<Student> getTutees() {
        return Collections.unmodifiableSet(tutees);
    }

    public String getName() {
        return name;
    }
}

Re-write the Tutor class to make it immutable(without modifying the Student class).

我知道name字段应该有final修饰符来保证线程安全。

如果学生 Set 包含可变的 Student 对象，并且 Student 类不能更改，那么我们如何使该类不可变？也许创建 Set 的克隆并在每次调用 getTutees 方法时清除 tutees 并将克隆的元素添加到其中？

或者尽管集合包含可变对象，它是否已经不可变？

Answer 1

集合的成员也需要是不可变的。不可修改仅意味着不能添加或删除任何元素。

即使集合是不可修改的，一旦访问它的代码引用了属于该集合的元素，那么如果该元素不是不可变的，那么它就可以被更改。

如果集合的 getter 返回一个防御副本，创建一个新的 Set 引用新的 Student 对象并返回它而不是原始集合，那么这将使集合不可变。

public Set<Student> getTutees() {
    Set newSet = new HashSet();
    for (Student tutee : tutees) {
        newSet.add(new Student(tutee.getName(), tutee.getCourse()));
    }
    return newSet;
}

在此处添加一个示例以表明使用 Collections.unmodifiableSet 是不够的:

import java.util.*;

public class ImmutabilityExample {
    public static void main(String[] args) {
        Student student = new Student("Joe", "Underwater Basketweaving 101");
        Tutor tutor = new Tutor("Bill", new Student[] {student});
        Set<Student> students = tutor.getTutees();
        System.out.println("before=" + students);
        students.iterator().next().setName("Mary");
        System.out.println("" + tutor.getTutees());
    }
}

class Student {
    private String name;
    private String course;

    public Student(String name, String course) {
        this.name = name;
        this.course = course;
    }

    public String getName() {
        return name;
    }

    public String getCourse() {
        return course;
    }

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

    public void setCourse(String course) {
        this.course = course;
    }
    public String toString() {
        return "Student, name=" + name + ", course=" + course;
    }
}

final class Tutor {
    private String name;
    private final Set<Student> tutees;

    public Tutor(String name, Student[] students) {
        this.name = name;
        tutees = new HashSet<Student>();
        for (int i = 0; i < students.length; i++) {
            tutees.add(students[i]);
        }
    }

    public Set<Student> getTutees() {
        return Collections.unmodifiableSet(tutees);
    }

    public String getName() {
        return name;
    }
}

这个的输出是:

before=[Student, name=Joe, course=Underwater Basketweaving 101]
[Student, name=Mary, course=Underwater Basketweaving 101]