java - 为什么在定义 hashCode() 和 equals() 后我的代码仍然比较链接

Question

我有 2 个类(class)“学生”和“小组”。小组有一组学生。 hashCode()和equals()两者都被覆盖。

然后我填充Set<Student>在组中。为什么两个相同的组(相同的 id、名称、Set<Student> 等)不同？当我删除Set<Student>时来自hashCode()和equals()组的组变得相同。但这是两个相同的Set<Student> 。我想在 Set<Student> 的情况下它比较链接。我不明白这个，例如String name是一个对象并且 Set<Student>是 Object 。当我将两者都包含到hashCode()时和equals()它比较 String name 的实际值以及 Set<Student> 的链接.

Student.java:

public class Student {  

    private int id;
    private String firstName;
    private String lastName;

    public Student(int id, String firstName, String lastName) {     
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
    }   

    public int getId() {
        return id;
    }

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

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    @Override
    public boolean equals(final Object other) {
        if (this == other) {
            return true;
        }
        if (other == null) {
            return false;
        }
        if (!getClass().equals(other.getClass())) {
            return false;
        }
        Student castOther = (Student) other;
        return Objects.equals(id, castOther.id) 
                && Objects.equals(firstName, castOther.firstName)
                && Objects.equals(lastName, castOther.lastName);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, firstName, lastName);
    }       
}

Group.java:

public class Group {

    private int id;
    private String name;    
    private Set<Student> students;

    public Group(int id, String name) {     
        this.id = id;
        this.name = name;
    }

    public int getId() {
        return id;
    }

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

    public Set<Student> getStudents() {
        return students;
    }

    public void setStudents(Set<Student> students) {
        this.students = students;
    }

    public String getName() {
        return name;
    }

    @Override
    public boolean equals(final Object other) {
        if (this == other) {
            return true;
        }
        if (other == null) {
            return false;
        }
        if (!getClass().equals(other.getClass())) {
            return false;
        }
        Group castOther = (Group) other;
        return Objects.equals(id, castOther.id) 
                && Objects.equals(name, castOther.name)
                && Objects.equals(students, castOther.students);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, name, students);
    }
}

GroupDaoImpl.java:

Set<Group> retrieveAll() throws DaoException {      
    String sql = "select * from groups;";               
    Set<Group> groupSet = new HashSet<>();

    try (Connection connection = daoFactory.getConnection();
            PreparedStatement preparedStatement = 
            connection.prepareStatement(sql)) {         
        preparedStatement.execute();
        ResultSet resultSet = preparedStatement.getResultSet();         

        while (resultSet.next()) {
            int id = resultSet.getInt("group_id");
            String name = resultSet.getString("name");              
            Group group = new Group(id, name);              
            group.setStudents(studentDao.retrieveByGroupId(id));                
            groupSet.add(group);
        }                       
    } catch (Exception e) {         
        throw new DaoException(e);          
    } 
    return groupSet;
}

StudentDaoImpl.java:

Set<Student> retrieveByGroupId(int groupId) throws DaoException {       
    String sql = "select * from student where group_id = ?;";       
    Set<Student> studentSet = new HashSet<>();

    try (Connection connection = daoFactory.getConnection();
            PreparedStatement preparedStatement = connection.prepareStatement(sql)) {
        preparedStatement.setInt(1, groupId);
        preparedStatement.execute();            
        ResultSet resultSet = preparedStatement.getResultSet();

        while (resultSet.next()) {
            int id = resultSet.getInt("student_id");
            String firstName = resultSet.getString("first_name");
            String lastName = resultSet.getString("last_name");             
            Student student = new Student(id, firstName, lastName);             
            studentSet.add(student);
        }                       
    } catch (Exception e) {
        throw new DaoException(e);          
    } 
    return studentSet;
}

GroupDaoImplTest.java:

public class GroupDaoImplTest {

private GroupDaoImpl groupDao = new GroupDaoImpl();
private static Set<Group> groupSet = new HashSet<>();
private static Group group;

@BeforeClass
public static void setUp() throws DaoException {
    StudentDaoImpl studentDao = new StudentDaoImpl();
    group = new Group(1, "Group 11");
    group.setStudents(studentDao.retrieveByGroupId(1));
    groupSet.add(group);        
    groupSet.add(new Group(2, "Group 12")); 
}

@Test
public void testInsert() throws DaoException {
    groupDao.insert(new Group(1, "Group 13"), 2);       
}

@Test
public void testUpdate() throws DaoException {
    groupDao.update(new Group(15, "Group 11"));     
}

@Test
public void testDelete() throws DaoException {
    groupDao.delete(new Group(16, "Group 11"));
}

@Test
public void testRetrieve() throws DaoException {        
    assertThat(groupDao.retrieve(1), is(group));        
}

@Test
public void testRetrieveByCathedraId() throws DaoException {        
    assertThat(groupDao.retrieveByCathedraId(1), is(groupSet));
}

@Test
public void testRetrieveAll() throws DaoException {             
    assertThat(groupDao.retrieveAll(), is(groupSet));
}

}

当我执行testRetrieveAll()时我得到:

java.lang.AssertionError: 
Expected: is <[university.domain.Group@1ef58100, university.domain.Group@1ef5849f]>
     but: was <[university.domain.Group@1ef58100, university.domain.Group@1ef5849f]> 

Answer 1

对我来说效果很好:

Group g1 = new Group(1, "a");
Set<Student> s1 = new HashSet<>();
s1.add(new Student(1, "a", "b"));
g1.setStudents(s1);

Group g2 = new Group(1, "a");
Set<Student> s2 = new HashSet<>();
s2.add(new Student(1, "a", "b"));
g2.setStudents(s2);

System.out.println(g1.equals(g2));