我有2个课程和小组。小组有一组学生。hashCode()和equals()两者都覆盖。
然后我在组中填充Set<Student>。为什么两个相同的组（相同的ID，名称，Set<Student>等）是不同的？当我从hashCode()和组的equals()删除Set<Student>时，组变得相同。但这是两个相同的Set<Student>。我想在Set<Student>的情况下，它会比较链接。我不明白这一点，例如String name是一个对象，Set<Student>是Object。当我同时将hashCode()和equals()包括在内时，它会比较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，我会得到：
java.lang.AssertionError:
Expected: is <[university.domain.Group@1ef58100, university.domain.Group@1ef5849f]>
but: was <[university.domain.Group@1ef58100, university.domain.Group@1ef5849f]>
它可以很好地工作：
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));
我认为问题是您的测试类中group的students字段是null，而groupDao.retrieveAll()返回的组的students字段始终具有非null students集。如果小组中没有学生，则其students字段是空的（不包含学生），而不是null。
Objects.hash()给出了null字段的哈希代码0，而空集的哈希代码也为0。因此，这两个组具有相同的哈希代码，但它们不相等。
呼叫时您期望的结果是什么对象
我猜，这将返回 true 仅当两个组实例的学生字段都恰好提及一个集合的一个实例时。