列表到哈希表

Question

So my problem is that I dont want to have repeated names in the list of objects.

So I tried to convert the list to hashset but it still has rows with the same name.

Basically I only want to get 1 row of Flow2 and another of TestFlow NOT 4. http://prntscr.com/hylkoc

public static Set<TestFlow> getFlows() {
    SessionFactory factory = HibernateUtil.GetSessionFactory();
    Session hibernateSession = factory.openSession();
    Transaction tx = null;
    List<TestFlow> testFlows;
    Set<TestFlow> testFlowSet = new HashSet<>();
    try {
        tx = hibernateSession.beginTransaction();
        testFlows = hibernateSession.createQuery("FROM TestFlow").list();
        testFlowSet = new HashSet<>(testFlows);
        tx.commit();
    } catch (HibernateException e) {
        if (tx != null) tx.rollback();
        e.printStackTrace();
    } finally {
        hibernateSession.close();
    }
    return testFlowSet;
}

Answer 1

TestFlow is an object, so a record having the same name will not mean it equals the other object.

You can use a Map instead:

Map<String, TestFlow>

The key will be the name, and the value will be the TestFlow record. That way if the name exists, you will not add the record to the map.

Another option is overriding the equals method, although if you use the object elsewhere and it has a bunch of fields, that may not be a good idea.

Answer 2

You can use TreeSet + Comparator :

Comparator:

public class TestFlowComparator implements Comparator<TestFlow> {
  @Override
  public int compare(TestFlow o1, TestFlow o2) {
    return o1.getName().compareTo(o2.getName());
  }
}

Test :

@Test
@DisplayName("stack")
public void srack(){
    List<TestFlow> listTestFlow = new ArrayList<>();
    listTestFlow.add(new TestFlow("a"));
    listTestFlow.add(new TestFlow("a"));
    listTestFlow.add(new TestFlow("a"));
    listTestFlow.add(new TestFlow("a"));
    listTestFlow.add(new TestFlow("ъ"));
    listTestFlow.add(new TestFlow("ъ"));

    Set<TestFlow> set = new TreeSet<>(new TestFlowComparator());
    set.addAll(listTestFlow);
    System.out.println(set.toString());
}

sout : [TestFlow{name = a } , TestFlow{name = ъ } ]

Answer 3

Inside TestFlow class override the two methods equals() and hashCode() .

For example if TestFlow class have int id property you can do that:

@Override
public boolean equals(Object obj) {
    if (this == obj) {
        return true;
    }
    if (obj instanceof TestFlow) {
        TestFlow tf = (TestFlow) obj;
        return tf.id == this.id;
    }
    return false;
}