Storing the keysets from a JPQL query result in a java list

I was successfully able to execute a jpql query and print the result which is stored in a queryResults variable. What I want to achieve next is storing just the IDs (primary key column) in a list without the date (value), but I am not too sure if this is possible; perhaps using something like a java map. Is it possible? If yes, how can this be easily achieved?

  private static final TestDao Test_DAO = new TestDao();

 @Test
 public void testById() {
List<TestEntity> queryResults = TEST_DAO.findById(""); //The record from the sql query is stored in queryResults and findById("") is the method that executes the query in a TestDao class and it is called here

for (TestEntity qResult: queryResults) { // looping through the query result to print the rows
System.out.println(qResult.getId());
System.out.println(qResult.getDate());
}

System.out.println("This is the sql result " + queryResults );
       
}

Output:
This is the result [TestEntity(id=101, date=2020-01-19 15:12:32.447), TestEntity(id=102, date=2020-09-01 11:04:10.0)]// I want to get the IDs 101 and 102 and store in a list without the Dates

I tried using a map this way:

Map<Integer, Timestamp> map= (Map<Integer, Timestamp>) queryResults.get(0); but I got an exception:

java.lang.ClassCastException: TestEntity cannot be cast to java.util.Map

There are some points before the implementation.

1. Why are you defining DAO as static? I think this is a bad implementation unless I am missing a particular reason you declared it static. You should define this as a member variable and not a static member
2. The naming of the method - findById() translated in English is - find Something by this Id, but you are fetching a list of Records, so naming is not correct.
3. Point 2 becomes invalid if ID property is not a Primary Key in your table, then it makes sense, but still naming is bad. Id is something we use to define Primary Key in the Database and should be and will be unique. But your comments suggest that ID is unique and the Primary Key. So read about how Databases work
4. And even if not unique, if you pass an Id to find some records, why will get different ids in the Records !!!

About implementation:

1. Changing in your existing code:

private TestDao Test_DAO = new TestDao();

@Test
public void testById() {
  List<TestEntity> queryResults = TEST_DAO.findById("");
  List<Long> listOfIds = new ArrayList<>(); // Assuming Id is Long type, same logic for any type
  for (TestEntity qResult: queryResults) {
    System.out.println(qResult.getId());
    listOfIds.add(qResult.getId());   // Just add it to the list
    System.out.println(qResult.getDate());
  }
       
}

In case you want to be efficient with the query:
You can use JPQL and hibernate

You can then write a query like:

String query = "select te.id from TestEntity te";
// Create the TypedQuery using EntityManager and then get ResultSet back
List<Long> ids = query.getResultList();

3. In case of using Spring-Data-Jpa, you can define the repository and define the method and pass the query with @Query annotation. Spring Data JPA