I concatenated a list of results from a DB query after a loop and got the expected output, but without any separator between the values:

dbDemo = ""
      List<EntityClass> queryResultlist = query.getResultList();
      for (EntityClass resultRow : queryResultlist) {
        dbDemo = dbDemo.concat(resultRow.getId());
      }
      System.out.println(dbDemo);

Output:Test1Test2Test3

Is there a way to add a line break separator to the values to have a pretty print? Something like this:

Test1
Test2
Test3

Test1, Test2, Test3 or "Test1", "Test2," "Test3"

CodePudding user response：

append newline at the end

dbDemo = ""
List<EntityClass> queryResultlist = query.getResultList();
for (EntityClass resultRow : queryResultlist) {
    dbDemo = dbDemo.concat(resultRow.getId() "\n");
}
System.out.println(dbDemo);

Output:

Test1
Test2
Test3

CodePudding user response：

You may want to use StringJoiner, which makes it easier to join strings with separators, prefixes and suffixes. You can use it as such:

StringJoiner stringJoiner = new StringJoiner(", "); // Use System.lineSeparator() for newlines
List<EntityClass> queryResultlist = query.getResultList();
for (EntityClass resultRow : queryResultlist) {
    stringJoiner.add(resultRow);
}
String dbDemo = stringJoiner.toString();
System.out.println(dbDemo);

Output:

Test1, Test2, Test3

CodePudding user response：

Another solution, using Streams:

final List<EntityClass> queryResultList = List.of(
    EntityClass.of(13),
    EntityClass.of(1337),
    EntityClass.of(42));
final String delimiter = ", ";
System.out.println(queryResultList.stream()
    .map(EntityClass::getId)
    .map(Object::toString)
    .collect(Collectors.joining(delimiter)));

Output:

13, 1337, 42

Ideone demo

We map each EntityClass to its id (which gets autoboxed from int to Integer), and from there transform it to a String by calling Object::toString. Finally, we concatenate each entry in the stream by appending ", ".

If we want to delimit each entry by a line break instead of ", ", we can just set delimiter = System.lineSeparator;, resulting in the following output:

13
1337
42

Ideone demo