import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class Test {

    private String id;

    public Test(String id) {
        this.id = id;
    }

    public static void main (String[] args) {

        List<Test> myList = new ArrayList<>();
        myList.add(new Test("foo"));
        myList.add(new Test("bar"));
        myList.add(new Test("foobar"));

        String ids = myList.stream().map(n -> n.id).collect(Collectors.joining(" "));

        System.err.println("'" + ids + "'");

    }
}