Reference confusion....

Question

<sigh>......I thought I was being clever here, but apparently not. :)
I had two methods that, apart from the List result variable and the String SQL were exactly the same, so I thought I'd write a general method (getResultList) that would do both in one, so to speak and just pass the result object references as parameters to the method. The method works, but the reference variables are still null when I try and use them.....specifically I get a NullPointerException.
Here is the code.......my thinking must be faulty here, because the two Lists (listBasePatientDetails and listNormalPatientIDs) which hold the results are null when I try and use them after calling loadData().
private OracleDataConnection theConnection;
private List listNormalPatientIDs;
private List listBasePatientDetails;
private boolean loadData() {
        if (getNormalPatientIDs() == false || getBasePatientDetails() == false) {
            return false;
        } else {
            return true;
    private boolean getNormalPatientIDs() {
        String sqlString = "SELECT PATID_REFNO FROM PATIENT_IDS";
        if (!getResultList(sqlString, listNormalPatientIDs)) {
            ErrorList.addError(new ErrorItem(ErrorList.DATABASE_QUERY_FAILED, 0, "RPatientID::getNormalPatientIDs()", "Could not retrieve normal patient IDs. Database query failed"));
            return false;
        } else {
            return true;
    private boolean getBasePatientDetails() {
        String sqlString = "SELECT PATID_REFNO, PITYP_REFNO, PATNT_REFNO, IDENTIFIER FROM PATIENT_IDS";
        if (!getResultList(sqlString, listBasePatientDetails)) {
            ErrorList.addError(new ErrorItem(ErrorList.DATABASE_QUERY_FAILED, 0, "RPatientID::getBasePatientDetails()", "Could not retrieve base patient details. Database query failed"));
            return false;
        } else {
            return true;
    private boolean getResultList(String sSQL, List returnList) {
        List resultList;
        if (theConnection.hasConnection()) {
            resultList = (List)theConnection.executeSQL(sSQL);
            if (resultList == null) {
                return false;
            } else {
                returnList = resultList;
                return true;
        } else {
            // Lost database connection.
            ErrorList.addError(new ErrorItem(ErrorList.DATABASE_CONNECTION_FAILED, 0, "RPatientID::getResultList", "Could not retrieve query data. Database connection lost."));
            return false;
    }So whats going wrong here? Is the result List reference used as a parameter going out of scope or something after returning from getResultList? I must have some gap in my thinking, because I thought references were pretty similar to pointers......ie. they just point to the objects rather than hold the objects themselves.
Suggestions? Thanks all.

Answer 1

public class ReturnTest {
  private static String s = "hio";
  public static boolean stuff(String str) {
    str = "YAHOOOOoooOOO";
    return false;
  public static void main(String[] args) {
    System.out.println(s);
    stuff(s);
    System.out.println(s);
}Try compiling that. When you run it you will notice that s still prints out as "hio." This is basically what you are doing with your method.
The parameter "str" (or returnList in your case) has a different address (pointer) than the original s. This is because Java makes copies of all the objects you pass to methods as paramters. To make your method work, you're either going to have to modify your two private lists directly or do something along the lines of this:
public class ReturnTest {
  private static String s = "hio";
  public static boolean stuff(String str) {
    str = "YAHOOOOoooOOO";
    s = str;
    return false;
  public static void main(String[] args) {
    System.out.println(s);
    stuff(s);
    System.out.println(s);
}Note how I put "s = str;" in the stuff() method. If you were to run this, you would see the output:
hio
YAHOOOOoooOOO
Like I said, there's probably a better way to do this, but that's my solution to your predicament.