weird behaviour when passing objects

[Tay Thotheolh]

public class XXXServer{

SQLManager sql = null;

   public XXXServer(){
      sql = new SQLManager();
      new Server(sql).start(); //start a server thread
   }

//Some other codes not important to this topic

   public doExit(){
      sql.close();
      System.exit(0);
   }

}
 

The error points that my sql object at doExit() method is null when I have apparently already had 'sql=new ...' done. I have tried to doExit(). I am curious if you pass the sql object to the Server for the starting of a server thread, did it somehow took away the sql in XXXServer class ?

[Rob Spoor]

Are you sure none of the other code resets your sql field to null? What is this Server class?

[Tay Thotheolh]

The XXXServer is some sort like a general server and the Server inside it is a more specific server. I wish I could dump the entire codes here so you can read through it but I doubt if this thing is going to take in so much codes I wrote.

I have checked for anything resetting it and no...nothing is resetting it according to what I know.

[David Newton]

Variables won't be randomly set to null--*something* is making it null, if that's actually what the error is. Are you *sure* that "sql" is null in the doExit() method? If it is, then *something* is setting it to null somewhere.

[Rob Spoor]

Do a search for "sql = null;" in your code, I'm sure you'll find something. And if not, please show us the stack trace you get.

[Ernest Friedman-Hill]

Since this is not your real code, but a paraphrase, are you absolutely sure the constructor doesn't look like

public XXXServer(){
      SQLManager sql = new SQLManager();
      new Server(sql).start(); //start a server thread
   }

That one word "SQLManager" on the first line makes all the difference: it makes that line declare a new, local variable also named "sql", leaving the member uninitialized. This is an extremely common mistake for beginners to make.

[Tay Thotheolh]

public class testServer{

   private testServer instance;
   private SQLManager sql;

   public testServer(){ instance = this.testServer }
   
   public testServer getInstance(){
      return instance;
   }

   public SQLManager getSQLManager(){
      return sql;
   }

}

Let's say the SQLManager is already instantiated and created. I have another class that wishes to get the SQLManager from testServer so I getInstance to get the testServer instance then I do a getSQLManager, does it break anything ? Because this is apparently another case where passing objects can be weird.

[David Newton]

I'm unclear regarding this line:

public testServer() {
    instance = this.testServer
}  

Does that actually compile (after adding the missing semi-colon)?

That aside, there's absolutely nothing weird about returning an object as the result of a method call from another class. Without understanding more about the actual code and how it's being used it's really tough to provide any meaningful input.

[Michael Angstadt]

Tay Thotheolh wrote:Because this is apparently another case where passing objects can be weird.

A reference to the SQLManager object is returned by calling the getSQLManager() method. So, the calling code can do things to the SQLManager like calling close() without the testServer knowing about it--which can have unintended behavior.

[Tay Thotheolh]

Hi. I managed to create an example. Let's say we have a Session here...

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

import java.io.File;
import org.jcurrency.cache.Cache;
import org.jcurrency.export.Export;
import org.jcurrency.xml.XMLParser;
import org.w3c.dom.Element;
import org.w3c.dom.NodeList;

/**
 *
 * @author thotheolh
 */
public class Session {

    private Cache cache; //1-online, 2-offline
    private Export export;
    private XMLParser parser;
    private String settings_File = "essentials/settings.xml";
    private static Session currentSession;

    //Settings Value
    private int MODE = 0;
    private String DISCONNECT = null;
    private File EXPORTFOLDER = null;
    private String EXPORTTYPE = null;
    private String GUI = null;
    private int TIMEDCACHE = 0;
    private String TIMEDCACHE_STATUS = null;
    private String CACHEDONE = null;

    public Session() {
        currentSession = this;
//        getSettings();
//        checkOnlineMode();
    }

    /**
     * Getters and Setters
     */
    /**
     * @return the cache
     */
    public Cache getCache() {
        return cache;
    }

    /**
     * @param cache the cache to set
     */
    public void setCache(Cache cache) {
        this.cache = cache;
    }

    /**
     * @return the export
     */
    public Export getExport() {
        return export;
    }

    /**
     * @param export the export to set
     */
    public void setExport(Export export) {
        this.export = export;
    }

    /**
     * @return the MODE
     */
    public int getMODE() {
        return MODE;
    }

    /**
     * @param MODE the MODE to set
     */
    public void setMODE(int MODE) {
        this.MODE = MODE;
    }

    /**
     * @return the DISCONNECT
     */
    public String getDISCONNECT() {
        return DISCONNECT;
    }

    /**
     * @param DISCONNECT the DISCONNECT to set
     */
    public void setDISCONNECT(String DISCONNECT) {
        this.DISCONNECT = DISCONNECT;
    }

    /**
     * @return the EXPORTFOLDER
     */
    public File getEXPORTFOLDER() {
        return EXPORTFOLDER;
    }

    /**
     * @param EXPORTFOLDER the EXPORTFOLDER to set
     */
    public void setEXPORTFOLDER(File EXPORTFOLDER) {
        this.EXPORTFOLDER = EXPORTFOLDER;
    }

    /**
     * @return the EXPORTTYPE
     */
    public String getEXPORTTYPE() {
        return EXPORTTYPE;
    }

    /**
     * @param EXPORTTYPE the EXPORTTYPE to set
     */
    public void setEXPORTTYPE(String EXPORTTYPE) {
        this.EXPORTTYPE = EXPORTTYPE;
    }

    /**
     * @return the GUI
     */
    public String getGUI() {
        return GUI;
    }

    /**
     * @param GUI the GUI to set
     */
    public void setGUI(String GUI) {
        this.GUI = GUI;
    }

    /**
     * @return the TIMEDCACHE
     */
    public int getTIMEDCACHE() {
        return TIMEDCACHE;
    }

    /**
     * @param TIMEDCACHE the TIMEDCACHE to set
     */
    public void setTIMEDCACHE(int TIMEDCACHE) {
        this.TIMEDCACHE = TIMEDCACHE;
    }

    /**
     * @return the TIMEDCACHE_STATUS
     */
    public String getTIMEDCACHE_STATUS() {
        return TIMEDCACHE_STATUS;
    }

    /**
     * @param TIMEDCACHE_STATUS the TIMEDCACHE_STATUS to set
     */
    public void setTIMEDCACHE_STATUS(String TIMEDCACHE_STATUS) {
        this.TIMEDCACHE_STATUS = TIMEDCACHE_STATUS;
    }

    /**
     * @return the CACHEDONE
     */
    public String getCACHEDONE() {
        return CACHEDONE;
    }

    /**
     * @param CACHEDONE the CACHEDONE to set
     */
    public void setCACHEDONE(String CACHEDONE) {
        this.CACHEDONE = CACHEDONE;
    }

    /**
     * Methods
     */
    private void checkOnlineMode() { //check if MODE value is correct
        //TODO: Check if mode is valid
        if (this.getMODE() != 1 || this.getMODE() != 2) {
            //TODO:Check if internet is connected, if yes, set to '1' else set '2'
        }
    }

    private void checkTimedCacheStatus() { //check if value for Timed-Cache is correct
    }

    private void getSettings() { //read 'settings.xml' settings value
        parser = new XMLParser();
        NodeList nodes = parser.doParse(settings_File, "settings");
        Element element;
        for (int i = 0; i < nodes.getLength(); i++) {
            element = (Element) nodes.item(i);
            setSettingsValue(element.getNodeName(), element.getTextContent(), element);
        }

    }

    private void setSettingsValue(String nodeName, String textContent, Element element) { //set settings to Settings Value from 'settings.xml'
        //TODO:Set and check each value
        if (nodeName.equals("mode")) {
        } else if (nodeName.equals("disconnect")) {
        } else if (nodeName.equals("export-folder")) {
        } else if (nodeName.equals("export-type")) {
        } else if (nodeName.equals("gui")) {
        } else if (nodeName.equals("timed-cache")) {
            element.getAttribute("status");
        } else if (nodeName.equals("cache-done")) {
        } else {
        }
    }

    public void writeSettings() { // writes Settings Value to 'settings.xml'. Usually called when program is exiting or some Settings Values are adjusted.
        //TODO:Use XMLMaker to write out all Settngs Value to 'settings.xml'
    }

    public void executeAllSettings() { // read all Settings Values and act accordingly to the values
        //TODO:Read through all Settings Values and handle each values
    }

    public void doCache() { // do Caching
        //TODO:Do caching of currency ratings
    }

    public void doConvert() { // do currency conversion
        //TODO:Move currency conversion functions from GUI functions to here
    }

    public void startCachingTimer() { //starts timer for timed caching function if enabled
    }

    public void stopCachingTimer() { //stops timer for timed caching function
    }

    public static Session getCurrentSession(){
        return currentSession;
    }
}

This session contains the session information of the program. I have another class called testSession. This is the main method class.

public class testSession {

    private testSession currentTest;
    private static Session session = new Session();

    public testSession() {
        currentTest = this;
        
        testSessionA sessA = new testSessionA();
        testSessionB sessB = new testSessionB();
        sessA.startSessionTest();
        sessB.startSessionTest();
    }

    public testSession getCurrentTest() {
        return currentTest;
    }
    
    public static Session getSession(){
        return session;
    }
    public static void main(String[] args) {
        // TODO code application logic here
        new testSession();
    }
}

I would have two seperate smaller classes called testSessionA and testSessionB. I would explain their uses later.

public class testSessionA {

    Session session;
    testSession sess;

    @SuppressWarnings("static-access")
    public testSessionA() {
        sess = new testSession().getCurrentTest();
        //session = new Session().getCurrentSession();
        session = sess.getSession();
    }

    public void startSessionTest() {
        session.setMODE(12345);
        System.out.println("Test Session A set MODE to 12345");
    }
}

public class testSessionB {

    Session session;
    testSession sess;
    @SuppressWarnings("static-access")
    public testSessionB() {
        sess = new testSession().getCurrentTest();
        //session = new Session().getCurrentSession();
        session = sess.getSession();
    }

    public void startSessionTest() {
        System.out.println("Mode: "+session.getMODE());
        if(session.getMODE()!=12345){
            System.out.println("Session Test Fail !");
        }
    }
}

What I want is that the main method class, testSession, to have a Session object. Then testSessionA tries to access testSession's Session object, update the MODE object in testSession to an int of 12345 and then testSessionB accessing testSession again, retrieving the Session object and attempt to print out the changed 12345 int from MODE object.

What I am getting now is the following StackOverFlowError...

Exception in thread "main" java.lang.StackOverflowError
        at org.jcurrency.test.testSessionA.<init>(testSessionA.java:20)
        at org.jcurrency.test.testSession.<init>(testSession.java:21)
        at org.jcurrency.test.testSessionA.<init>(testSessionA.java:20)
        at org.jcurrency.test.testSession.<init>(testSession.java:21)
        at org.jcurrency.test.testSessionA.<init>(testSessionA.java:20)
        at org.jcurrency.test.testSession.<init>(testSession.java:21)
Java Result: 1

How do I resolve it ? If this can be resolved, I think the other SQLManager thing (which have similar structures) would also be resolved.

[Henry Wong]

What I am getting now is the following StackOverFlowError...

How do I resolve it ? If this can be resolved, I think the other SQLManager thing (which have similar structures) would also be resolved.

Well, you do know why it is happening right?

Exception in thread "main" java.lang.StackOverflowError   
        at org.jcurrency.test.testSessionA.<init>(testSessionA.java:20)   
        at org.jcurrency.test.testSession.<init>(testSession.java:21)   
        at org.jcurrency.test.testSessionA.<init>(testSessionA.java:20)   
        at org.jcurrency.test.testSession.<init>(testSession.java:21)   
        at org.jcurrency.test.testSessionA.<init>(testSessionA.java:20)   
        at org.jcurrency.test.testSession.<init>(testSession.java:21)   
Java Result: 1  

Basically, the constructor of your TestSession class is creating a TestSessionA local variable, which in turn, the constructor of your TestSessionA class is creating a TestSession local variabla, which in turn, the constructor of your TestSession class is creating a TestSessionA local variable, which in turn, the constructor of your TestSessionA class is creating a TestSession local variabla, which in turn, the constructor of your TestSession class is creating a TestSessionA local variable, which in turn, the constructor of your TestSessionA class is creating a TestSession local variabla, which in turn, the constructor of your TestSession class is creating a TestSessionA local variable, which in turn, the constructor of your TestSessionA class is creating a TestSession local variabla, which in turn, the constructor of your TestSession class is creating a TestSessionA local variable, which in turn, the constructor of your TestSessionA class is creating a TestSession local variabla, which in turn, the constructor of your TestSession class is creating a TestSessionA local variable, which in turn, the constructor of your TestSessionA class is creating a TestSession local variabla, which in turn, etc. etc. etc. etc. etc. etc. etc. etc. etc.etc. etc. etc. etc.etc. etc. etc. etc. etc. etc. etc. etc. etc. etc. etc. etc.etc. etc. etc. etc.etc. etc. etc. etc. etc. etc. etc. etc. etc. etc. etc. etc.etc. etc. etc. etc.etc. etc. etc. etc. etc. etc. etc. etc. etc. etc. etc. etc.etc. etc. etc. etc.etc. etc. etc. etc.... until your stack overflows.

How do you resolve it, is simple -- don't do that.

Henry