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How can I assign instance of a class from an array to a class initializer in java?

In a Android application I am making I have an array of instances of a certain class I made, and later in the program I need to use the getter and setter methods from that class on an instance of the class from the array. Do I need to assign the instance of the class from the array to a new class initializer? Here is some code to clear this up:
Class
public class ProfileInformation {
private String console;
private String gamertag;
public String getConsole() {
return console;
}
public void setConsole(String console) {
this.console = console;
}
public String getGamertag() {
return gamertag;
}
public void setGamertag(String gamertag) {
this.gamertag = gamertag;
}
}
Array
ArrayList<ProfileInformation> ProfTags = new ArrayList<>();
Some instances of ProfileInformation are then added to arraylist, and then I get one of the instances from the arraylist and try to use getGamertag() to set it to a string:
ProfileInformation profNew = ProfTags.get(ProfTags.size()-1);
String example = profNew.getGamertag();
The problem is example will equal null. Why is this?

First, an Arraylist is a List, try not to confuse that with actual arrays.
Do I need to assign the instance of the class from the array to a new class initializer?
You don't need to get an element out of the Arraylist, no. You can chain many methods together
String example = ProfTags.get(ProfTags.size()-1).getGamertag();
example will equal null. Why is this?
For the same reason any object is null... You never set it equal to anything else

This code runs on my laptop:
public static void main(String[] args) {
ArrayList<ProfileInformation> ProfTags = new ArrayList<>();
element = new ProfileInformation();
element.setGamertag("Actual Gamer tag value");
ProfTags.add(element);
ProfileInformation profNew = ProfTags.get(ProfTags.size()-1);
String example = profNew.getGamertag();
}
Output is:
Actual Gamer tag value
I guess you didn't call setGamertag(String).

Create an object and put in an array

I want to create just an object,i am not sure if i am creating and it is right.My example ,lets say i have 2 class(Userinput and paper).0fcourse there is and the main.In this example No inherit (just 2 simple class).Am i create an object right?How i put in it in an array or in the same array?
package exercise;
public class Exercise{
static int N; //from keyboard .I have a class userinput.It doesnt need to write it here ,i have in the other class the problem
public static void main(String[] args) { // main class
Paper[] pin = new Paper[N]; //i create an array
Paper.setpencil(3); // i wrote the 3 .In this way i create 3 pencil?
Paper.getpencil(3);
Paper.setsomething(4); // i wrote the 4 .I create 4 ?
Paper.getsomething(4);
} }
public class Paper{ //in this class i am confused
public Paper(){} //default constructor
private int pencil;
private String something;
public int getpencil(){
return pencil;
}
public void setpencil(){
pencil=UserInput.getInteger():
}
public int getsomething(){
return something;
}
public void setsomething(){
something=UserInput.setInteger();
}
}

with this statement:
Paper[] pin = new Paper[N];
you create an array of object of class Paper.
You must also create an object for each array element like:
for (int i=0; i < N, i++)
{
pin[i] = new Paper();
}
And next you should refer to an element (e.g. first element that with index 0) of the array in this way:
pin[0].setpencil(3);

Manipulating an object inside a method

I am trying to manipulate an object inside a method like this:
My class Problem:
public class TaxiProblem {
public Problem(final World world, final Agent agent) {
_world = world;
_world.setRandomAgent(_agentPlace);
}
private Place _agentPlace;
// Various other functions
}
and in the function .setRandomAgent() in the class World I am trying to manipulate the Place object in what I want it to be like this:
public void setRandomAgent(Place agentPlace) {
int rand = _random.nextInt(25);
agentPlace = _places.get(rand);
agentPlace.setHasAgent(true);
}
I basically want to grab a random Place from the List _places and have it in the variable agentPlace in .setRandomAgent() which in turn will have it in _agentPlace in the Problem class. I thought this would work since Java passes objects by reference in methods but it didn't and _agentPlace remains null.

By doing this
agentPlace = _places.get(rand);
you are overwriting the reference that was passed to the method and losing access to the object you want to alter.
In your setRandomAgent method, agentPlace is indeed a reference that points to your _agentPlace field, not the field itself. In the line I pasted above, what you do is make that reference point to a different object.

_agentPlace = _world.getRandomAgent();
public Place getRandomAgent() {
int rand = _random.nextInt(25);
Place agentPlace = _places.get(rand);
agentPlace.setHasAgent(true);
return agentPlace();
}
When you pass agentPlace to the method, you are creating a copy of the reference. So if you modify the object, then it would work when you return up the stack. But reassigning the variable makes you lose the object you were working with.

I suspect that your problem lies in the implementations as your understanding of pass by reference I believe is correct. The following code will produce the results you expect - That is, it will first print "Before change", then "I am changed!".
class Program
{
static void Main(string[] args)
{
var problem = new Problem();
}
}
public class Problem
{
public Problem()
{
var toChange = new ClassToChange();
toChange.ChangeMe = "Before change";
Console.WriteLine(toChange.ChangeMe);
var changer = new ClassThatChanges();
changer.ChangeSomething(toChange);
Console.WriteLine(toChange.ChangeMe);
Console.ReadLine();
}
}
public class ClassToChange
{
public string ChangeMe { get; set; }
}
public class ClassThatChanges
{
public void ChangeSomething(ClassToChange classToChange)
{
classToChange.ChangeMe = "I am changed!";
}
}

Calling Field Names from a List of Strings

I have a class as such:
public interface ControlService extends RemoteJsonService
{
public void myValues( String [] Names, AsyncCallback<Map<String,RegisterValues>> callback);
public class RegisterValues
{
int FirstField;
int SecondField;
//etc. 200 times
I have a string as such:
String[] fieldValues = new String[]{"FirstField", "SecondField", ....}; //200 of these
My attempt at calling upon these fields (the important part is towards the end):
public class RegistersPanel implements TestStandPanel, ChangeHandler
{
private ControlService service_;
public RegistersPanel(){
service_.RegisterValues( Names, new AsyncCallback<Map<String,ControlService.RegisterValues>>()
{
public void onSuccess( Map<String,ControlService.RegisterValues> result )
{
for( String Name : result.keySet() ){
for (String Values : fieldValues){
message+=result.get(Name).getField(Values); //something like this but I can't quite figure out the syntaxt
//message+=result.get(Name).FirstField; this is tedious
}
As you can see, I can't just use getField, and Eclipse is giving me no helpful options - any advice?

I think this is something what you are looking for inside the double for loop,
RegisterValues regValue=reuslt.get(Name);
regValue.getClass().getDeclaredField(Values).getInt(regValue);
This is not the exact solution. This just gets you access to the fields value inside that class.
P.S: that Values would look nice if changed to value

Dynamically return a list of all class variable values in Java

I am creating a helper class in parsing XML elements, so the developer do not need to know the exact name and capitalization of the XML fields.
private static class TagNames{
public static String RESOURCE_ID = "ResourceId";
public static String RESOURCE_NAME = "ResourceName";
public static String RESOURCE_PRICE = "ResourcePrice";
}
This makes it easier to do things like:
someXMLParser.getValueByTagName(TagNames.RESOURCE_ID);
My question is this. If I want to iterate over all the fields declared in class TagNames, how do I do that? Pseudocode:
For tag in TagNames:
someXMLParser.getValueByTagName(tag)
I know I will probably have to restructure all of this. But I can't figure out a way to make the names easily accessible as well as iterable, without any duplication.
Any suggestions?

You're literally asking for a solution based on reflection, but I think a Java Enum may be a better choice in this case. Building on Frederick's example:
public class EnumTest {
public enum Tags {
RESOURCE_ID("ResourceId"),
REOURCE_NAME("ResourceName"),
RESOURCE_PRICE("ResourcePrice");
private final String tagName;
Tags(String tagName) {
this.tagName = tagName;
}
public String getTagName() {
return tagName;
}
}
public static void main(String[] args) {
for(Tags tag : Tags.values()) {
System.out.println("const:" + tag.name()
+ " tagName:" + tag.getTagName());
}
// API user might do e.g.:
// document.getValueForTag(Tags.REOURCE_NAME);
}
}

Although I agree that you should probably use enums or ResourceBundles, here's a solution to your actual question. A method that generates a Map name -> value from all public constants in a given class (the only thing that's missing should be try / catch or throws)
public static Map<String, Object> getConstantValues(Class<?> clazz){
Map<String, Object> constantValues = new LinkedHashMap<String, Object>();
for(Field field : clazz.getDeclaredFields()){
int modifiers = field.getModifiers();
if(Modifiers.isPublic(mod)
&& Modifiers.isStatic(mod) && Modifiers.isFinal(mod)){
constantValues.put(field.getName(), field.get(null));
}
}
return constantValues;
}

You may want to consider using a ResourceBundle instead of a class to store the tag names. May require a little bit of reworking of your code but it will be easier to produce a list of tags compared to what you are doing now, and adding a new tag won't require much work other then adding a line to the properties file.

You can do this quite easily using enum and an accompanying array:
public class Main {
public enum TagName { RESOURCE_ID, REOURCE_NAME, RESOURCE_PRICE }
private static String[] tags = {"ResourceID", "ResourceName", "ResourcePrice"};
public static String getValueByTagName(TagName tag) {
return tags[tag.ordinal()];
}
public static void main(String[] args) {
System.out.println("Calling by getValueByTagName:");
System.out.println(getValueByTagName(TagName.RESOURCE_ID));
System.out.println("Calling TagName.values() for loop:");
for (TagName t : TagName.values()) {
System.out.println(getValueByTagName(t));
}
}
}

Using an enum is a good fit, especially if you use a custom constructor and the built in "values" method:
public class Main {
public static enum TagName {
RESOURCE_ID("ResourceId"),
RESOURCE_NAME("ResourceName"),
RESOURCE_PRICE("ResourcePrice"),
;
private String s;
private TagName(String s) { this.s = s; }
public String toString() { return this.s; }
public static String[] strings() {
List<String> ss = new ArrayList<String>();
for (TagName tagName : TagName.values()) {
ss.add(tagName.toString());
}
return ss.toArray(new String[ss.size()]);
}
}
public static void main(String[] args) {
// Use TagName.values() for the enums, or for strings...
for (String s : TagName.strings()) {
System.out.println(s);
}
}
}
This way you can simply add new tags and they'll automatically get picked up by the "strings" method; for extra performance you could compute that string array just once, statically, since you can't change the set of enums dynamically. You could get even fancier by auto-generating the tag strings from their constant values, if they are really normalized...