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understand Array Object, and what I want to do is call my array in every class I have

I'm trying to understand Array Object, and what I want to do is call my array in every class I have.
this is my code:
projectProva.java
public class ProjecteProva {
Scanner sc = new Scanner(System.in);
private final int maxContador = 4;
private final DadeArr LlistaUsuari[] = new DadeArr[maxContador];
int ContadorActual;
}
DadeArr.java
public class DadeArr {
private String nomUsuari;
private String cognomUsuari;
public DadeArr(String nU, String nC){
nomUsuari = nU;
cognomUsuari = nC;
}
Right now I'm working in projectProva.java , I have some method that saves into array a data input with scanner.
Here is an example of one of my method:
public int inserir(int aContadorActual){
ContadorActual = 1;
for (int i=1;i<=ContadorActual;++i){
System.out.println("Introdueix el nom del usuari: ");
String nU = sc.nextLine();
//sd.setNomUsuari(Name);
System.out.println("Introdueix el teu cognom : ");
String nI = sc.nextLine();
LlistaUsuari[ContadorActual] = new DadeArr(nU,nI);
System.out.println("El teu usuari s'ha creat satisfactoriament");
}
ContadorActual++;
return ContadorActual;
}
This method asks user his name and surname and saves it in array LlistaUsuari.
Then, I want to use this array(with the data in) in another .java file from the same package, but i don't know how to properly call the array.
I just started to learn this type of array, and i want to understand it.
After solving this , im looking forward to take all array info and send it to a data base or text file.
If I can't proceed with it i will switch to Array 2D.
Plus, I'm wondering if this type of Array ( array object ) is very usefull or not.
Thanks.
I also made this question at https://www.reddit.com/r/javahelp/comments/dsyu4b/array_object/?

You have multiple gotchas in your code.
1. you should always iterate an array from index 0 (unless you have special needs or you are programming in a language like Python where arrays start from 0)
2. You should set the condition in the loop to be less than the exact length of the array. So in your case it will be i < LlistaUsuari.length. You are getting a null because you are only filling up index #1 of the array.
All you need is a public getter method for the array that you want to access.
Something like:
public DadeArr[] getLlistaUsuari() {
return this.LlistaUsuari;
}
And that should do it.
In other classes, create an instance of ProjecteProva (lets's call it pPI) and to get the array there, simply do pPI.getLlistaUsuari() and you'll have it,

Shallow copy of object not correctly updating original object

I would like to model a graph, and to do so :
I have a class A that contains a LinkedList of instances of A and has a setter method associated :
class A {
private LinkedList<A> list;
[...]
public setList(LinkedList<A> l) {
this.list = l;
}
}
And in an other class NetA I have a method genCon that takes a LinkedList of instances of A then sets their list attribute to be a shuffled SubList of rlist :
static void genCon (LinkedList<A> rlist) {
for(int i=0; i<rlist.size(); i++) {
A temp = rlist.get(i);
LinkedList<A> slist = new LinkedList<A>(rlist.subList(0, rlist.size()));
temp.setList(slist);
}
}
Then genCon(rlist) is called in main, but altough all the objects of rlist should have their list initialized (and being equal to a shuffled version of rlist) some appear to be empty, with no consistent pattern (i.e. not every n or repeatable pattern), but completely at random.
At first I thought that A temp = rlist.get(i) was not giving me a shallow copy of the object at index i, but the check for identity with == returns true, so, if I am not mistaken that means that both variable hold the same reference and that should not be what is causing the issue?
Then I thought that it might be an optimization issue, maybe eclispe tries to do the operations in parallel and that somehow messes up the initialization at random?
I have tried to process step by step, but I can't seem to find where I messed up.
What did I miss?
Edit :
The main function looks like this :
public static void main(String[] args) {
LinkedList<A> a_list = generateAList(20);
genCon(a_list);
for(int i=0; i<a_list.size(); i++) {
System.out.println(a_list.get(i).toString());
}
}
a_list is correctly initialized. The issue happens in the following loop, when trying to print the objects.
Since it's only for testing main is located in the same class as genCon() atm.
generateAList() looks like this :
static public LinkedList<A> generateAList(int n) {
LinkedList<A> a_list = new LinkedList<A>();
for(int i=0; i<n; i++) {
ap_list.add(A.rand()); // A.rand() is just a static function that return an instance of A with randomly set values and an unitialized list.
}
return ap_list;
}

Using Runnable to run a method by multiple threads

In my program, I want to create multiple threads in one of the methods where each thread has to run a specific method with a given input. Using Runnable, I have written this snippet.
class myClass {
public myClass() { }
public void doProcess() {
List< String >[] ls;
ls = new List[2]; // two lists in one array
ls[0].add("1"); ls[0].add("2"); ls[0].add("3");
ls[1].add("4"); ls[1].add("5"); ls[1].add("6");
// create two threads
Runnable[] t = new Runnable[2];
for (int i = 0; i < 2; i++) {
t[ i ] = new Runnable() {
public void run() {
pleasePrint( ls[i] );
}
};
new Thread( t[i] ).start();
}
}
void pleasePrint( List< String > ss )
{
for (int i = 0; i < ss.size(); i++) {
System.out.print(ss.get(i)); // print the elements of one list
}
}
}
public class Threadtest {
public static void main(String[] args) {
myClass mc = new myClass();
mc.doProcess();
}
}
Please note, my big code looks like this. I mean in one method, doProcess(), I create an array of lists and put items in it. Then I want to create threads and pass each list to a method. It is possible to define the array and lists as private class members. But, I want to do that in this way.
Everything seems to be normal, however, I get this error at calling pleasePrint():
error: local variables referenced from an inner class must be final or effectively final
pleasePrint( ls[i] );
How can I fix that?

The reason you are getting this error is straightforward and clearly mentioned - local variables referenced from an inner class must be final or effectively final. This is, in turn, because, the language specification says so.
Quoting Guy Steele here:
Actually, the prototype implementation did allow non-final variables
to be referenced from within inner classes. There was an outcry from
users, complaining that they did not want this! The reason was interesting: in order to support such variables, it was necessary to
heap-allocate them, and (at that time, at least) the average Java
programmer was still pretty skittish about heap allocation and garbage
collection and all that. They disapproved of the language performing
heap allocation "under the table" when there was no occurrence of the
"new" keyword in sight.
As far as your implementation goes, instead of using an array of list, I'd rather use a list of lists.
private final List<List<String>> mainList = new ArrayList<>();
You can create new lists and insert them into the main list in the constructor depending on the number of lists you want.
public ListOfLists(int noOfLists) {
this.noOfLists = noOfLists;
for (int i = 0; i < noOfLists; i++) {
mainList.add(new ArrayList<>());
}
}
You can then change your doProcess() method as follows:
public void doProcess() {
for (int i = 0; i < noOfLists; i++) {
final int index = i;
// Using Lambda Expression as it is much cleaner
new Thread(() -> {
System.out.println(Thread.currentThread().getName());
pleasePrint(mainList.get(index)); // Pass each list for printing
}).start();
}
}
Note: I used an instance variable named noOfLists to (as the name suggests) store the number of lists I need. Something as follows:
private final int noOfLists;
To populate the list, you could do:
mainList.get(0).add("1");
mainList.get(0).add("2");
mainList.get(0).add("3");
mainList.get(1).add("4");
mainList.get(1).add("5");
mainList.get(1).add("6");
// And so on...
And you'll get the output something as:
Thread-0
1
2
3
Thread-1
4
5
6
Hope this helps :)

First to that, you will get a NullPointerException here:
ls[0].add("1"); ls[0].add("2"); ls[0].add("3");
ls[1].add("4"); ls[1].add("5"); ls[1].add("6");
Before, yo must instantiate the lists:
ls[0] = new ArrayList<>();
ls[1] = new ArrayList<>();
About the compiler error, try to define the array as final. Change:
List< String >[] ls;
ls = new List[2]; // two lists in one array
By:
final List< String >[] ls = new List[2]; // two lists in one array
This is because you can't access to non-final (or effectively final) variables from a local class.
'ls' is effectively final but probably, since you have defined it in two lines, the compiler is not able to notice that.

How to add to ArrayList in a For loop

I have a sequence of information being randomly generated. I would like to save that information in a variable of some kind so that it can be recalled elsewhere. i think I want to use an ArrayList, but I'm not sure how to add the information while inside a for loop (which is where it is being created). The code I have is as follows:
public static ArrayList<String> phoneList
public static void main(String[] args){
Random randomNumber = new Random();
int howMany = randomNumber.nextInt(11);;
String holding;
for (int i=0; i < howMany; i++){
int itemRandNum = randomNumber.nextInt(11);//for all Item Categories
int priceRandNum = randomNumber.nextInt(11);//Prices for all categories
holding = phones[itemRandNum]+" $"+ priceOfPhones[priceRandNum];
//System.out.println(holding);
phoneList.add("holding"); //here is where I would like to add the information
//contained in "holding" to the "phoneList" ArrayList.
}
System.out.println(phoneList);
}
I am getting a NullPointerException. If an ArrayList is not the best thing to use here, what would be?
Any help you can give is appreciated.

public static void String Main(String[] args) suggests this doesn't have much to do with Android.
First, instantiate the list (Outside your for loop):
phoneList = new ArrayList<>(howMany); //Adding "howMany" is optional. It just sets the List's initial size.
Then, add the values:
for (int i = 0; i < howMany; i++) {
//...
phoneList.add(holding);
//Don't place holding in quotation marks, else you'll just add "holding" for every entry!
}

You get a NullPointerException because ArrayList phoneList is null since you didn't initialize it. Therefore write
public static ArrayList<String> phoneList = new ArrayList<>();

As it is you are just declaring the class ArrayList not instantiating it. it is necessary for all the time you want to use a class to create an instance of the same class and thats simple, just do:
public static ArrayList phoneList = new ArrayList() (if you are running older versions of java), otherwise use public static ArrayList phoneList = new ArrayList<>().

Delete object from array

I've got this constructor in the class Music:
protected String Title;
protected String Autor;
protected String Type;
protected int Code;
public Music (String title, String autor, String type, int code){
this.setTitle(title);
this.setAutor(autor);
this.setType(type);
this.setCode(code);
}
#Override
public String toString(){
return this.Title + " " + this.Autor + " " + this.Type + " " + this.Code;
}
Then, in other class called ManageMusic I create some methods to then use them on the main class. I also define a String array refered to the Music class which I will use in the main class:
private final Music[] musicList;
private int counter;
public ManageMusic(int maxSize) {
musicList= new Music[maxSize];
counter= 0;
public void add(Music m){
musicList[counter] = m;
counter++;
}
Here, I have to create a delete method which would delete a especific object from the musicList and return this list without that object.
This is the way I add music elements to the musicList on the main class:
static ManageMusic musiclist = new ManageMusic(20);
musicList.add(new Music(title, autor, format, code));
My approach for the delete method in the ManageMusic class is to copy this list into a new String[] and then copy it back to the list. But as I'm using an objet from Music instead of from String, I cannot make the copy back because it does not cast the String to the musicList:
public void delete(int code){
String[] newString = new String[musicList.length];
int index_1 = 0;
for (int i=0; i<musicList.length; i++){
if(i != code){
newString[index_1] = musicList[i].toString();
index_1++;
}
}
int index_2 = 0;
for (int i=0; i<newString.length; i++){ //THIS IS WHERE IT SAYS: Cannot convert
// from String to Music
musicList[index_2] = newString[i];
index_2++;
}
}
I have to do something not far from this, because then I've got a method that list elements from the musicList, so I cannot set a return statement for the method.

Why you can do it without an ArrayList
As some people suggested in the comments, you should probably use ArrayLists or similar stuff from the java.util.collection package.
However I assume you want to learn how such things work, so I will not provide you with code (First because I'm too lazy, second to encourage you to learn it yourself) but with some explanation.
edit: First: Your problem is that you copy strings, not references. Instead of trying to use the toString method, try to handle it with the "objects" (i.e. their references) themselves.
Error checking
As you might have noticed your add will cause an IndexOutOfBoundsException if you try to add another entry after your list reached your max_size. You should check for that. You should also check lots of things in the following explanations, I'll provide a few suggestions.
Simple deletion with your exact example
Just use Music[] instead of String[] and copy the reference of the temp Music[] to your musicList.
Better way to handle it: dynamic array structure
So what I suggest is to make use of a dynamic array structure. You will have to copy arrays around a lot, which can be a bit difficult. But basically it's what an ArrayList does.
Constructor
But how to implement that dynamic structure? Let's first start with the initialization, i.e. your constructor. You will not need a parameter for a maximum size (unless you want to restrict your music collection for some reason). Just create an array with size 0. (Of course you can also implement copy constructors and other things, but for the start keep it simple.)
add()
To add more music, you simply create a new array with the size of your current collection + 1, copy all references (this is probably the answer you were looking for. You take the strings, but just take the objects themselves) from the current array to the new array, add the new Music and change the reference of your member variable to your newly created, bigger array (i.e. assigning musicList = tempArray; or something similar). (Error checking: is the object null?)
delete()
For deletion you can do just the same. Create a new temporary array (this time with a reduced size), copy all values over but leave out the one you want to delete. To determine which shall be deleted you can either check for indices or even check the objects for equality. (Error checking: size of temp array should never be smaller than 1, so check if your array is empty - consider to use a method like isEmpty() for that.)
Why should I do this?
Once you got those concepts you will be able to manage your array in whatever way you like. Search through it, delete all elements, copy your music collection to your friend's, etc. etc.
And beyond that?
And after you learned this, go ahead and try the ArrayList - you will figure out it works very much like what you have just written. And now you can be proud that you not only can use ArrayLists, but also know how and why they behave like they do.

Its better to use ArrayList than writing own logic to delete object from existing array. Here is how you can use ArrayList :
{
ArrayList<Music> list = new ArrayList<Music>();
Music m1 = new Music(title, autor, format, code);
list.add(m1);
// similarly you can check whether object is present in ArrayList or not using
if(list.contains(m1)){ // This check whether object is present in ArrayList or not
//Do whatever you want
}
}

ArrayList example:
List<Music> musicList = new ArrayList<Music>();
adding to end of list list:
musicList.add(new Music(...));
adding to specified position in list (later ones all move up one place)
musicList.add(index, new Music(..));
remove from list:
musicList.remove(index);
or
musicList.remove(someMusic);
Size of list:
int size = musicList.size();
Get first music:
Music first = musicList.get(0);
Get last music:
Music last = musicList.get(musicList.size()-1);
Loop:
for (Music : musicList) {
//do stuff
}

do like this
public void delete(int code){
List<Music> list = new ArrayList<Music>(); //creating new empty list
for (Music m:musicList){
if(m.code != code){ // removing condition
list.add(m); // adding music to new list
}
}
musicList = list.toArray(new Music[list.size()]); // assigning back list to musicList
}