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Why does this simple code throw ArrayIndexOutOfBoundsException?

This is just a dummy code.
I fail to understand what is wrong as I am new to JAVA.
I have already referred:
What causes a java.lang.ArrayIndexOutOfBoundsException and how do I prevent it?
The answers there only pertain to using wrong length indices.
Code:
class abc{
static int n;
static int[] arr=new int[n];
static void print_arr(){
for(int x: arr) System.out.print(x+" ");
}
}
class Main {
public static void main(String[] args) {
abc.n=5;
for(int i=0;i<abc.n;i++){
abc.arr[i]=10;
}
abc.print_arr();
}
}
I want this code to print 10 five times.

One possible way.
class abc{
static int n;
static int[] arr;
static void init(int size) {
arr=new int[size];
}
static void print_arr(){
for(int x: arr) System.out.print(x+" ");
} }
class Main {
public static void main(String[] args) {
abc.n=5;
init(abc.n);
for(int i=0;i<abc.n;i++){
abc.arr[i]=10;
}
abc.print_arr();
} }
Better way
class ABC{
private int size;
private int[] arr;
ABC(int n) {
size = n;
arr = new int[n];
}
public void print_arr(){
for(int x: arr)
System.out.print(x+" ");
}
public int getSize() {
return size;
}
public int[] getArray() {
return java.util.Arrays.copyOf(arr,arr.length);
}
public void setArray(int [] array) {
arr = array.clone();
} }
class Main {
public static void main(String[] args) {
int size = 5;
ABC abc = new ABC(size);
int [] array = new int[size];
for(int i=0;i<abc.getSize();i++){
array[i]=10;
}
abc.setArray(array);
abc.print_arr();
} }

class abc
{
static int n=5;
static int[] arr=new int[n];
static void print_arr()
{
for(int x: arr) System.out.print(x+" ");
}
}
class Main
{
public static void main(String[] args)
{
for(int i=0;i<abc.n;i++)
{
abc.arr[i]=10;
}
abc.print_arr();
}
}
In your case ArrayIndexOutOfBounds exception occurs because you are trying to initialize the array by a variable which has not been initialized yet. So either initialize n with a value before array initialization or use dynamic sized array.

How to access an array from another class

I want to operate on the array called "players" that is declared in the main method. I want to use "players" in my class called "Glucksspielthread"
I know that I can't access "players" because it is declared in the main method and is not visible for other classes.
How can I solve this problem? Here is my code:
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Glucksspieltest {
public static void main(String[] args) {
int numPlayers = Integer.parseInt(args[0]);
int threadSize = Integer.parseInt(args[1]);
ExecutorService es = Executors.newFixedThreadPool(threadSize);
Glucksspielthread[] players = new Glucksspielthread[numPlayers];
for (int i = 0; i < numPlayers; i++) {
players[i] = new Glucksspielthread(i);
es.execute(players[i]);
}
}
}
class Thinker {
public static void think(int Millisekunden) {
try {
Thread.sleep(Millisekunden);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void randomThink(int minMillisekunden, int maxMillisekunden) {
System.out.println("test");
}
}
class Glucksspielthread implements Runnable {
public int playerNumber;
Glucksspielthread(int number) {
playerNumber = number;
}
#Override
public void run() {
for (int i = 0; i <= playerNumber; i++) {
// here, I want to operate on array called "players" that is declared in the main method
}
}
}

Just for your test purpose make your players variable static and public in the Glucksspieltest class, like this:
public class Glucksspieltest {
public static Glucksspielthread[] players;
Then acces it in the Glucksspielthread class like this:
for (int i = 0; i <= playerNumber; i++) {
// here, I want to operate on array called "players" that is declared in the main method
Glucksspieltest.players
}

Add a method to class Glucksspieltest, and make the players array global:
public class Glucksspieltest {
private static Glucksspielthread[] players;
public static Glucksspielthread[] getPlayers(){
return players;
}
public static void main(String[] args) {
int numPlayers = Integer.parseInt(args[0]);
int threadSize = Integer.parseInt(args[1]);
ExecutorService es = Executors.newFixedThreadPool(threadSize);
players = new Glucksspielthread[numPlayers];
for (int i = 0; i < numPlayers; i++) {
players[i] = new Glucksspielthread(i);
es.execute(players[i]);
}
}
}
This way you can get the array by calling the getPlayers() method.
(Note that, it would be adviced to add a constructor to initialize and fill the players array, and separate the player management from the main method as well.)

Make players as private global referance variable
public class Glucksspieltest {
//Make a Global reference variable players
private static Glucksspielthread[] players;
// Make a getter Method to get players
public static Glucksspielthread[] getPlayers(){
return players;
}
public static void main(String[] args) {
int numPlayers = Integer.parseInt(args[0]);
int threadSize = Integer.parseInt(args[1]);
ExecutorService es = Executors.newFixedThreadPool(threadSize);
players = new Glucksspielthread[numPlayers];
for (int i = 0; i < numPlayers; i++) {
players[i] = new Glucksspielthread(i);
es.execute(players[i]);
}
}
}
And access it by Glucksspieltest.getPlayers();
class Glucksspielthread implements Runnable {
public int playerNumber;
private static Glucksspielthread[] players;
Glucksspielthread(int number) {
playerNumber = number;
}
#Override
public void run() {
for (int i = 0; i <= playerNumber; i++) {
// here, I want to operate on array called "players" that is declared in the main method
players= Glucksspieltest.getPlayers(); // play with players
}
}
}

Setting ArrayList items in one Class from another Class

There is a foo class with an ArrayList of double msg called msgstoboo as well as a method setMsg(int index, double input) to alter individual messages in msgstoboo.
There is a networkoffoos class with an ArrayList of foo objects called listoffoos. There is an updatefoomsg method:
public void updatefoomsg (ArrayList<ArrayList<Foo>> Foonetwork)
{
for(int foolayer = 0; foolayer< foonetwork.size(); foolayer++)
for(int fooinlayer = 0; fooinlayer< foonetwork.get(foolayer).size(); fooinlayer++)
for(int msginfoo = 0; msginfoo < foonetwork.get(foolayer).get(fooinlayer).msgstoboo.size(); msginfoo++)
Foonetwork.get(foolayer).get(fooinlayer).setMsg(msginfoo,somerandomvalue)
}
The goal of updatefoomsg is to change the values of individual msgs in msgstooboo. However, no values in the foo class ArrayList `msgstoboo' are altered. Why is this and how do I fix it? Thank you in advance.
UPDATE: Here are the whole foo and networkoffoos classes
public class foo
{
ArrayList<Double> msgstoboo = new ArrayList<Double>(Double);
public foo(int numofmessages)
{
for (int i = 0; i < numofmessages; i++)
{
msgstoboo.add(1);
}
}
public void setMsg(int index, double input)
{
msgstoboo.set(index,input);
}
&&
public class networkoffoos
{
ArrayList<ArrayList<foo>> foonetwork = new ArrayList<ArrayList<foo>>();
public void networkoffoos(int numoffoos)
{
for(int i = 0; i < numoffoos; i++)
foonetwork.add(new foo(somenumberofmsgs))
}
//**AND THE "updatefoomsg" method included in this post**
}

The following works for me. Made some small changes because your code wouldn't compile. Hope it helps.
import java.util.ArrayList;
public class test {
private static ArrayList<ArrayList<Foo>> foonetwork = new ArrayList<ArrayList<Foo>>();
public static void main(String[] args){
networkoffoos(5);
updatefoomsg(foonetwork);
}
public static void networkoffoos(int numoffoos) {
for(int i = 0; i < numoffoos; i++) {
ArrayList<Foo> fooArrayList = new ArrayList<Foo>();
fooArrayList.add(new Foo(10));
foonetwork.add(fooArrayList);
}
}
public static void updatefoomsg (ArrayList<ArrayList<Foo>> foonetwork) {
for(int foolayer = 0; foolayer< foonetwork.size(); foolayer++)
for(int fooinlayer = 0; fooinlayer< foonetwork.get(foolayer).size(); fooinlayer++)
for(int msginfoo = 0; msginfoo < foonetwork.get(foolayer).get(fooinlayer).msgstoboo.size(); msginfoo++)
foonetwork.get(foolayer).get(fooinlayer).setMsg(msginfoo,3);
}
}
and your Foo class
import java.util.ArrayList;
public class Foo {
ArrayList<Double> msgstoboo = new ArrayList<Double>();
public Foo(int numofmessages) {
for (int i = 0; i < numofmessages; i++) {
msgstoboo.add(Double.valueOf(1));
}
}
public void setMsg(int index, double input) {
msgstoboo.set(index, input);
}
}

I get errors, any suggestions?

When compiling this java program, I get errors like cannot find symbol... any suggestions?
import java.io.*;
import java.util.Scanner;
public class joel001
{
public int d;
// find the smallest number of an array
public static int small(int a[])
{
int smallest=0;
for(int i=0;i<a.length();i++)
{
if(a[i]<smallest)
{
smallest=a[i];
}
}
return smallest;
}
// subtract the smallest number of an array from all its elements
public static int[] sub(int a[],int d)
{
this.d=d;
for(int i=0;i<a.length();i++)
{
a[i]=a[i]-d;
}
return a;
}
// count the array's non zero elements
public static int count(int a[])
{
int countn=0;
for(int i=0;i<a.length();i++)
{
if(a[i]!=0)
{
countn=countn+1;
}
}
return countn;
}
public static void main(String args[])
{
int b,c,z,k=1;
int a[]=new int[1000];
Scanner s=new Scanner(System.in);
b=s.nextInt(); //input
for(i=0;i<b;i++)
{
a[i]=s.nextInt();
}
while(k==1)
{
z=count(a);
if(z==0)
{
break;
}
System.out.println(z);
c=small(a);
a=sub(a,c);
}
}
}

For a start, the length of an array is arr.length, not arr.length().
Secondly, in sub(), there is no this because it's a static function.
Thirdly, in main(), you need to declare i before trying to use it.
That will take care of all your compile-time errors. Run-time, or logic, errors are something you need to learn to fix in a debugger.

You are declaring all your methods of joel001 as static. Static methods are not related to an instance or object of the joel001 class. You need to research OOP before you continue.
Try something like this:
import java.io.*;
import java.util.Scanner;
public class joel001
{
public int d;
// find the smallest number of an array
public int small(int a[])
{
int smallest=0;
for(int i=0;i<a.length();i++)
{
if(a[i]<smallest)
{
smallest=a[i];
}
}
return smallest;
}
// subtract the smallest number of an array from all its elements
public int[] sub(int a[],int d)
{
this.d=d;
for(int i=0;i<a.length();i++)
{
a[i]=a[i]-d;
}
return a;
}
// count the array's non zero elements
public int count(int a[])
{
int countn=0;
for(int i=0;i<a.length();i++)
{
if(a[i]!=0)
{
countn=countn+1;
}
}
return countn;
}
public void main2() {
int b,c,z,k=1;
int a[]=new int[1000];
Scanner s=new Scanner(System.in);
b=s.nextInt(); //input
for(int i=0;i<b;i++)
{
a[i]=s.nextInt();
}
while(k==1)
{
z=count(a);
if(z==0)
{
break;
}
System.out.println(z);
c=small(a);
a=sub(a,c);
}
}
public static void main(String args[])
{
joel001 obj = new joel001();
obj.main2();
}
}

Trouble with Sort program using objects and constructors

Really new to java wanted to make my separate sorting methods (they all work hopefully i did them right)
Also very new to objects and constructors hopefully what im talking about is an object
so here are the constructors
public class sort{
public int[] selectsort(int[] num)
{
int j,i,key,min;
for(j = 0; j<num.length; j++)
{
key = num[j];
min = j;
for(i=j+1; i<num.length; i++)
{
if(num[i]<num[min])
{
min = i;
}
}
num[j] = num[min];
num[min] = key;
}
return num;
}
public int[] insertsort(int[] num)
{
int j,i,key;
for(j = 1; j<num.length; j++)
{
key = num[j];
for(i=j-1; i>=0 && num[i]>key; i--)
{
num[i+1]=num[i];
}
num[i+1]=key;
}
return num;
}
public static int[] bubblesort(int[] num)
{
int i,j,ini;
for(i = num.length-1; i>1; i--)
{
for(j=0;j<i; j++)
{
if(num[j]>num[j+1])
{
ini = num[j];
num[j]=num[j+1];
num[j+1]=ini;
}
}
}
return num;
}
}
and the program/test
import java.util.Arrays;
public class sorttest{
public static void main(String[] args)
{
int[] num = new int[]{9,1,4,5,6,2,3,7,8};
System.out.println(Arrays.toString(selectsort(num)));
}
}
javac sort.java compiles but javac sorttest.java doesnt
error:
sorttest.java:9: cannot find symbol
symbol : method selectsort(int[])
location: class sorttest
System.out.println(Arrays.toString(selectsort(num)));
^
1 error

The method selectsort is not a part of the sorttest class - it's a public static method in the sort class. This means you need to qualify it by its class name:
import java.util.Arrays;
public class sorttest{
public static void main(String[] args) {
int[] num = new int[]{9,1,4,5,6,2,3,7,8};
System.out.println(Arrays.toString(sort.selectsort(num)));
}
}
Alternatively, you could use a static import:
import static sort.selectsort;
import java.util.Arrays;
public class sorttest{
public static void main(String[] args) {
int[] num = new int[]{9,1,4,5,6,2,3,7,8};
System.out.println(Arrays.toString(selectsort(num)));
}
}