When I try to create an array it gives me an error "This array is read from only", why can't I write in it?

Question

The task is : Write a java program that takes an input stream of integers from the user and finds the smallest distance between two neighboring numbers in the stream. It should output the indices of the two neighboring numbers.

This is the code, I stored some number in an arraylist, then i took the last index of the arraylist and tried to create an array with the same number of elements

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Pr6 
{
public Pr6 ()
{
    Scanner sc = new Scanner (System.in);
    String x = "";
    int Distance =0;
    int Index1 = 0;
    int Index2 = 0;
    List<Integer> AllNum = new ArrayList<>();
    do 
    {
        System.out.println ("Please enter a number");
        int NewNumber = sc.nextInt();
        AllNum.add (NewNumber);
        System.out.println ("Do you want to enter a new number? Enter Y for yes and N for no");
        x = sc.nextLine();
    } while ("Y".equals(x));

    int Index = AllNum.size();
    int [] AllNumArray = new int [Index];

    for (int i = 0; i < Index; i++)
    {
        AllNumArray[i] = AllNum.get(i);
    }

    for (int i = 0; i < Index; i++)
    {
        int NewDistance = AllNumArray[i] - AllNumArray [i+1];
        if (NewDistance > Distance)
        {
            Distance = NewDistance;
            Index1 = i;
            Index2 = i+1;
        }
    }

    System.out.println ("The smallest distance between two neighboring numbers is: " + Distance);
    System.out.println("The first Nnumber is " + AllNumArray[Index1] + ", and its index is " + Index1);
    System.out.println("The second Nnumber is " + AllNumArray[Index2] + ", and its index is " + Index2);
}

}

```x``` is String type. So ```AllNum.lastIndexOf(x)``` always returns -1. — , Dec 19 '15 at 13:45
@SMA I edit the code, but now when I run it for the second time it gave me negativearraysize exception — TheGrimBoo, Dec 19 '15 at 14:01
@saka1029 the problem that the code doesn't allow me to enter the numbers in the arraylist — TheGrimBoo, Dec 19 '15 at 16:03
Is the error message exactly "This array is read from only"? Which is actual problem arraylist or array? — , Dec 19 '15 at 16:23
there is a new error the error: Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1 — TheGrimBoo, Dec 19 '15 at 16:37

Bahramdun Adil · Accepted Answer · 2015-12-19T16:49:46.927

You can get the last object as below:

ArrayList<Integer> AllNum = new ArrayList<>();
        do
        {
            System.out.println ("Please enter a num");
            AllNum.add (sc.nextInt());
            System.out.println ("Do you want to enter a new sentence? Enter Y for yes and N for no");
            x = sc.nextLine();
        } while ("Y".equals(x));
        int Index = AllNum.get(AllNum.size() - 1); // change is here
        int [] AllNumArray = new int [Index];

You get index of x from list which x is string Y, so it is not in the list, so it returns -1. Look to the JavaDoc

public int lastIndexOf(@org.jetbrains.annotations.Nullable java.lang.Object o) Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the highest index i such that (o==null ? get(i)==null : o.equals(get(i))), or -1 if there is no such index.

Edit: Rewrite the code:

Scanner sc = new Scanner(System.in);
String x; // no need to init here String x = "";
int Distance = 0;
int Index1 = 0;
int Index2 = 0;
List<Integer> AllNum = new ArrayList<>();
do {
    System.out.println("Please enter a number");
    int NewNumber = sc.nextInt();
    AllNum.add(NewNumber);
    System.out.println("Do you want to enter a new number? Enter Y for yes and N for no");
    x = sc.next(); // here changed sc.nextLine() -> sc.next()
} while (x.equalsIgnoreCase("y")); // here changed equals() -> equalsIgnoreCase()

int Index = AllNum.size();
int[] AllNumArray = new int[Index];
for (int i = 0; i < Index; i++) {
    AllNumArray[i] = AllNum.get(i);
}
for (int i = 0; i < Index; i++) {
    // here changed, and the main problem was here,
    int nextIndex = i < AllNumArray.length - 1 ? i + 1 : 0;
    int NewDistance = AllNumArray[i] - AllNumArray[nextIndex];
    if (NewDistance > Distance) {
        Distance = NewDistance;
        Index1 = i;
        Index2 = i + 1;
    }
}
System.out.println("The smallest distance between two neighboring numbers is: " + Distance);
System.out.println("The first Nnumber is " + AllNumArray[Index1] + ", and its index is " + Index1);
System.out.println("The second Nnumber is " + AllNumArray[Index2] + ", and its index is " + Index2);

the problem is that the code doesn't allow me to enter the numbers in the arraylist — TheGrimBoo, Dec 19 '15 at 16:02
But the code which you posted in the question has the problem which I solved in the answer, when i run the code it works well, with no problem — Bahramdun Adil, Dec 19 '15 at 16:06

When I try to create an array it gives me an error "This array is read from only", why can't I write in it?

1 Answers1