Why am I getting index out of bounds exception?

I’ve made this code to convert whole base 10 numbers into binary. The code I believe is everything that it needs to be but I can’t get ArrayList to work.

I’ve spent a few hours on this site and other trying countless changes to no avail.

I’ve gotten the code to compile without errors but as soon as I run it and enter an int the program crashes.

Here is the code:

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

public class BinaryConverter {
   public static void main(String[] args) {
      Scanner in = new Scanner(System.in);
      ArrayList<Integer> binary = new ArrayList<Integer>();
      int a = in.nextInt();
      binary = binaryConverter(a);
      for (int i = binary.size(); i > -1; i = i - 1){
  public static ArrayList<Integer> binaryConverter(Integer n) {
   ArrayList<Integer> remainder = new ArrayList<Integer>();
   while (n/2 != 0) {
      n = n/2;
   return remainder;

These are the exceptions java throws at me when I input a number.

Jons-iMac:java jon$ java BinaryConverter

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index 8 out-of-bounds for length 8
    at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)
    at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)
    at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:248)
    at java.base/java.util.Objects.checkIndex(Objects.java:372)
    at java.base/java.util.ArrayList.get(ArrayList.java:440)
    at BinaryConverter.main(BinaryConverter.java:16)

I hope this is enough information.


In your code you get the value by index which is evaluated incorrectly.

  for (int i = binary.size(); i > -1; i = i - 1){

Assignment of binary.size() indeed out of bounds, because indexes counted from binary.size() to zero, but it is out of range which is bound from zero to binary.size()-1.

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