How do I make a variable array size?

Question

So my program is functional. I just want it to use a binary search of a text file and determine the highest index number it can be found in. If it cannot be found it finds the next largest and makes it a negative. My problem is right now the array size that gets made is 11. That is because it is how many integers are in the text file I am using. What if I will not know the number of integers? I understand people suggest making a list but will that not accept repetitions? why does this section of code:

while(scanner.hasNextInt()){
        arr[i++] = scanner.nextInt();}
    scanner.close();

not change my array size as necessary?

import java.util.Scanner;
import java.io.File;
import java.io.IOException;

public class BinarySearch {   
 public static int binarysearch(int[] arr, int key)
{
int lo = 0, hi = arr.length - 1;
{
    while (lo < hi) 
    {
        int mid = (lo + hi) / 2;
        if (arr[mid] <= key)
            lo = mid + 1;
        if (arr[mid] > key)
            hi = mid;
    }
    if (arr[lo] == key) {
        return lo;
    }
    else if ((arr[lo] != key) && (arr[lo-1] == key)){
        return lo - 1;
    }
    else{
    return (binarysearch( arr,(key-1))* - 1); 
} 
    }
} 



public static void main(String[] args) throws IOException{ {

    int k = Integer.parseInt(args[1]);
    Scanner scanner = new Scanner(new File(args[0]));

    int[] arr = new int[11];
    int i = 0;
    while(scanner.hasNextInt()){
        arr[i++] = scanner.nextInt();}
    scanner.close();

    System.out.println(binarysearch(arr, k));

}}
}

Possible duplicate of [Resize an Array while keeping current elements in Java?](https://stackoverflow.com/questions/13197702/resize-an-array-while-keeping-current-elements-in-java) — Logan, Sep 03 '18 at 00:29
Lists allow repeated values. Array lists resize arrays exactly the way the question is requesting. — Sean F, Sep 03 '18 at 00:34

Basil Bourque · Answer 1 · 2018-09-03T06:50:39.540

`List`

A List is a Collection often used instead of arrays. Specifically, the ArrayList implementation of List is backed by arrays internally, and handles re-sizing or re-building the internal array as needed, automatically.

List< Integer > integers = new ArrayList<>() ;
integers.add( 7 ) ;
integers.add( 11 ) ;

To find the highest, sort using the Collections utility class.

Collections.sort( integers ) ;

For reverse order:

Collections.reverse( integers ) ;

If you know roughly the size your list might end up needing, you can specify the initial capacity when instantiating your ArrayList. Setting the initial capacity can help with making ArrayList run a little more efficiently, but you need not worry about this.

int initialCapacity = ( myNumberOfThis * myNumberOfThat ) + 1 ;  // Whatever clues you have about possible size.
List< Integer > integers = new ArrayList<>( initialCapacity ) ;

See the Oracle Tutorial.

If you want to eliminate duplicates, use a Set instead. Search Stack Overflow as this has been covered many many times already.

How do I make a variable array size?

1 Answers1

List

`List`